Seminars this semester

Oct 27	Tue	Sam Dolan (Sheffield)	Cosmology, Relativity and Gravitation
15:00		On the Riemann tensor, Lorentz transformations and SO(3,C)
		LT5 Hicks
	Abstract: This is a pedagogical talk for researchers interested in Einstein's theory of General Relativity. I will highlight the isomorphism between $SO+(3,1,R)$ [the group of proper orthochronous Lorentz transformations] and $SO(3,C)$ [the group of complex 3x3 orthogonal matrices with unit determinant]. The latter representation arises naturally when considering Lorentz transformations of bivectors (two-forms). Using the Hodge dual, bivectors can be written as complex 3-vectors. We may then write the Riemann tensor as a 3x3 complex matrix; and the Weyl tensor as a traceless symmetric matrix. Considering the eigenproblem leads us to the Petrov classification, and the various ways that gravitational fields can affect (e.g.) Szekeres' gravitational compass.
	Slides
Nov 3	Tue	Jurgen Mifsud (Sheffield)	Cosmology, Relativity and Gravitation
15:00		The consequences of disformal couplings on the fine-structure constant
		LT5, Hicks
Nov 17	Tue	David Dempsey (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Analogue Black Holes and the Draining Bathtub Vortex
		LT5, Hicks
	Abstract: I introduce the draining bathtub vortex; an analogue model of a Kerr black hole which may be observed in a laboratory. Surface waves on water obey the same wave equation as a scalar field on curved spacetime with a certain effective metric. The system we consider has both an analogue event horizon and an analogue ergosphere. We consider the behaviour of a wave created by a Gaussian far from the vortex and investigate its interaction with the analogue black hole. The wavefront travels along null geodesics of the metric and its height is related to the Van Vleck determinant.
Nov 24	Tue	Jake Shipley (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Double-Black-Hole Solutions and Chaos in General Relativity
		LT5
	Abstract: In this talk I will present an overview of the relativistic problem of two fixed extremal Reissner-Nordstrom black holes, which is a special case of the Majumdar-Papapetrou solution to the Einstein-Maxwell theory. In particular, I will discuss the null geodesics, with a particular focus on the unstable eternal orbits around the two black holes. We find that the initial conditions corresponding to these eternal orbits form a Cantor set. I will highlight the fractal nature of this set and how the intricate mixing of initial conditions leads to chaos around the two black holes.
Dec 1	Tue	Peter Dunsby (University of Cape Town)	Cosmology, Relativity and Gravitation
15:00		f(R) Theories of Gravity and the Emergence of Late Time Cosmological Acceleration
		LT5, Hicks
	Abstract: I will review various approaches to cosmological modelling in f(R) theories of gravity, using both top-down and bottom-up constructions. The top-down models are based on Robertson-Walker geometries and employ techniques such as Dynamical Systems methods and the reconstruction of the gravitational action from the expansion history of the Universe. The bottom-up constructions are built by patching together sub-horizon-sized regions of perturbed Minkowski space. The results obtained suggest that these theories do not provide a theoretically attractive alternative to the standard Concordance model of cosmology.
Dec 8	Tue	Jack Morrice (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Topology & Large Scale Structure
		LT5, Hicks
	Abstract: On the scales of galaxy clusters, galaxies and below, the fundamental structures, or countable objects, are aggregations of matter. Galaxies and their clusters form disconnected regions of high density, surrounded by a connected low density sea. In contrast, the countable objects on the largest observable scales seem to be the voids, or absences of matter. Somewhere in between these scales, the topology of structure completely reverses. In this very open-ended talk I will discuss how topology can provide new insights into the formation and evolution of the largest known structures, and how this may uncover some big new challenges to the standard model of cosmology.
Oct 19	Wed		Cosmology, Relativity and Gravitation
16:00		Research updates
		LT9, Hicks
Oct 26	Wed		Cosmology, Relativity and Gravitation
16:00		Models of Gravity conference update (Elizabeth) & research update (Sam)
		LT9, Hicks
Nov 16	Wed	Jake Shipley (Sheffield)	Cosmology, Relativity and Gravitation
16:00		Conference (Gravitational Lensing and Black Hole Shadows Workshop) and research visit updates
		LT9, Hicks
Sep 19	Tue	Elizabeth Winstanley (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Quantum field theory on anti-de Sitter space-time
		Hicks Seminar Room J11
Sep 21	Thu	Sam Dolan (Sheffield)	Cosmology, Relativity and Gravitation
14:00		Geometrical optics and spin-helicity effects.
		Hicks Seminar Room J11
	Abstract: The Belem group have recently shown a "spin-helicity effect" in the absorption of circularly-polarized electromagnetic & gravitational waves by a Kerr black hole, in which the counter-rotating helicity is more absorbed than the co-rotating helicity. The difference in the absorption cross sections scales with the inverse wavelength, so the helicity-dependence disappears in the zero-wavelength limit. The aim of this talk is to extend geometricaloptics beyond leading order to understand this effect.
Sep 21	Thu	Christopher Fewster (University of York)	Cosmology, Relativity and Gravitation
15:00		Preferred states in quantum field theories, ancient and modern
		Hicks Seminar Room J11
	Abstract: The vacuum state of Minkowski space quantum field theory is distinguished as a state of maximal symmetry. General curved spacetimes have no nontrivial symmetry and therefore lack an obvious candidate vacuum state. Nonetheless, one might wonder whether there is still a way of selecting a preferred state and there have been many attempts in that direction. I will discuss various aspects of this issue, describing a general model-independent no-go theorem that excludes the existence of a local and covariant choice of preferred state. I will also discuss recently-introduced class of "SJ states" and its extensions.
Oct 10	Tue	Antonin Coutant (Nottingham)	Cosmology, Relativity and Gravitation
16:00		The draining bathtub experiment
		Hicks Seminar Room J11
Oct 31	Tue	Nicola Franchini (Nottingham)	Cosmology, Relativity and Gravitation
16:00		Black Holes with Light Boson Hair and QPOs
		Hicks Seminar Room J11
	Abstract: A complex bosonic field minimally coupled to gravity, can give rise to black holes solutions that evade the no-hair theorem. These solutions are spinning black holes that can be drastically different from Kerr ones. A phenomenological way to distinguish hairy and Kerr black holes is to measure the quasi-periodic oscillations, which are peaks in the X-ray flux , emitted in the inner region of the accretion disk. Interpreting these peaks with the relativistic precession model, one can predict the emission around hairy black holes. Future generation X-ray telescopes will be able to measure with high precision the quasi-periodic oscillations, hence giving a way to test the no-hair theorem.
Nov 7	Tue	Theo Torres (Nottingham)	Cosmology, Relativity and Gravitation
16:00		Non-shallow water waves on a vortex: A model for dispersive fields around rotating black holes
		Hicks Seminar Room J11
	Abstract: Shallow water waves scattering on a draining and rotating potential flow constitute the analogue of a rotating black hole. In such a spacetime, it has been shown theoretically that, at low frequency, waves can extract energy from black holes. Such a process in known as superradiance. Our recent observation of this effect in an experiment at the University of Nottigham (T.T. et al. Nature Phys. 13 (2017) 833-836 arXiv:1612.06180 [gr-qc]) suggests that superradiance persists beyond the shallow water regime. In this talk, I will present the experiment we conducted and I will extend some features of analogue rotating black holes to the dispersive regime. Especially I will focus on light rings and quasi-normal modes.
Nov 14	Tue	Christopher Berry (University of Birmingham)	Cosmology, Relativity and Gravitation
16:00		Binary observations with LIGO and Virgo
		Hicks Seminar Room J11
	Abstract: Gravitational waves provide a new method for exploring the properties of black hole and neutron star binaries. I'll review the discoveries of LIGO and Virgo, and in particular GW170817, the first observation of a binary neutron star coalescence and the first gravitational wave signal to have a confirmed electromagnetic counterpart.
Nov 21	Tue	Vladimir Toussaint (University of Nottingham)	Cosmology, Relativity and Gravitation
16:00		Massive conformal zero-mode for the untwisted, free scalar field in the (1+1)-dimensional, spatially compactified Friedmann-Robertson-Walker (FRW) cosmological spacetimes.
		Hicks Seminar Room J11
	Abstract: We consider a massive scalar field in the (1 + 1)- dimensional, spatially compactified Friedmann-Robertson-Walker (FRW) cosmological spacetimes. We consider both twisted and untwisted fields. The issue of the massive conformal zero mode arises for the untwisted field whenever the effective mass vanishes at early or late times. More precisely we show that this occurs whenever the zero-momentum mode of the untwisted field reduces to a massive conformal zero-mode in the corresponding asymptotic region(s). To resolve this issue, we develop a new scheme for quantizing the zero momentum mode. This new quantization scheme introduces a family of two real parameters for every zero-momentum mode with an associated two-real-parameter set of in/out vacua. Moreover, we show that the zero momentum ground state's wave functional corresponds to a family of two-real parameter Gaussian wave packets that do not spread out in time. For applications, we examine the finite-time detector’s response to a massive scalar field in the (1 + 1)-dimensional, spatially compactified Milne spacetime. Explicit analytic results are obtained for the comoving and inertially non-comoving trajectories. Numerical results are provided for the comoving trajectory. The numerical results suggest that when the in-vacuum is chosen to be very far from the conventional Minkowski vacuum state, then it contains particles. As result, spontaneous excitation of the comoving detector occurs.
Nov 28	Tue	Markus Fröb (University of York)	Cosmology, Relativity and Gravitation
16:00		Algebraic quantum field theory, the Hadamard condition and Ward identities
		Hicks Seminar Room J11
	Abstract: In (perturbative) algebraic quantum field theory, the construction of interacting fields, including renormalisation, is done independently of the quantum state of the system. A crucial ingredient in the construction is a Green's function of Hadamard form. For gauge theories, so far one has been restricted to a special gauge (Feynman gauge in Yang-Mills theories), where this Green's function was explicitly known in a general curved background. We show how to construct Green's functions in a general linear covariant gauge, both for Yang-Mills theories and linearised gravity. These functions fulfil certain divergence and trace identities, which can be interpreted as Ward identities in the free theory, and are a prerequisite for the gauge independence of the interacting quantum theory.
Dec 5	Tue	Hannah Middleton (University of Birmingham)	Cosmology, Relativity and Gravitation
16:00		Constraints on the gravitational wave background from massive black hole binaries using pulsar timing arrays
		Hicks Seminar Room J11
	Abstract: Pulsar timing arrays are searching for the stochastic gravitational wave background from the merging population of massive black hole binaries. Upper limits on the background are beginning to reach astrophysically interesting sensitivities, however no detection has been reported so far. The recent upper limit from the Parkes Pulsar Timing Array has been interpreted as casting into doubt the standard model of binary assembly through galaxy mergers and hardening via stellar interactions, suggesting that their evolution must be accelerated or stalled. We use a Bayesian analysis to consider the implications of the upper limit for a range of astrophysical scenarios. Weak constraints can be placed on the population parameters, however we find that these astrophysical scenarios are as yet fully consistent with the current observations.
Dec 12	Tue	Lucas Lombriser (Royal Observatory Edinburgh)	Cosmology, Relativity and Gravitation
16:00		Cosmic Self-Acceleration from Modified Gravity before/after GW170817
		Hicks Seminar Room J11
	Abstract: Scalar-tensor modifications of gravity have long been considered as an alternative explanation for the late-time accelerated expansion of our Universe. I will first show that a rigorous discrimination between acceleration from modified gravity and from a cosmological constant or dark energy was not possible with observations of the large-scale structure alone. I will then demonstrate how the measurement of the cosmological speed of gravitational waves with GW170817 breaks this dark degeneracy and how the combination of the two challenges the concept of cosmic acceleration from one of the most general scalar-tensor modifications of gravity. Even more general theories, however, reintroduce the dark degeneracy and I will show how a more conclusive result will only be possible with a large number of Standard Sirens. (Refs: 1509.08458; 1602.07670; https://arstechnica.com/science/2017/02/theoretical-battle-dark-energy-vs-modified-gravity).
Feb 13	Tue	Tommi Tenkanen (Queen Mary UL)	Cosmology, Relativity and Gravitation
16:00		Primordial Black Holes as Dark Matter
		Hicks Seminar Room J11
	Abstract: I revisit the cosmological and astrophysical constraints on the fraction of dark matter (DM) in primordial black holes (PBHs). I consider a variety of production mechanisms and mass functions for PBHs and discuss whether they can constitute the observed DM abundance or not. I also discuss how one can constrain the physics of the early Universe with the constraints on PBHs, presenting e.g. constraints on the running of the inflaton spectral index which are comparable to those from the Planck satellite.
May 15	Tue	Damien Trinh (Manchester)	Cosmology, Relativity and Gravitation
16:00		The dark sector in light of GW170817
		Hicks Seminar Room J11
	Abstract: Recent observations in the propagation of gravitational waves have seemingly severely restricted viable alternatives to the cosmological constant. Indeed, the stringent constraint that gravitational waves must propagate at the speed of light is not something which many modified gravity theories predict. Using the Equation of State approach, I will discuss its implications to some theories and also highlight some caveats to the recent observations which may yield a slightly more optimistic outlook for this field.
May 22	Tue	Joseph Fernandez (Liverpool John Moores)	Cosmology, Relativity and Gravitation
16:00		Tidal encounters of compact binaries with massive black holes as a source of gravitational waves
		Hicks Seminar Room J11
	Abstract: Massive black holes (MBHs) are ubiquitous in galactic centres. The extreme potential due to these objects dominates the surrounding dynamics, giving rise to physics not possible in other regions. These regions are of particular interest for gravitational wave astronomy, as several dynamical processes which can give rise to black hole (BH) binary mergers have been postulated. We show that compact binaries can survive close encounters with the MBH without being disrupted, and that they tend to become hard and eccentric. Since the GW merger time of binaries is sensitive to the semi-major axis length and eccentricity, we find that this leads to a reduction of the merger time by several orders of magnitude in some cases. Therefore, tidal encounters of stellar mass BH binaries with a MBH at the centre of galaxies can provide a new formation channel of BH mergers. We use Monte Carlo simulations to evaluate the effective spin of the binaries after the encounter We also provide a description of a simple scenario to understand how this process could take place the a larger astrophysical context.
May 29	Tue	Jessie Durk (Queen Mary UL)	Cosmology, Relativity and Gravitation
16:00		Black hole lattices as inhomogeneous cosmological models
		LT09, Hicks
	Abstract: The standard model of cosmology, ΛCDM, is based on the assumption that the Universe can be described by the homogeneous and isotropic FLRW solution to the Einstein field equations. The need to test whether the large-scale expansion of space is that of FLRW, or is instead modified by the presence of inhomogeneities, has lead to this assumption being relaxed. An interesting subset of inhomogeneous cosmologies include those dubbed black hole lattices. These are exact, fully-relativistic treatments of universes with a discretised matter content. We generalise an existing family of these to include a cosmological constant, structure formation and electric charge. For each new generalisation, we find a common behaviour of tending towards FLRW-like as the number of masses is increased, and for the addition of structures, we investigate the effect of gravitational interaction energies between clustered masses.
Oct 10	Wed	Visakan Balakumar & Jake Percival (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Report from QFT Energy Inequalities conference
		Hicks Seminar Room J11
Oct 17	Wed	Jack Morrice (Cape Town)	Cosmology, Relativity and Gravitation
00:00		Streams: a small, mesh-based look at large structure formation
		Hicks Seminar Room J11
	Abstract: Today, cosmology's N-body paradigm looks a bit like a power monopoly. The codes (GADGET, RAMSES...) have huge computational overheads; descriptions of their underlying algorithms are old and hard to find; and the outputs of these codes are very difficult to analyze. These factors make the study of cosmic large scale structure accessible only to those with easy access to a lot of supercomputer cores and a familiarity with old imperative programming languages like C and Fortran. In this talk, we will look at Streams, a modest attempt to address these issues. Streams is a small package written for the Julia programming language that uses Lagrangian perturbation theory to significantly reduce overheads, and is built from a computer geometry (mesh) perspective which, together, make it very well suited to studying the formation of folds and caustics in the dark matter fluid on a personal computer.
Oct 24	Wed	Elizabeth Winstanley (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Quantum expectation values on black hole space-times
		Hicks Seminar Room J11
	Abstract: The renormalized expectation value of the stress energy tensor (RSET) is an object of central importance in quantum field theory in curved space-time, but calculating this on black hole space-times is far from trivial. The standard methodology was developed in the 1980s and 1990s and successfully applied to a range of quantum fields on Schwarzschild black holes. The subject received an impetus in the last few years with to the development of two novel approaches to computing the RSET and renormalized vacuum polarization (VP). These advances have enabled calculations on a wider range of black hole space-times to be performed. In this talk we will review both the standard and novel methodologies and some results for the RSET and VP on asymptotically flat, de Sitter and anti-de Sitter black holes.
Nov 7	Wed	Nicola Rendell (York)	Cosmology, Relativity and Gravitation
15:00		Infrared divergences in cosmological spacetimes
		Hicks Seminar Room J11
	Abstract: We study the infrared divergences of the graviton propagator in FLRW spacetime. We show that, through the use of a 'large' gauge transformation, this divergence is a gauge effect.
Nov 14	Wed	Eleni Kontou (York)	Cosmology, Relativity and Gravitation
15:00		Strong quantum energy inequality and the Hawking singularity theorem
		Hicks Seminar Room J11
	Abstract: Hawking's singularity theorem concerns matter obeying the strong energy condition (SEC), which means that all observers experience a nonnegative effective energy density (EED), thereby guaranteeing the timelike convergence property. However, for both classical and quantum fields, violations of the SEC can be observed in some of the simplest of cases, like the massive Klein-Gordon field. Therefore there is a need to develop theorems with weaker restrictions, namely energy conditions averaged over an entire geodesic and quantum inequalities, weighted local averages of energy densities. We have derived lower bounds of the EED in the presence of both classical and quantum scalar fields allowing nonzero mass and nonminimal coupling to the scalar curvature. In the quantum case these bounds take the form of a set of state-dependent quantum energy inequalities valid for the class of Hadamard states. Finally, we discuss how these lower bounds are applied to prove Hawking-type singularity theorems asserting that, along with sufficient initial contraction at a compact Cauchy surface, the spacetime is future timelike geodesically incomplete. Talk is based on: DOI:10.1007/s10714-018-2446-5, arXiv:1809.05047 and a manuscript in preparation.
Nov 21	Wed	Konstantinos Dimopoulos (Lancaster)	Cosmology, Relativity and Gravitation
15:00		Cosmic Inflation and Dark Energy from the Electroweak Phase Transition
		Hicks Seminar Room J11
	Abstract: Cosmic inflation is a period of accelerated expansion in the Early Universe. Inflation is the most compelling proposal for the formation of of the observed structures in the Universe like galaxies and galactic clusters. It also makes the Universe uniform and spatially flat in agreement with observations. To drive inflation an exotic substance is needed, with pressure negative enough to cause the expansion of the Universe to accelerate, when this substance is dominant. Observations suggest that the late Universe is also undergoing accelerated expansion, which is assumed to be due to another exotic substance called dark energy. Can this be the one and the same with the substance behind inflation? In this talk I present a novel idea, in which inflation leaves behind a minute potential density, which can become the dark energy observed today. The field responsible for inflation (scalaron), is trapped in a local minimum of its scalar potential until the electroweak phase transition. The transition releases the field and allows it to vary slowly down a shallow potential tail, becoming dark energy. This behaviour is facilitated by a suitable coupling between the scalaron field and the electroweak Higgs field. The model is successful without fine-tuning, because it makes use of the curious fact that the electroweak energy scale is roughly the geometric mean of the Planck scale and the dark energy scale.
Nov 28	Wed	Paul McFadden (Newcastle)	Cosmology, Relativity and Gravitation
00:00		Conformal field theory in momentum space
		Hicks Seminar Room J11
	Abstract: Conformal symmetry places strong constraints on the properties of a quantum field theory, fixing the form of all 2- and 3-point functions up to constants. The resulting expressions are well-known in position space, yet surprisingly their counterparts in momentum space have only recently been identified. We review these developments, introducing an elementary method for solving the momentum-space conformal Ward identities. In special cases, divergences arise and we must renormalise giving rise to beta functions and anomalies. Our results have interesting new applications ranging from condensed matter physics to holographic cosmology.
Dec 5	Wed	Thomas Morley (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Renormalised vacuum polarisation on topological black holes
		Hicks Seminar Room J11
	Abstract: Anti-de Sitter spacetime is a solution of Einstein's equations with a negative cosmological constant. This fact allows for unusual black hole solutions with non-spherical horizon topology. We calculate the renormalised vacuum polarisation for black holes with spherical, flat and hyperbolic event horizons, following the “extended coordinates” method, which uses a mode-sum representation for the Hadamard parametrix. Renormalisation counter terms are subtracted from the Green’s function mode-by-mode, leaving each individual term manifestly finite.
Dec 5	Wed	Jake Percival (Sheffield)	Cosmology, Relativity and Gravitation
15:30		Semiclassical gravity for static spacetimes: Universality and structure dependence.
		Hicks Seminar Room J11
Dec 12	Wed	Sebastian Trojanowski (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Looking forward to new physics with FASER: ForwArd Search ExpeRiment at the LHC
		Hicks Seminar Room J11
	Abstract: One of the most rapidly developing areas of research in particle physics nowadays is to look for new, light, extremely weakly-interacting particles that could have avoided detection in previous years due to the lack of luminosity. These, so-called intensity frontier searches, have also broad cosmological connections to e.g. dark matter, as well as can help to unravel the mystery of neutrino masses. In this talk, we will summarize the current status of this field with a particular emphasis on a newly proposed experiment to search for such particles produced in the far-forward region of the LHC, namely FASER, the ForwArd Search ExpeRiment. FASER has been proposed as a relatively cheap detector to supplement traditional experimental programmes searching for heavy new physics particles in the high-pT region and, therefore, to increase the whole BSM physics potential of the LHC. On top of potentially far-reaching implications to BSM particle physics and cosmology, the newly proposed detector can also be used to measure high-energy SM neutrino cross sections.
Oct 2	Wed	Peter Millington (University of Nottingham)	Cosmology, Relativity and Gravitation
15:00		Quasi-normal modes and fermionic vacuum decay around a Kerr black hole
		Hicks Seminar Room J11
	Abstract: The non-rotating fermion vacuum in Kerr spacetimes is unstable to a spontaneous vacuum decay, which leads to the formation of a co-rotating Dirac sea. This decay, which amounts to the fermionic pendant of the black hole bomb instability, has an analogue in the electrodynamics of supercritical fields, and we show that the decay process is encoded by the set of quasi-normal fermion modes.
	Slides
Oct 9	Wed	Adam Moss (University of Nottingham)	Cosmology, Relativity and Gravitation
15:00		Accelerated Bayesian inference using deep learning
		Hicks Seminar Room J11
	Abstract: I introduce a novel Bayesian inference tool that uses a neural network to parameterise efficient Markov Chain Monte-Carlo (MCMC) proposals. The target distribution is first transformed into a diagonal, unit variance Gaussian by a series of non-linear, invertible, and non-volume preserving flows. Neural networks are extremely expressive, and can transform complex targets to a simple latent representation from which one can efficiently sample. Using this method, I develop a nested MCMC sampler, finding excellent performance on highly curved and multi-modal analytic likelihoods. I also demonstrate it on Planck 2015 data, showing accurate parameter constraints, and calculate the evidence for simple one-parameter extensions to LCDM in $\sim20$ dimensional parameter space.
Oct 30	Wed	Thomas Stratton (University of Sheffield)	Cosmology, Relativity and Gravitation
15:00		Scattering of gravitational waves by a neutron star
		Hicks Seminar Room J11
	Abstract: Since the direct detection of gravitational waves in 2015, a new window on the physical Universe has begun to open. A region of spacetime with large enough curvature, such as a black hole or neutron star, may scatter a freely propagating gravitational wave. I will consider scattering of gravitational waves by a compact star modelled with a polytropic equation of state. Within the framework of perturbation theory, I calculate the differential scattering cross section and discuss the interference effects present, namely rainbow and glory scattering. I will show how the star’s properties, such as the equation of state, imprint themselves on the cross section, and compare our results with black hole scattering.
	Slides
Nov 6	Wed	Lasse Schmieding (University of York)	Cosmology, Relativity and Gravitation
15:00		Scalar Fields in two dimensional de Sitter Space
		Hicks Seminar Room J11
	Abstract: Unlike higher dimensional de Sitter spaces, two dimensional de Sitter space is not simply connected. The behaviour of the fields on making a full rotation of the spatial direction must therefore be specified. Previously, Epstein and Moschella have shown that anti-periodic real scalar fields have no analogue of a Bunch-Davies vacuum state. For complex scalar fields, more general behaviour is possible. I will discuss complex scalar field theories in two dimensional de Sitter space and then comment on the existence of de Sitter invariant and Hadamard states for these theories. Along the way, I will review aspects of the representation theory of SL(2,R), the symmetry group relevant for two dimensional (anti-)de Sitter space.
Nov 22	Fri	Eleonora Di Valentino (University of Manchester)	Cosmology, Relativity and Gravitation
14:00		Cosmology in tension
		LT9, Hicks
	Abstract: The Cosmic Microwave Background (CMB) temperature and polarization anisotropy measurements from the Planck mission have provided a strong confirmation of the LCDM model of structure formation. However, there are a few interesting tensions with other cosmological probes and anomalies in the data that leave the door open to possible extensions to LCDM. The most famous ones are the Hubble constant and the S8 parameter tensions, the Alens anomaly and a curvature of the Universe. I will review all of them, showing some interesting extended cosmological scenarios, in order to find a new concordance model that could explain the current cosmological data.
Dec 4	Wed	Natalie Hogg (University of Portsmouth)	Cosmology, Relativity and Gravitation
15:00		Constraining the interacting vacuum model of dark energy
		Hicks Seminar Room J11
	Abstract: There are well-known problems within the LambdaCDM model of cosmology, such as the H0 tension, that motivate the search for alternative dark energy models. In this talk, I will present one such alternative, known as the interacting vacuum scenario. In this scenario, the vacuum is free to exchange energy with the cold dark matter. Models of this type have the potential to resolve the H0 tension. I will start by discussing LCDM and its problems, then introduce the theory of the interacting vacuum model. I will present the results of a recent work (1902.10694) in which we constrained this model with observational data and conclude with a model comparison between the interacting model and LCDM.
Dec 11	Wed	Theo Torres (University of Sheffield)	Cosmology, Relativity and Gravitation
15:00		Hydrodynamic simulations of rotating black holes
		Hicks Seminar Room J11
	Abstract: Wave scattering phenomena are ubiquitous in almost all Sciences, from Biology to Physics. Interestingly, it has been shown many times that different physical systems are the stage to the same processes. One stunning example is the observation that waves propagating on a flowing fluid effectively experience the presence of a curved space-time. In this talk we will use this analogy to investigate, both theoretically and experimentally, fundamental effects occurring around vortex flows and rotating black holes. In particular, we will focus on light-bending, superradiance scattering, and quasi-normal modes emission
Jan 30	Thu	Kento Osuga (University of Sheffield)	Cosmology, Relativity and Gravitation
15:00		Topological Recursion and Supersymmetry
		Hicks Seminar Room J11
	Abstract: Topological recursion is an abstract recursive formalism which was originally introduced to solve matrix models to all order in the large N expansion. Somewhat surprisingly, however, topological recursion has its own life beyond matrix models and its applications appear in both physics and mathematics such as 2d quantum gravity and Gromov-Witten invariants. Then an interesting question arises: does a similar story hold with supersymmetry? In this talk, I will first review the notion of topological recursion and briefly explain how it can be used to solve matrix models. I will then introduce a supersymmetric analogue of matrix models called supereigenvalue models and discuss their recursive structure. This is a joint work with Vincent Bouchard.
Feb 5	Wed	Axel Polaczek (University of Sheffield)	Cosmology, Relativity and Gravitation
15:00		Quantum Gravity and Cosmology
		Hicks Seminar Room J11
	Abstract: In this talk I will give an overview of quantum gravity with an emphasis on approaches that involve discretisations of spacetime. I will then focus on group field theory (GFT) which is an approach aiming to describe the dynamics of the microscopic degrees of freedom. In particular, I will discuss cosmology in the context of GFT, where one of the main results is the resolution of the big bang singularity.
Feb 12	Wed	Chris Fewster (University of York)	Cosmology, Relativity and Gravitation
15:00		Singularity theorems with weakened energy hypotheses inspired by QFT
		Hicks Seminar Room J11
	Abstract: The original singularity theorems of Penrose and Hawking were proved for matter obeying the Null Energy Condition or Strong Energy Condition respectively. Various authors have proved versions of these results under weakened hypotheses, by considering the Riccati inequality obtained from Raychaudhuri's equation. Here, we give a different derivation that avoids the Raychaudhuri equation but instead makes use of index form methods. We show how our results improve over existing methods and how they can be applied to hypotheses inspired by Quantum Energy Inequalities. In this last case, we make quantitative estimates of the initial conditions required for our singularity theorems to apply. The talk will be largely based on arXiv:1907.13604 (joint work with E.-A. Kontou) and will introduce index form methods from the start.
Feb 19	Wed	Leong Khim Wong (Cambridge)	Cosmology, Relativity and Gravitation
15:00		Dynamics of black holes with induced scalar charges
		Hicks Seminar Room J11
	Abstract: While stringent no-hair theorems forbid isolated black holes from possessing permanent moments beyond their mass, spin and electric charge, the presence of an external scalar field can endow a black hole with additional multipole moments even when the field is minimally coupled to gravity. Recent advancements in effective field theory (EFT) techniques make it possible to study how these induced scalar multipoles affect the dynamics of black holes in binary systems. I will present an overview of the EFT approach and will discuss some interesting phenomena that arise due to this effect, including a novel guise of superradiance.
Feb 27	Thu	Rodrigo Panosso Macedo (Queen Mary University of London)	Cosmology, Relativity and Gravitation
15:00		Revisiting black-hole perturbation theory: the hyperboloidal slice approach
		Hicks Seminar Room J11
	Abstract: After reviewing the well-stablished notion of black-hole perturbation theory and the concept of quasinormal modes, we present a spectral representation of solutions to relativistic wave equations based on a geometrical approach in which the constant-time surfaces extend until future null infinity. Here, we restrict ourselves to an asymptotically flat, spherical symmetric spacetime (with focus on the Reisnner-Nordstrom solution), though the geometrical framework extends also to the Kerr spacetime. With the help of a Laplace transformation on the wave equation in question, we provide a geometrical interpretation to known algorithms (i.e. Leaver’s approach) apart from deriving an algorithm for obtaining all ingredients of the desired spectral decomposition, including quasi-normal modes, quasi-normal mode amplitudes as well as the jump of the Laplace-transform along the branch cut.
Mar 6	Fri	Cameron Thomas (University of Sheffield)	Cosmology, Relativity and Gravitation
14:00		Predictions of interacting dark energy
		Hicks Seminar Room J11
	Abstract: Dark energy is a form of energy with an unknown physical origin that makes up nearly 70% of the universe and is causing our universe to expand at an accelerating rate. A popular candidate for dark energy is a classical scalar field with a self-interaction potential (quintessence), the other most popular candidate being the cosmological constant Λ which explains cosmological observations very well, but suffers from numerous theoretical problems which quintessence, at least in part, appears to solve. If a quintessence scalar field exists, then it is expected to dynamically couple to all matter species, both baryonic and dark, unless forbidden by some hidden symmetry. Direct coupling between dark energy and baryonic matter is severely constrained by so-called “fifth-force” experiments, however, a direct coupling between dark energy and dark matter is not constrained by conventional experiments, and remains open for investigation. I will review models with conformal couplings and their cosmological consequences, as well as their status in light of the swampland conjectures.
Apr 29	Wed	Benjamin Elder (University of Nottingham)	Cosmology, Relativity and Gravitation
15:00		Dynamical friction in superfluids
		https://app.vscene.net/r/KjKhl4yTE4
	Abstract: A number of modern theories of dark matter hypothesize that it can condense into a superfluid phase. Such a phase could be distinguished from ordinary particle dark matter by its lack of dynamical friction, a process by which moving objects are slowed by gravitational attraction to their own wake. This process involves both the Jeans instability and the “quantum pressure” of superfluid dark matter. I will present a treatment of this phenomenon via two equivalent formalisms: (i) a standard hydrodynamical description of superfluid density waves, and (ii) a novel quasiparticle description, wherein massive perturbers lose energy via the radiation of phonons. Somewhat surprisingly, we find that even subsonic perturbers can lose energy to dynamical friction. This theoretical work resolves a long-standing puzzle associated with dynamical friction in fluids, and paves the way for more detailed study of superfluid dark matter phenomenology.
	Slides
May 6	Wed	Suddhasattwa Brahma (McGill University)	Cosmology, Relativity and Gravitation
15:00		Theory confronts Observations: Cosmology in the era of the Swampland
		https://app.vscene.net/r/KjKhl4yTE4
	Abstract: It is well-known that accelerating spacetimes form the basis of our understanding of early and late-time cosmology. On the other hand, there has been a pile of mounting evidence, mainly based on numerous results from String Theory (but not limited to them), that de Sitter space is difficult to embed in a quantum theory of gravity. Thus, these theoretical constraints that any consistent effective field theory must satisfy in order to have a UV-completion -- the so-called "Swampland conjectures" -- form a new challenge for phenomenologically viable model-building in cosmology. In this talk, I shall discuss some aspects of these conjectures, evidence in support of them and how to reconcile them with astronomical observations with a special focus on inflation. The importance of non-perturbative quantum corrections in constructing quasi de-Sitter backgrounds shall also be demonstrated.
May 13	Wed	Lucia Menendez-Pidal De Cristina (University of Nottingham)	Cosmology, Relativity and Gravitation
15:00		Singularity resolution depends on the clock
		https://app.vscene.net/r/KjKhl4yTE4
	Abstract: We study the quantum cosmology of an FLRW universe filled with a free massless scalar field and a perfect fluid that can be interpreted either as dark energy, radiation or dust. As general relativity does not have a preferred time coordinate, we study two versions of the quantum theory using different dynamical variables as clocks. The general covariance of the classical theory is not conserved after quantisation. The two theories exhibit different behaviour regarding singularity resolution and hence this cosmological model serves as an illustration of the problem of time.
	Slides
May 20	Wed	Kieran Finn (University of Manchester)	Cosmology, Relativity and Gravitation
15:00		Frame Covariance in Quantum Gravity
		https://app.vscene.net/r/KjKhl4yTE4
	Abstract: The laws of physics should not depend on how we choose to describe them. However, this is exactly what happens in the standard formulation of quantum field theories. The effective action receives different quantum corrections depending on how we parametrise our fields and even Feynman diagrams can yield results that depend on the definition of the fields we choose to work with. In this talk I will rectify these problems by introducing the notion of frame covariance, in which the quantum fields are treated as coordinates on a manifold, known as the field space. Field redefinitions are then simply diffeomorphisms of this manifold and thus we can impose reparametrisation invariance using well-known techniques from differential geometry. This talk is based on arXiv:1910.06661.
	Slides
May 27	Wed	Philippe Brax (Universite Paris-Saclay)	Cosmology, Relativity and Gravitation
15:00		Charged Dark matter and H0 tension
		https://app.vscene.net/r/KjKhl4yTE4
	Abstract: I will describe recent models of non-linear electrodynamics with charged dark matter and their cosmological consequences. In particular, I will emphasize the inhomogeneous nature of the resulting cosmology. This follows from the screening of the extra U(1) in a way akin to the K-mouflage mechanism of modified gravity. I will eventually argue that this may have some applications to the local dynamics vs large scale structure of the Universe and the H0 tension.
Jun 3	Wed	Visakan Balakumar (University of Sheffield)	Cosmology, Relativity and Gravitation
00:00		Hadamard renormalisation for a charged scalar field
		https://app.vscene.net/r/KjKhl4yTE4
	Abstract: The Hadamard renormalisation method provides a powerful and axiomatic approach to renormalising the stress-energy tensor in the study of quantum fields in curved spacetime. This procedure has been developed by Decanini and Folacci for massive neutral scalar fields in a general spacetime of arbitrary dimension. Motivated by the study of superradiant scattering in Reissner–Nordström black holes, we extend their work to include charged scalar fields in spacetimes with a classical, background gauge field and explicitly demonstrate the Hadamard renormalisation procedure in four dimensions.
	Slides
Jun 17	Wed	Hal Haggard (Bard College)	Cosmology, Relativity and Gravitation
15:00		The Black Hole Spin Puzzle, Black Hole Entropy, and Gravitational Wave Observations
		https://app.vscene.net/r/KjKhl4yTE4
	Abstract: Black hole entropy is a robust prediction of quantum gravity with no observational test to date. This entropy can be used to determine the probability distribution of the spin of black holes at equilibrium in the microcanonical ensemble. This ensemble, relevant for black holes formed in the early universe, predicts the existence of a population of black holes with zero spin. Observations of such a population at LIGO, Virgo, and future gravitational wave observatories would elucidate the statistical nature of black hole entropy and the origin of stellar mass black holes.
	Slides
Jun 24	Wed	Ippocratis Saltas (CEICO)	Cosmology, Relativity and Gravitation
15:00		New tests of gravity at large and small scales in the Universe
		https://app.vscene.net/r/KjKhl4yTE4
	Abstract: In this talk, I will explain the implications of the recent measurement of the gravitational waves’ speed for large and small scales in the Universe. I will pay special focus on how the physics of stellar structure, and in particular helioseismology, can provide us with tight constraints on the residual scalar-gravity interactions.
Jul 1	Wed	Nelson Nunes (University of Lisbon)	Cosmology, Relativity and Gravitation
15:00		Disformal couplings with a LambdaCDM background
		https://app.vscene.net/r/KjKhl4yTE4
	Abstract: The discovery of the accelerated expansion of the Universe is now very well established and the LambdaCDM paradigm is the strongest candidate to explain it. Nonetheless, there are tensions between the relatively high level of clustering found in cosmic microwave background experiments and the smaller one obtained from large-scale observations in the late Universe. A way to alleviate this issue is to consider a scalar field dark energy component conformally coupled to dark matter maintaining a LambdaCDM background cosmology. In this presentation we extend these studies to allow in addition disformal couplings which we explore by performing a dynamical system analysis.
Jul 8	Wed	Robert Brandenberger (McGill University)	Cosmology, Relativity and Gravitation
15:00		Is Inflationary Cosmology Consistent with Fundamental Physics?
		https://app.vscene.net/r/KjKhl4yTE4
	Abstract: The inflationary scenario is currently the paradigm of early universe cosmology. However, an embedding of inflation into a fundamental theory is missing. I will first show that there are alternative early universe scenarios which are consistent with current observations. Hence, we do not require inflation to explain the data. Then I will discuss recent challenges which indicate that standard inflation is NOT consistent with fundamental physics. Specifically, I will discuss the "Swampland Criteria" and the "Trans-Planckian Censorship Conjecture".
Sep 30	Wed	Visakan Balakumar (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Quantum superradiance on static black hole space-times
		Blackboard Collaborate
	Abstract: We study the quantum analogue of the classical process of superradiance for a massless charged scalar field on a charged black hole space-time. We show that an “in” vacuum state, which is devoid of particles at past null infinity, contains an outgoing flux of particles at future null infinity. This radiation is emitted in the superradiant modes only, and is nonthermal in nature.
Oct 7	Wed	Katy Clough (Oxford)	Cosmology, Relativity and Gravitation
15:00		Initial conditions for inflation
		Blackboard Collaborate
	Abstract: Inflation solves a number of problems in early universe cosmology but potentially introduces some new ones regarding how it was able to get started in the first place. In this talk I will explain these issues in the context of single field slow roll inflationary models, and discuss how they might restrict the phase space of initial conditions and early universe models that we consider valid. I will describe work I have done using numerical relativity to investigate the problem in the non-linear regime.
Oct 14	Wed	Jose Cembranos (Complutense, Madrid)	Cosmology, Relativity and Gravitation
15:00		Disformal dark matter from brane-worlds
		Blackboard Collaborate
	Abstract: Scalar particles coupled to the Standard Model fields through a disformal coupling arise in different theories, such as massive gravity or brane-world models. We will review the main phenomenology associated with such particles. Distinctive disformal signatures could be measured at colliders and with astrophysical observations. The phenomenological relevance of the disformal coupling demands the introduction of a set of symmetries, which may ensure the stability of these new degrees of freedom. In such a case, they constitute natural dark matter candidates since they are generally massive and weakly coupled. We will illustrate these ideas by paying particular attention to the branon case, since these questions arise naturally in brane-world models with low tension, where they were first discussed.
Oct 21	Wed	Atsushi Higuchi (York)	Cosmology, Relativity and Gravitation
15:00		The Hartle-Hawking vacuum state in the Schwinger-Keldysh formalism for interacting scalar field theory
		Blackboard Collaborate
	Abstract: Schwarzschild and de Sitter spacetimes are spacetimes with bifurcate Killing horizons. It is known that the preferred vacuum state (the Hartle-Hawking state) in quantum field theory in these spacetimes is a thermal state in the static patch. For example, it is the thermal state with the Hawking temperature outside the horizon in the Schwarzschild case. It is also accepted that this state is obtained by analytic continuation from the corresponding Euclidean theory. In this talk, I will make this statement more precise in the context of the Schwinger-Keldysh (or in-in) perturbation theory. (This is a joint work in progress with William C C de Lima.)
Oct 28	Wed	Harry Desmond (Oxford)	Cosmology, Relativity and Gravitation
15:00		Fifth force searches in galaxies
		Blackboard Collaborate
	Abstract: Fifth forces generically follow from new dynamical fields, and hence are ubiquitous in extensions to the standard model. Broad classes of Lagrangian exhibit "screening mechanisms" which hide the fifth force in high-density environments such as the Milky Way, while keeping it operative on larger scales. I will describe the search for fifth forces which act differently on different components of galaxies, e.g. through screening. First, I model the gravitational environments of the local Universe to determine the screening properties of real galaxies and the strength of the fifth-force field over space. I then use this information to forward-model two signals in galaxy morphology -- displacements of stars and gas and warping of stellar disks -- and hence infer fifth-force parameters with a Bayesian likelihood framework. Taking ~16,000 HI-cross-optical detections from the ALFALFA and SDSS surveys and ~4,000 galactic disk images from the Nasa Sloan Atlas, I set the strongest constraints to date on astrophysical fifth forces. Two particularly interesting applications are to f(R) and models such as coupled quintessence in which the fifth force acts only in the dark sector: for the former I require f_R0 < 1.4x10^-8 in the Hu-Sawicki model, and for the latter a fifth-force strength <10^-4 times that of gravity.
Nov 11	Wed	Lisa Glaser (Vienna)	Cosmology, Relativity and Gravitation
15:00		Simulating spectral triples
		Blackboard Collaborate
	Abstract: A spectral triple consists of an algebra, a Hilbert space and a Dirac operator, the information contained in these is equivalent to that of a differential manifold. However it also offers avenues for generalization, since a general algebra can be non-commutative, which leads us to non-commutative geometry. In this talk I will introduce spectral triples and non-commutative geometry, and then from there move on to talking about my own work, using computer simulations to better understand the structure of spectral triples, and to visualize them.
Nov 18	Wed	Elisa Maggio (Sapienza, Rome)	Cosmology, Relativity and Gravitation
15:00		Gravitational wave signatures of exotic compact objects
		Blackboard Collaborate
	Abstract: Gravitational waves from the coalescence of compact binaries provide a unique opportunity to test gravity in strong field regime. In particular, the postmerger phase of the gravitational signal is a proxy for the nature of the remnant. This is of particular interest in view of some quantum-gravity models which predict the existence of horizonless exotic compact objects that overcome the paradoxes associated to black holes. Such exotic compact objects can emit a modified ringdown with respect to the black hole case and late-time gravitational wave echoes as characteristic fingerprints. In this talk, I develop a generic framework to the study of the ringdown of exotic compact objects and provide a gravitational-wave template for the echo signal. Finally, I assess the detectability of exotic compact objects with current and future gravitational-wave detectors.
Nov 25	Wed	Ivonne Zavala (Swansea)	Cosmology, Relativity and Gravitation
15:00		Multifield Inflation: Fat inflatons, large turns and the η-problem
		Blackboard Collaborate
	Abstract: I will discuss multifield models of inflation where all scalar fields are heavier than the Hubble scale, thus evading the η-problem. I will show how this is achieved in multifield inflation thanks to large turns in the field space, which I will introduce. I will then illustrate this scenario in a D5-brane model in Type IIB string flux compactifications, where the brane moves along the angular and radial directions in a warped throat driving fat D-brane natural-like inflation, with interesting cosmological predictions.
Dec 2	Wed	Andrew Gow (Sussex)	Cosmology, Relativity and Gravitation
15:00		A History of the Universe in 100 Primordial Black Holes
		Blackboard Collaborate
	Abstract: A summary of the work completed during the course of my PhD so far, centred on the hypothesised Primordial Black Holes (PBHs). The talk will begin at the curvature power spectrum, and advance forward in time through the creation of PBHs and the intricacies of their mass distribution, culminating in the possibility of the LIGO gravitational wave signals being from PBH mergers, instead of astrophysics.
Dec 16	Wed	Antonia Micol Frassino (ICC, Barcelona)	Cosmology, Relativity and Gravitation
15:00		Exotic and quantum BTZ black holes
		Blackboard Collaborate
	Abstract: In this talk, I will discuss some black hole solutions in three-dimensional gravity that manifest interesting properties. In particular, I will consider the three-dimensional Einstein-AdS action in the presence of a gravitational Chern-Simons term and focus on a family of geometries that goes from the BTZ black hole to its 'exotic' counterpart. Then, in the context of braneworld holography, I will describe the holographic construction of the quantum rotating BTZ black hole (quBTZ) using an exact four-dimensional bulk solution. I will present some of the thermodynamic properties of these black holes, focus on the generalized first law and discuss possible further developments.
Jan 27	Wed	Theo Torres Vicente (Nottingham)	Cosmology, Relativity and Gravitation
15:00		Electromagnetic self-force on a charged particle on Kerr spacetime
		Blackboard Collaborate
	Abstract: In this talk, we consider the electromagnetic radiation-reaction/self-force process for a charged particle orbiting a rotating black hole. We will present and complement the existing results for the scalar and gravitational cases, to give a full picture of integer spins in the Kerr spacetime. We restrict ourselves to the case of circular orbits and we will compute the dissipative and conservative components of the electromagnetic self-force numerically, by solving the inhomogeneous Teukolsky equations using the BHperturbation toolkit. The results will be compared to the scalar and gravitational cases found in the literature.
Feb 10	Wed	Luke Hart (Manchester)	Cosmology, Relativity and Gravitation
15:00		Reading between the lines: the hidden secrets of recombination
		Blackboard Collaborate
	Abstract: Cosmological recombination has been widely regarded a solid pillar of understanding the cosmic microwave background (CMB) and its anisotropies. For many years, the questions have been answered over the accuracy of these calculations due to exceptional codes as CosmoRec and HyRec as well as numerous publications on the intricate atomic processes. However, the era that dawned the formation of hydrogen and helium atoms has still given us brilliant insights into exotic physics as well as tribalistic disputes in the various pockets of modern cosmology. In this talk, we will briefly recap the physics of recombination before highlighting extensions to the standard model (parametric and non-parametric) that affect the surface of last scattering. Finally, we will look to the future probes that provide a direct, spectral handprint of the atomic transitions in hydrogen and helium: the recombination radiation. Here we will conclude with the feasibility of studying these lines with prospective missions such as SuperPIXIE, Voyage 2050 and what happens when the exotic physics modifications that we can test with the CMB anisotropies are propagated through to the SEDs from recombination.
Feb 17	Wed	Daniele Oriti (LMU Munich)	Cosmology, Relativity and Gravitation
15:00		Emergent relational cosmology from quantum gravity
		Blackboard Collaborate
	Abstract: We overview recent results on the extraction of an effective cosmological dynamics from fundamental quantum gravity formalisms in which spacetime is not fundamental, focusing on so called tensorial group field theories (strictly related to lattice quantum gravity and loop quantum gravity). This line of research is inspired by the idea of our universe as a quantum gravity condensate, and at the same time realizes it concretely. We emphasize how reaching the desired objective requires addressing several outstanding issues in quantum gravity: identifying quantum states in the fundamental theory with a good geometric interpretation, performing some form of coarse graining of the fundamental dynamics, defining diffeomorphism invariant observables to express the resulting coarse grained dynamics in physically transparent language. We also discuss what the theory says about the fate of the big bang singularity at the beginning of our universe.
Mar 3	Wed	Elsa Teixeira (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Cosmological Predictions of a Disformal Dark Sector
		Blackboard Collaborate
	Abstract: In this talk I will give an overview of interacting dark energy, with emphasis on disformal couplings and its cosmological implications. I will then focus on the general Dark D-Brane setting, for which the interaction in the dark sector arises naturally through the induced metric on a moving brane. In particular, I will discuss the background and linear perturbation equations in this setting, together with a numerical analysis, with brief connection to observational constraints. Testing gravity in the dark sector will be an exciting topic in the upcoming decade, with next-generation cosmological data probing gravitational phenomena in finer detail.
Mar 10	Wed	Aaron Held (Imperial College)	Cosmology, Relativity and Gravitation
15:00		Shadows and Binaries: Stationary and dynamical spacetimes beyond GR
		Blackboard Collaborate
	Abstract: The recent wealth of experimental data from the LIGO/Virgo as well as from the EHT collaboration is fully consistent with GR. However, the true nature of the observed compact objects must involve some new physics (quantum or classical) to ameliorate the singularities present in the interior of the respective GR description. In the spirit of local EFTs, the talk will be based on the main assumption that the new physics, whatever its origin, is tied to local curvature scales. Based on this locality principle, I construct a new class of everywhere-regular, stationary spacetimes parameterized by a mass function in horizon-penetrating coordinates. This construction allows me to identify characteristic image features of the shadows of this class of regular black holes, in particular, in distinction to other models not following the locality principle. Moving on to dynamical spacetime evolution, still following the principles of local EFT, I will present first results on a fully non-linear but well-posed numerical simulation of (quadratic) higher-derivative gravity in the spherically-symmetric sector.
Mar 17	Wed	Richard Daniel (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Scale-invariant, $R^2$ Inflation
		Blackboard Collaborate
	Abstract: In this talk, I will briefly recap slow roll inflation, before demonstrating that an $f(R^2)$ theory of inflation is able to dynamically generate a Planck mass from the vacuum expectation values of the scalar fields. We see that in such models if the self interaction is non-zero, a potentially large cosmological constant will emerge. To avoid this problem we introduce another scalar field, producing a Higgs-like potential. This naturally drives the cosmological constant to zero soon after inflation. We will analyse both models in the Einstein frame, where we find a conserved Noether current simplifying the model to a N-1 scalar field model. Finally, I will discuss the non-trivial features in the power spectrum, which produce testable parameters for future cosmological experiments.
Apr 21	Wed	Bianca Dittrich (Perimeter Institute, Waterloo)	Cosmology, Relativity and Gravitation
15:00		Quantization of spacetime and its (effective) dynamics
		Blackboard Collaborate
	Abstract: General relativity taught us that spacetime geometry is dynamical and quantum theory posits that dynamical objects are quantum. In this talk I will sketch the notion of quantum geometry, which arises in loop quantum gravity. Somewhat surprisingly, this quantum geometry, although it arises from a quantization of a torsion-free theory, does include torsion degrees of freedom. I will then introduce an effective dynamics for such quantum geometries and sketch how to derive corrections that arise due to the inclusion of torsion degrees of freedom.
Apr 28	Wed	Eleni Kontou (Amsterdam)	Cosmology, Relativity and Gravitation
15:00		A singularity theorem for evaporating black holes
		Blackboard Collaborate
	Abstract: The classical singularity theorems of General Relativity rely on energy conditions that are easily violated by quantum fields. In this talk I will provide motivation for an energy condition obeyed by semiclassical gravity: the smeared null energy condition (SNEC), a proposed bound on the weighted average of the null energy along a finite portion of a null geodesic. I will then then present the proof of a semiclassical singularity theorem using SNEC as an assumption. This theorem extends the Penrose theorem to semiclassical gravity and has interesting applications to evaporating black holes. Based on: arXiv: 2012.11569
May 12	Wed	Isha Kotecha (Okinawa Institute for Science and Technology)	Cosmology, Relativity and Gravitation
13:30		Generalised Gibbs States and Application in Discrete Quantum Gravity
		Blackboard Collaborate
	Abstract: Thermal states are absolutely important for statistical descriptions of physical systems; likewise also in discrete quantum gravity, where classical continuum spacetime is thought to emerge from the collective physics of some underlying quantum structure. But what equilibrium even means in a background independent context is a foundational open issue. In this talk, I will discuss a generalisation of Gibbs states for use in such contexts, while emphasising on the maximum entropy principle characterisation. The resulting setup is then applied in quantum gravity, by modelling a quantum spacetime as a many-body system of candidate quanta of geometry, and utilising their field theoretic formulation of group field theory (GFT). This leads to concrete examples of quantum gravitational generalised Gibbs states. I will then present non-perturbative thermofield double vacua, and their inequivalent thermal representations, as induced by these Gibbs states. An interesting class of thermal condensates are defined, which encode fluctuations in the underlying quantum geometry. These are subsequently applied in GFT cosmology to extract an effective FLRW universe at late times, with a bounce and accelerated expansion at early times.
May 26	Wed	Bruno Barros (Lisbon)	Cosmology, Relativity and Gravitation
15:00		Dark energy interactions: phenomenology and observations
		Blackboard Collaborate
	Abstract: In this talk, I will thoroughly explore the phenomenology of dark energy couplings, by exposing two distinct models where dark energy interacts with dark matter. We will journey through three different cosmological phases, so as to study the main influence of the coupling on formation of structure processes. The background cosmology is analysed by resorting to numerical and dynamical system techniques. We will follow closely the linear behaviour of the matter perturbations and test their growth against redshift space distortions data. We also shed some light on the sensitivity of future missions to constrain the dark coupling. Finally, we are going to witness the collapse of matter overdensities by inspecting the physics of the spherical collapse, tracking the evolution of perturbations along the first stages of their nonlinear regime, and compute the number of bound structures formed.
Jun 9	Wed	Noemi Frusciante (Lisbon)	Cosmology, Relativity and Gravitation
15:00		Probing modified gravity with cosmology and solutions to the Hubble tension
		Blackboard Collaborate
	Abstract: The late time cosmic acceleration is one of the most puzzling phenomena in modern cosmology. Its modeling within General Relativity (GR) through the cosmological constant (L) results in the LCDM scenario. Although the latter gives a precise description of the Universe, it is known that it still contains a number of unresolved problems. These lead researchers to look for modified gravity models, for example by including additional degrees of freedom. In this talk I will present the phenomenology and the cosmological bounds of theories consistent with the gravitational-wave event GW170817. In particular I will discuss models which solve the Hubble tension between Planck and local measurements and for which data show a statistically significant preference over LCDM.
Sep 29	Wed	Leanne Durkan (University College Dublin)	Cosmology, Relativity and Gravitation
15:00		Modelling Extreme Mass Ratio Inspirals for LISA
		Seminar Room B19, 301 Glossop Road
	Abstract: Extreme mass-ratio inspirals (EMRIs) are a key source of gravitational waves for the future space based detector, LISA. To model EMRI waveforms to the accuracy required for parameter estimation by LISA, we require black hole perturbation theory and gravitational self-force theory, where the perturbing quantity is the small mass ratio. In my talk I will provide an overview of how we model EMRIs, using the example of a Schwarzschild background and circular orbits. I will justify why we must calculate the metric perturbation to second-order in the small mass ratio, why we use the Lorenz gauge and why, to first-order, we can treat the smaller compact body as a point-like particle. I will present some recent results of the gravitational wave energy flux at future null infinity, discussing in more detail the contribution of the slow-time derivative of the first-order metric perturbation, which appears in the second-order source to the field equations. I will also briefly mention current research being explored to extend the calculation of the Lorenz gauge metric perturbation to eccentric orbits and to Kerr, required to develop physically realistic models for LISA.
Oct 6	Wed	Christiane Klein (Leipzig)	Cosmology, Relativity and Gravitation
15:00		Charged scalar fields inside charged black holes
		Blackboard Collaborate
	Abstract: The strong cosmic censorship conjecture states that black hole spacetimes cannot be continued beyond their inner horizon due to the divergence of local observables, such as the stress-energy tensor of a classical or quantum scalar field, at that horizon. In the case of a spherically symmetric, charged black hole, numerical and analytical studies indicate, that this conjecture is violated classically, even for charged scalar fields, but that the conjecture can be restored by quantum effects in the real scalar case. Here, we present a study on the behaviour of quantum charged scalar fields in a charged, non-rotating black hole. Apart from an extension of the results for real quantum fields, we focus on the charge current induced by this field. We derive an expression for the renormalized current in the Unruh vacuum. In addition, we demonstrate numerically, that the quantum scalar field can charge, instead of discharge, the black hole near the inner horizon.
Oct 13	Wed	Alexander Smith (Saint Anselm College)	Cosmology, Relativity and Gravitation
13:30		Relational dynamics and quantum time dilation
		Blackboard Collaborate
	Abstract: The lesson of general relativity is background independence, which results in a Hamiltonian constraint. This presents a challenge for quantum gravity because the quantisation of this constraint demands that physical states of geometry and matter are frozen, leading to one aspect of the problem of time. We must then explain how the conventional notion of time evolution emerges, which motivates the need for a relational description of quantum dynamics. Using quantum clocks and covariant time observables, I will introduce a formulation of relational quantum dynamics that allows for a probabilistic notion of relativistic time dilation and a proper time / rest mass uncertainty relation. This framework will then be used to describe a quantum time dilation effect that occurs when a clock moves in a superposition of different relativistic momenta. I will argue that this time dilation effect may be observable with present-day technology and offers a new test of relativistic quantum mechanics.
Oct 29	Fri	Silvia Nagy (Queen Mary)	Cosmology, Relativity and Gravitation
16:00		Different approaches to gravity from Yang-Mills squared
		Seminar Room B19, 301 Glossop Road
	Abstract: The idea of writing various quantities in gravity as double copies of the analogous objects in Yang-Mills gauge theory has been gaining a lot of traction in recent years - I will give an overview of the numerous different formulations that have arisen from this drive. Then I will focus on 2 particular ones: the first is based on certain double copy replacement rules in the self-dual sector, and the second links with twistor theory. These have allowed for a recent expansion of the remit of the double copy in the context of symmetries and classical solutions. Finally, I will make some comments on a more ambitious question: is it possible to unify the various different formulations into a single framework?
Nov 10	Wed	Eugene Lim (King's College)	Cosmology, Relativity and Gravitation
15:00		Gravitational Waves from Exotic Compact Objects
		Blackboard Collaborate
	Abstract: If dark matter is made out of light bosonic fields, then they can collapse to form highly compact self-gravitating objects collectively known as exotic compact objects (ECOs). Since they are compact, they can merge to either form a more massive ECO, or a black hole, producing gravitational waves which can be probed by GW detectors. In this talk, I will discuss the physics of such objects and their expected signatures from a hypothetical merger scenario. I will talk about the technical challenges that are needed to be overcome before we can make sufficiently precise predictions that can then be used to search for them.
Nov 17	Wed	Silvia Pla García (Valencia)	Cosmology, Relativity and Gravitation
15:00		Quantum Field Theory in FLRW universes: vacuum choices and renormalization
		Hicks LT9
	Abstract: In this talk I will review two of the main issues in the theory of quantum fields in curved space-times, with special attention to FLRW universes and charged scalar fields. One is the ambiguity in the choice of a preferred vacuum state. The second is the explicit evaluation of the vacuum expectation value of composite operators of the quantum fields (e.g., the stress energy tensor), which requires the application of renormalization techniques. I will focus on the adiabatic regularization method. Then, I will discuss its connection with other renormalization techniques (Hadamard renormalization, De Witt–Schwinger method). Finally, I will illustrate the importance of all previous considerations with a simple physical example.
Nov 24	Wed	Julio Arrechea (IAA, Granada)	Cosmology, Relativity and Gravitation
15:00		All cats are grey in the dark: semiclassical relativistic stars
		Blackboard Collaborate
	Abstract: Quantum vacuum polarization violates energy conditions in the spacetime external to a compact star. As such an object is made to approach the black hole limit, semiclassical corrections become capable of producing new equilibrium end-states in stellar evolution. The semiclassical contribution is modelled by a massless quantum scalar field in the Boulware vacuum state, and its renormalized stress-energy tensor is firstly approached by an analytic Polyakov approximation. This already reveals a crucial difference with respect to classical stellar equilibrium: We find families of solutions that exhibit bounded pressures and mass up to a central core of Planckian radius. A minimal deformation of the Polyakov approximation inside this central core is sufficient to produce regular ultracompact configurations that surpass the Buchdahl compactness bound. We review the main features of these semiclassical relativistic stars.
Dec 15	Wed	Özenç Güngör (Case Western)	Cosmology, Relativity and Gravitation
15:00		A classical, non-singular bounce and its stability analysis
		Hicks LT9
	Abstract: Bouncing cosmological models offer a viable alternative to Big-Bang cosmology and have gained recent attention. In a bouncing cosmology, the universe is initially contracting towards a minimum size before expanding. Such cosmological models are geodesically complete by construction and offer simple solutions to problems such as the Horizon problem. I will present a model that realizes such a cosmology and discuss its analytical and numerical properties. I will then focus more on cosmological perturbations in a positively curved universe and discuss the stability of said perturbations for the model in question.
Feb 2	Wed	David Benisty (Cambridge)	Cosmology, Relativity and Gravitation
15:00		Lorentzian Quintessential Inflation
		Seminar Room B19, 301 Glossop Road
	Abstract: From the assumption that the slow roll parameter $\epsilon$ has a Lorentzian form as a function of the e-folds number N, a successful model of a quintessential inflation is obtained. The form corresponds to the vacuum energy both in the inflationary and in the dark energy epochs. The form satisfies the condition to climb from small values of $\epsilon$ to 1 at the end of the inflationary epoch. In the late universe $\epsilon$ becomes small again and this leads to the Dark Energy epoch. The observables that the models predict fits with the latest Planck data: r ∼ 10−3 , ns ≈ 0.965. Naturally a large dimensionless factor that exponentially amplifies the inflationary scale and exponentially suppresses the dark energy scale appears, producing a sort of cosmological seesaw mechanism. We find the corresponding scalar Quintessential Inflationary potential with two flat regions - one inflationary and one as a dark energy with slow roll behavior.
Feb 9	Wed	David Stefanyszyn (Cambridge)	Cosmology, Relativity and Gravitation
15:00		Bootstrapping primordial fluctuations
		Hicks LT4
	Abstract: Our understanding of observables in AdS space and Minkowski space is well developed. We can construct boundary observables in AdS space, and the S-matrix in Minkowski space, using fundamental physical principles such as symmetries, locality and unitarity while avoiding the numerous redundancies associated with local Lagrangians. Our understanding of observables in dS space is however significantly less well developed even though dS space is an integral part of our best descriptions of the early and late universe. In this talk I will present recent progress on our efforts to bootstrap cosmological correlation functions in dS space. I will mostly concentrate on inflationary correlators and explain how we can construct them directly using symmetries, locality and unitarity without having to work with specific models. I will illustrate the power of these bootstrap methods by showing how gravitational three-point functions are heavily constrained by these physical principles despite the complications of the corresponding Lagrangian descriptions.
Feb 23	Wed	Justin Vines (Albert Einstein Institute Potsdam; UCLA)	Cosmology, Relativity and Gravitation
15:00		Scattering waves and/or particles off spinning black holes
		Blackboard Collaborate
	Abstract: We will explore some questions, and some (historical and recent) beginnings of answers to them, concerning the scattering of classical and/or quantum waves/fields (massless/massive; spin-0, spin-1/2, ...) off of a spinning black hole, i.e., in a background Kerr spacetime --- and the relationships of such processes to the scattering (and bound states) of various particle-like objects around black holes.
Mar 9	Wed	Susanne Schander (Perimeter Institute )	Cosmology, Relativity and Gravitation
15:00		Backreaction in Quantum Cosmology
		Blackboard Collaborate
	Abstract: Several approaches to quantum gravity suggest that the big bang singularity is resolved and is often replaced by a big bounce. This raises the question of whether there are phenomenological consequences of such a scenario. As the physics involved in particular concerns the Planck era before inflation, one expects that quantum backreaction between the homogeneous and inhomogeneous degrees of freedom cannot be neglected in this regime. After a brief introduction to these concepts and the current status of the phenomenology involved, we review space adiabatic perturbation theory (SAPT) which was invented by Panati, Spohn and Teufel as an extension of the well known Born-Oppenheimer approach (BOA) for quantum mechanical systems with backreaction. We explain why BOA is insufficient for quantum cosmology and why an extension of SAPT to quantum field theory is non-trivial. Finally, we apply SAPT to quantum backreaction in cosmology, list which challenges had to be overcome and present the current status of our calculations.
Mar 23	Wed	Antonella Palmese (UC Berkeley)	Cosmology, Relativity and Gravitation
15:00		Probing the Universe’s expansion and the origin of compact object binaries with multi-messenger astronomy
		Blackboard Collaborate
	Abstract: The synergy between gravitational wave (GW) experiments, such as LIGO/Virgo, and optical surveys, such as the Dark Energy Survey (DES), is most prominent in the discovery of electromagnetic counterparts to GW events and the application of the standard siren method, which has already enabled several measurements of the Hubble Constant. Our DES follow-up observations of the first binary neutron star merger detected by LIGO/Virgo enabled the discovery of the first optical counterpart to a GW event and the first standard siren measuement, while also providing information about the origin of the binary. We have later extended the standard siren analysis to compact object binary merger events without electromagnetic counterparts using galaxy catalogs, for which I will present the latest results. These measurements are a promising tool to shed light on the Hubble constant tension in the coming years. In the last part of the talk, I will present some interesting possibilities for the formation of the most massive binary black hole mergers detected so far which are related to galaxies’ central black holes, in particular those in dwarf galaxies and Active Galactic Nuclei.
Apr 6	Wed	Antonio Ferreiro (Dublin City University)	Cosmology, Relativity and Gravitation
15:00		Particle production in expanding universes
		Blackboard Collaborate
	Abstract: The spontaneous production of particles due to a gravitational field is one of the cornerstones of the theory of quantum fields in curved spacetime. This effect has an important role in our current understanding of the early phases of our Universe. I will introduce this phenomenon and point out the difficulties that arise when computing physical magnitudes, e.g. the stress-energy tensor. Finally, I will show the connection between this effect and the generation of matter during the reheating phase in the inflationary scenario.
Apr 27	Wed	Adrián del Rio (Penn State)	Cosmology, Relativity and Gravitation
15:00		Electric-Magnetic duality anomaly induced by gravitational waves
		Blackboard Collaborate
	Abstract: Since the discovery in 1969 of the anomalous non-conservation of the chiral fermion current in QED, it is known that classical symmetries of field theories may fail to survive the quantization, leading to what is known as quantum anomalies. In this talk I will introduce a new example in the context of quantum electrodynamics and gravity. I will first show that the classical electric-magnetic symmetry of Maxwell theory breaks down quantum-mechanically in curved spacetimes. Then I will show that solutions of Einstein's equations can trigger this effect if and only if they admit a flux of gravitational radiation with net circular polarization. I will argue that typical scenarios where this occurs include binary black holes that break spacetime mirror symmetries, as for instance in precessing systems. The emergence of this anomaly physically implies that some astrophysical systems can spontaneously generate a flux of photons from the quantum vacuum with net helicity. This is not predicted by the Hawking effect and could have interesting implications.
May 4	Wed	Lissa de Souza Campos (Pavia)	Cosmology, Relativity and Gravitation
16:00		Sensible dynamics on Static Spacetimes
		Blackboard Collaborate
	Abstract: I will consider a free, scalar, massive quantum field theory on static spacetimes with a timelike boundary. Invoking the works of Ishibashi and Wald, Sturm-Liouville theory and concepts from algebraic quantum field theory, I will outline the steps to take for the explicit construction of physically-sensible two-point functions admitting generalized Robin boundary conditions. Each boundary condition determines an inequivalent dynamics, but they are all equivalently physically-sensible. I will comment on an ongoing investigation aimed at highlighting an extra ambiguity regarding the choice of boundary conditions at an irregular singularity: there might be not just one or a one-parametric family of, but rather infinite admissible boundary conditions to be taken into account.
May 11	Wed	Sunny Vagnozzi (Cambridge)	Cosmology, Relativity and Gravitation
16:00		Searching for dark energy off the beaten track
		Blackboard Collaborate
	Abstract: Most of the efforts in searching for dark energy (DE) have focused on its gravitational signatures, and in particular on its equation of state. However, there is a lot to be learned by getting off the beaten track. I will first focus on non-gravitational interactions of (screened) DE with visible matter, leading to the possibility of “direct detection of dark energy”, analogous to direct detection of dark matter: I will argue that such interactions can and potentially may already have been detected in experiments such as XENON1T, while discussing some of their complementary cosmological and astrophysical signatures. I will then discuss early- and late-time consistency tests of LCDM, and how these may shed light on (early and late) DE, particularly in relation to the Hubble tension, presenting two such tests based on the early ISW effect and the ages of the oldest astrophysical objects in the Universe. If time allows, I will present new ways of probing more general ultralight particles (which may be related to either dark matter or DE), using black hole shadows and planetary objects such as asteroids.
May 18	Wed	Francesco Sartini (ENS Lyon)	Cosmology, Relativity and Gravitation
15:00		Hidden symmetries in black holes
		Blackboard Collaborate
	Abstract: The spacetime in the interior of a black hole can be described by a homogeneous line element, for which the Einstein–Hilbert action reduces to a one-dimensional mechanical model. We have shown that this model exhibits a symmetry under the (2+1)-dimensional Poincaré group. The existence of this symmetry unravels new aspects of symmetry for black holes and opens the way toward a rigorous group quantization of the interior, which in turn provides a powerful tool to discriminate between different regularization schemes. Remarkably, the physical ISO(2,1) symmetry can be seen as a broken infinite-dimensional symmetry. This is done by reinterpreting the action for the model as a geometric action for the BMS3 group, where the configuration space variables are elements of the algebra bms3 and the equations of motion transform as coadjoint vectors.
Jun 8	Wed	Sivakumar Namasivayam (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Vacuum polarization on three-dimensional anti-de Sitter space-time with Robin boundary conditions
		Hicks LT4
	Abstract: Quantum field theory in curved space-time is a theory of quantum fields propagating on a background classical curved space-time. We study a quantum scalar field, with general mass and coupling, on a background three dimensional anti-de Sitter space-time (adS3) with a view to determining the vacuum and thermal expectation values of the square of the scalar field, $\langle\Phi^2\rangle$, known as the vacuum polarisation (VP). Anti-de Sitter space-time is a maximally symmetric solution to Einstein’s field equations with a constant negative curvature which plays a pivotal role in the Ads-CFT (conformal field theory) correspondence. However the presence of a time-like boundary at spatial infinity means that information can be lost to the boundary in finite time, meaning that adS is not a globally hyperbolic space-time. Thus, we need to impose appropriate boundary conditions in order to have a well-posed quantum field theory. Applying Dirichlet, Neumann and Robin boundary conditions at the spacetime boundary, we have found that the vacuum expectation values of the VP with either Neumann or Dirichlet boundary conditions are constant and respect the maximum symmetry of the background spacetime. This is not seen with Robin boundary conditions however, which depend on the spacetime location. We have also found that both the vacuum and thermal expectation values of the VP, for all Robin parameters (except Dirichlet), converge to the Neumann value at the spacetime boundary whereas the Dirichlet expectation values have a different limit.
Jun 15	Wed	Lisa Mickel (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Entanglement entropy of a thermalized black hole in group field theory
		Hicks LT4
	Abstract: Finding a description of black holes in theories of quantum gravity is a common challenge, where one of the topics of interest is to derive the entropy of a black hole. In this talk we will work in the framework of coloured group field theories (GFTs). In previous work, coloured GFT states with a suitable topology were introduced to obtain a foliation for the Schwarzschild black hole and calculate the entanglement entropy across a (horizon) surface. We extend this construction by thermalizing the interior and exterior of the black hole using the framework of thermofield dynamics before constructing the state. The state we consider is obtained from a so-called seed state with the appropriate topology that is then consecutively refined in a topology preserving manner. Since this is work in progress, I will conclude with some final remarks on possible challenges that we might face during the computation of the entanglement entropy.
Jun 22	Wed	Andrea Calcinari (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Towards anisotropic cosmology in group field theory
		Hicks LT4
	Abstract: In cosmological group field theory models for quantum gravity coupled to a massless scalar field, the total volume follows the classical Friedmann dynamics of a flat FLRW Universe at low energies while resolving the Big Bang singularity at high energies. An open question is how to generalise these results to other homogeneous cosmologies. In this talk I will show the first steps taken towards studying Bianchi models in group field theory, based on the introduction of a new anisotropy observable analogous to the β variables in Misner’s parametrisation. In a model based on coupling three Peter-Weyl modes, we find that in an expanding Universe β initially behaves like its classical analogue before “decaying”, showing a previously studied isotropisation. I will conclude with some potential future developments about defining relational dynamics in group field theory without the need of matter, just like one can do in a classical setting thanks to the anisotropy degrees of freedom.
Sep 28	Wed	Steffen Gielen (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Unitarity and clock dependence in quantum cosmology
		Hicks LT10
	Abstract: General relativity gives us the freedom to choose different time coordinates to describe evolution, but also says that only relational notions of evolution (such as "what is the value of quantity A when quantity B takes the value $b_0$"?) are meaningful, i.e., possibly related to observation. When we quantise, this leads to various basic technical and conceptual questions known under the heading of the "problem of time": depending on the viewpoint we seem to have either no dynamics at all or too many, potentially inequivalent, ways of defining time evolution in quantum theory. The standard quantum-mechanical demand of unitary time evolution becomes ambiguous in the general-relativistic context, as it may refer to different notions of time. Here I will discuss some of these issues in a simple cosmological model, where three inequivalent quantum theories can be defined. Demanding unitarity in these simple theories leads to quite radically different "predictions" for the resulting cosmology, illustrating the problem of time.
Oct 5	Wed	Robert Santacruz (New Brunswick)	Cosmology, Relativity and Gravitation
15:00		Do black holes emit matter?
		Hicks LT10
	Abstract: It is known that the classical mathematical description of black holes contains some undesired properties, such as the singularity. In this seminar, I will explain how to construct an effective LQG model of black hole collapse that: i) avoids the singularity and divergent observables; ii) follows a bounce resulting in an outgoing shock wave; iii) determines the lifetime of these objects to be proportional to the mass squared; iv) provides a conformal diagram that strongly modifies the “information loss” picture.
Oct 14	Fri	Chad Briddon (Nottingham)	Cosmology, Relativity and Gravitation
14:00		Using SELCIE to investigate screened scalar field models sourced by complex systems
		Hicks LT6
	Abstract: The mechanism that produces the dark energy driving the expansion of the universe remains a mystery. One popular proposal is to have a scalar field play the role of dark energy. Such a field would have, at least, indirect coupling to matter and so result in a new fundamental force which could be used as a probe to detect these fields. However, no such 'fifth force' has been detected so far, placing strong constraints on models of this type. The 'chameleon' is a scalar field that couples to matter but due to its nonlinear effective potential it possesses a screening mechanism which allows it to evade detection in high density regions such as our solar system, while still having a measurable effect on cosmological scales. The difficulty of this and similar models is that the nonlinear equations lack known analytic solutions except in highly symmetric cases. To this end we have developed a Python package named SELCIE (Screening Equations Linearly Constructed and Iteratively Evaluated) which allows the user to construct systems with arbitrary density profiles and solve for the resulting chameleon field profile. It accomplishes this by using the gmsh and FEniCS software packages. This software has already been used to investigate which properties of NFW halos maximise the likelihood of detecting fifth forces generated by the chameleon field. Using this tool, we have been investigating how the chameleon fifth forces in vacuum chamber experiments depend on the shape of the source used.
Oct 19	Wed	Anna Tokareva (Imperial College London)	Cosmology, Relativity and Gravitation
15:00		Gravitational wave production after non-local $R^2$ inflation
		Hicks LT10
	Abstract: Gravity can be embedded into a renormalizable theory by means of adding quadratic in curvature terms. However, this at first leads to the presence of the Weyl ghost. It is possible to get rid of this ghost if the locality assumption is weakened and the propagator of the graviton is represented by an entire function of the d'Alembertian operator without new poles and zeros. Models of this type admit a cosmological solution describing the $R^2$, or Starobinsky, inflation. We study graviton production after inflation in this model and show that it is negligible despite the presence of the higher derivative operators which could potentially cause instabilities.
Oct 26	Wed	Elliot Nash (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Chiral Connection Formulations, Reality Conditions and Cosmology
		Hicks LT10
	Abstract: Many attempts have been made to reformulate GR in terms of different mathematical objects. One such attempt was made by J.Plebanski in the 1970's which involved rewriting GR as a theory of connections and differential forms, taking advantage of a certain geometric duality. This work has been developed over time, leading to so-called "chiral connection" reformulations (Krasnov 2011). They reformulate GR as a diffeomorphism invariant gauge theory with structure group SO(3,C). These theories are of complex fields and have manifestly complex valued actions. Extra conditions - Reality Conditions - are needed to select solutions that correspond to real Lorentzian GR. Even in highly symmetric cosmological models the reality conditions can interact with the dynamical theory in a highly non-trivial way and need to be treated carefully. The role of the reality conditions in the classical theory must be well understood so that we can understand the role they play in the quantum theory.
Nov 2	Wed	Guilherme Franzmann (Stockholm )	Cosmology, Relativity and Gravitation
15:00		Is our Universe geometrical after all?
		Hicks LT10
	Abstract: After decades, we still lack a proper understanding of the quantum nature of gravity. Nonetheless, we have already seen many theoretical hints that gravity does not easily fit in the quantum mechanical framework. In this talk, I will discuss the issues associated with gravitating vacuum energy and take that as empirical evidence of the breakdown of QFT in the presence of gravity. Then, I will argue for a radical alternative where space(-time) is completely emergent from quantum mechanics alone, defined for finite-dimensional Hilbert spaces. After briefly reviewing how spacetime can be emergent, I will sketch a new research program that establishes experimental signatures to test the emergent nature of spacetime.
Nov 23	Wed	Silvia Pla Garcia (King's College London)	Cosmology, Relativity and Gravitation
15:00		Ultraviolet regularity, CPT, and the big bang quantum vacuum
		Hicks LT 10
	Abstract: In this talk, I will explain how to construct CPT-invariant quantum states in a radiation dominated-universe. I will show the boundary conditions that these states must satisfy to become ultraviolet regular [Hadamard]. Finally, I will propose some examples and briefly discuss one of the physical motivations of this analysis: the two-sheeted universe with time-reversal symmetry.
Dec 7	Wed	Robyn Muñoz (Portsmouth)	Cosmology, Relativity and Gravitation
15:00		Simulations of a quasi-spherical cosmological collapse and gravito-electromagnetic and Petrov invariant characterisation
		Hicks LT10
	Abstract: We look into whether the spherical collapse model (SCM) is a good approximation in a numerical relativity cosmological simulation, and describe the spacetime's evolution during nonlinear structure formation. In the simulation, we evolve a quasi-spherical collapsing structure, where fully nonlinear initial conditions are provided by perturbing the $\Lambda$CDM model with the comoving curvature perturbation $\mathcal{R}_c$, defined as a 3-dimensional sinusoidal. We then have a grid of quasi-spherical over-densities connected through filaments and surrounded by under-densities. This is implemented in the synchronous comoving gauge, using a dust perfect fluid description of cold dark matter, and then it is fully evolved with the Einstein Toolkit code. We find that the SCM is an excellent approximation at the peak of the over-density, where we observe no shear. Additionally, we characterise the expansion of the turn-around boundary and show how it depends on the initial distribution of matter. Then for the spacetime, we find the electric and magnetic parts of the Weyl tensor to be strongest along and around the filaments respectively. We classify the spacetime as Petrov type I everywhere but identify the leading order behaviour. Along the filaments, it's of type D, while the centre of the over-density remains conformally flat, type O, in line with the SCM. The surrounding region demonstrates a sort of peeling-off in action, with the spacetime transitioning between different Petrov types as non-linearity grows with the production of gravitational waves.
Feb 8	Wed	Matteo Lucca (Brussels)	Cosmology, Relativity and Gravitation
15:00		What can cosmology tell us about primordial black holes?
		Blackboard Collaborate
	Abstract: In this seminar I will review the main cosmological constraints that can be imposed on primordial black holes (PBHs), one of the most appealing and popular dark matter candidates to date. In particular, I will cover the limits that we can infer from the impact of PBH formation, evaporation and accretion on observables such as e.g., the CMB, the cosmic ray background and the 21 cm lines. I will also quickly discuss microlensing constraints. For each of these bounds I will provide a brief pedagogical introduction, an overview of the following constraints and a discussion about the many caveats that inevitably come with them.
Feb 20	Mon	Rita Neves (Madrid)	Cosmology, Relativity and Gravitation
14:00		Observational Imprints of Loop Quantum Cosmology
		Hicks LT C
	Abstract: In Loop Quantum Cosmology, the big-bang singularity is resolved in terms of a quantum bounce, connecting a contracting epoch of the Universe with an expanding one. After giving way to a period of decelerated expansion, standard slow-roll inflation begins. Cosmological perturbations will be affected by the pre-inflationary dynamics, as well as by quantum corrections to their equations of motion, leading to features in the primordial power spectra. In this talk we will analyse these features against CMB data, obtaining constraints for the parameter of the model and investigating alleviations of the lensing and power suppression anomalies.
Mar 8	Wed	Benjamin Berczi (Nottingham)	Cosmology, Relativity and Gravitation
15:00		Gravitational collapse of quantum fields
		Hicks LT9
	Abstract: The behaviour of quantum fields around black holes has been in the forefront of research for almost half a century, since the discovery of Hawking radiation in 1974. However, we still know remarkably little about the details of the evaporation of black holes beyond first order approximations. My talk will introduce our novel formalism using which a fully quantum mechanical field can be simulated to collapse into a black hole, and discuss how the quantum effects can be studied. It will be explained how we relate the semiclassical simulations to purely classical ones using a coherent state as the chosen quantum state. Initial results of the quantum effects around the formed black hole will be presented as well.
Mar 29	Wed	Alessia Platania (Perimeter Institute)	Cosmology, Relativity and Gravitation
15:00		Sifting quantum black holes through the principle of least action
		Hicks LT9
	Abstract: We tackle the question of whether regular black holes or other alternatives to the Schwarzschild solution can arise from an action principle in quantum gravity. Focusing on an asymptotic expansion of such solutions and inspecting the corresponding field equations, we demonstrate that their realization within a principle of stationary action would require either fine-tuning, or strong infrared non-localities in the gravitational effective action. This points to an incompatibility between large-distance locality and many of the proposed alternatives to classical black holes.
Apr 26	Wed	Jonathan Betts (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Machine Learning & Cosmological Structure Formation in Modified Gravity
		Hicks LT9
	Abstract: In recent years progress in the field of high performance computing systems has increased the availability of cosmological N-body simulations. These datasets have been computed for Lambda-CDM and a variety of modified gravity cosmologies. One behaviour these data can interrogate is the formation of cosmological structures. Structure formation is largely solved in Lambda-CDM using Extended-Press-Schechter spherical collapse. Screened modified gravities introduce extra forces that may mean this formalism is no longer valid. Machine learning is known to be useful in elucidating empirical information about highly non-linear systems from large quantities of training data. We used the machine learning method of random forest classifiers to study the formation of structures in screened modified gravities, in particular f(R) and nDGP gravity. We examine the differences between models that have learned structure formation from each gravity, as well as a model that has learned from Lambda-CDM data. We also test the generalisability of the Lambda-CDM model on data from f(R) and nDGP gravities of varying strengths, and therefore the generalisability of Extended-Press-Schechter spherical collapse to these types of modified gravity.
May 10	Wed	Louis Hamaide (King's College London)	Cosmology, Relativity and Gravitation
15:30		Black Hole Information Recovery from Gravitational Waves
		Hicks LT10
	Abstract: We study the classical and quantum black hole information in gravitational waves from a black hole's history. We review the necessary concepts regarding quantum information in many-body systems to motivate information retrieval and content in gravitational waves. We then show the first step in an optimal information retrieval strategy is to search for information in gravitational waves, compared to searching for correlations in Hawking radiation. We argue a large portion of the information of the initial collapsing state may be in the gravitational waves. Using the Zerilli equation for particles falling radially into Schwarzschild black holes, we then describe a method to retrieve full classical information about infalling sources, including masses, infall times and angles.
May 17	Wed	Nuno M Santos (Aveiro and Lisbon)	Cosmology, Relativity and Gravitation
15:00		Synchronized bosonic hair: equilibrium solutions
		Blackboard Collaborate
	Abstract: Bosonic fields can spin down rotating black holes (BHs) via superradiance. If massive, they may remain trapped in the vicinity of a BH and endow it with hair co-rotating in synchrony with the event horizon. An illustrative example of this mechanism is the family of BHs with synchronized hair, that can co-exist with Kerr BHs and emerge dynamically from them at some scales. In this talk, I will first explore the features of BHs with vanishingly little (scalar and vector) hair, drawing their similarity to the atomic orbitals of the electron in a hydrogen atom. Then, I will discuss how hairy such BHs can become from the growth and saturation of superradiant instabilities. Finally, I will address the thermodynamic stability of BHs with synchronized hair in the canonical ensemble.
May 22	Mon	Sam Dolan (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Metric perturbations of Kerr spacetime in Lorenz gauge: a new method
		Hicks LT10
	Abstract: Black hole perturbation theory is a key tool for modelling compact-binary inspirals, and their associated gravitational wave signatures, particularly in the case where the ratio of the masses of the two bodies is far from unity. For many applications, it is advantageous to work in the Lorenz gauge, so that the metric perturbation is governed by a tensor wave equation of a manifestly hyperbolic form (i.e. hyperbolic PDEs). In this talk, I will describe a new separation-of-variables method for obtaining the metric perturbations of Kerr spacetime, that is, for a rotating black hole. In this scheme, the metric perturbation is constructed in the frequency domain from scalar variables that satisfy decoupled *ordinary* differential equations. For the case of a particle moving on a circular equatorial orbit of Kerr spacetime, I will compare the results of the new method with existing numerical results from a 2+1D time-domain code developed in Southampton. This talk is based on work with Barry Wardell, Chris Kavanagh and Leanne Durkan.
May 25	Thu	Carsten van de Bruck (Sheffield)	Cosmology, Relativity and Gravitation
15:00		How to link primordial perturbations to CMB anisotropies - I
		Hicks LT10
	Abstract: I give a brief overview on how to link processes in the very early universe (think inflation) to CMB anisotropies. In my first presentation I will briefly talk about the issue of gauges in cosmological perturbation theory, how the CMB anisotropies are described (i.e. what are the famous C_l’s?) and discuss the Sachs-Wolfe effect. In the second presentation I will discuss how primordial perturbations are produced in single field inflation and how the primordial power spectrum is calculated. I will also summarise the changes which happen if inflation is driven by more than one field.
May 26	Fri	Carsten van de Bruck (Sheffield)	Cosmology, Relativity and Gravitation
15:00		How to link primordial perturbations to CMB anisotropies - II
		Hicks LT10
	Abstract: I give a brief overview on how to link processes in the very early universe (think inflation) to CMB anisotropies. In my first presentation I will briefly talk about the issue of gauges in cosmological perturbation theory, how the CMB anisotropies are described (i.e. what are the famous C_l’s?) and discuss the Sachs-Wolfe effect. In the second presentation I will discuss how primordial perturbations are produced in single field inflation and how the primordial power spectrum is calculated. I will also summarise the changes which happen if inflation is driven by more than one field.
Jun 7	Wed	Gaspard Poulot (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Scalar field dark matter and dark energy: A hybrid model for the dark sector
		TBA
	Abstract: Diverse cosmological and astrophysical observations strongly hint at the presence of dark matter and dark energy in the Universe. One of the main goals of Cosmology is to explain the nature of these two components. It may well be that both dark matter and dark energy have a common origin. In this talk, I describe a model in which the dark sector arises due to an interplay between two interacting scalar fields. Employing a hybrid inflation potential, I show that the model can be described as a system of a pressureless fluid coupled to a light scalar field. I then discuss this setup's cosmological consequences and the observational signatures in the cosmic microwave background radiation and the large-scale structures.
Jun 30	Fri	Marc Schiffer (Perimeter Institute)	Cosmology, Relativity and Gravitation
15:00		Using matter to explore asymptotic safety with functional and lattice methods
		Hicks LT10
	Abstract: Asymptotically safe quantum gravity might provide a unified description of the fundamental dynamics of quantum gravity and matter. The realization of asymptotic safety, i.e., of scale symmetry at high energies, constraints the possible interactions and dynamics of a system. In this talk, I will first introduce the scenario of asymptotic safety for gravity with matter, and explain how it can be explored using functional methods. I will then emphasize, how the constraints on the microscopic dynamics of gravity and matter arising from quantum scale symmetry can turn into constraints on fundamental parameters of our universe, when combined with phenomenological observations. Finally, I will highlight how non-dynamical scalar fields can be used to investigate whether lattice methods, in particular Euclidean dynamical triangulations, are a suitable tool to investigate asymptotic safety.
Oct 4	Wed	Mariaveronica De Angelis (Sheffield)	Cosmology, Relativity and Gravitation
16:00		Multi-field inflation with kinetic couplings: theoretical predictions and observational constraints
		Hicks Seminar Room J11
	Abstract: In the two-field inflationary paradigm, it is commonly assumed that the kinetic coupling between the fields, resulting from a non-minimal coupling in the Jordan frame and leading to a curved field manifold in the Einstein frame, depends solely on one field. Our study delves into the situation where the kinetic coupling can instead vary with both fields. The aim of this study is to investigate adiabatic and isocurvature perturbations within these extended theories. Our analysis reveals that, while the evolution equation for the curvature perturbation remains unchanged when allowing coupling dependence on both fields, the effective mass of the entropy perturbation undergoes modifications. We analytically study the correlations between the models’ free parameters and present also a novel numerical method tailored to the study of general multi-field models. Our algorithm captures the dynamics of the fields throughout the entire inflationary phase, providing accurate predictions for observables such as the spectrum of primordial scalar perturbations, primordial gravitational waves, isocurvature modes, and the transfer of entropy to scalar modes after the horizon crossing. By sampling over the initial conditions of the fields and the free parameters of the model, we enable a Monte Carlo analysis, testing the theoretical predictions against observational data.
Oct 11	Wed	Marco de Cesare (Naples)	Cosmology, Relativity and Gravitation
15:00		Interacting dark sector from the trace-free Einstein equations: cosmological perturbations with no instability
		Hicks Seminar Room J11
	Abstract: In trace-free Einstein gravity, the energy-momentum tensor of matter is not necessarily conserved and so the theory offers a natural framework for interacting dark energy models where dark energy has a constant equation of state w=-1. From the point of view of quantum gravity phenomenology, it has been argued that such violations of energy-momentum conservation might originate from discreteness at the Planck scale. We show that within this framework it is possible to build models where cosmological perturbations are free from instabilities, which are known to affect a large class of interacting dark energy models. We will also comment on the possibility that the models here considered may help alleviate the Hubble tension.
Oct 13	Fri	Maria Giovanna Dainotti (National Astronomical Observatory of Japan)	Cosmology, Relativity and Gravitation
15:00		On the Hubble constant tension
		Hicks Seminar Room J11
	Abstract: The difference from 4$\sigma$ to 6$\sigma$ in the Hubble constant ($H_0$) between the values observed with the local probes (Cepheids and Supernovae Ia, SNe Ia) and the probes at high-z (Cosmic Microwave Background obtained by the Planck data) still challenges the astrophysics and cosmology community. Here, we investigate this tension obtained by using the SNe Ia gathered in the Pantheon sample and the Baryon Acoustic Oscillations, assuming $H_0=73.5 $ and $H_0=70 $ as the local value, and dividing the Pantheon sample in 3, 4, and 10 bins ordered in redshift.} For each bin, we run a Monte Carlo Markov-Chain (MCMC) analysis obtaining the value of $H_0$. Subsequently, the values of $H_0$ are fitted with the model $g(z)=\tilde{H_0}/(1+z)^\alpha$, where $\tilde{H_0}$ is $H_0(z=0)$ and $\alpha$ is the evolutionary parameter. Our results show that a decreasing trend with $\alpha\sim10^{-2}$ is still visible in this sample.} The $\alpha$ coefficient is compatible with zero between 1.1$\sigma$ and 2.2$\sigma$. This trend, if not due to statistical fluctuations, could be explained through a hidden astrophysical bias, such as the effect of stretch evolution, or it requires new theoretical models such as the $f(R)$ theories of gravity. Assuming a specific $f(R)$ model in the Jordan frame, we find that the results of our analysis remain unchanged. We conclude that this specific model is not appropriate for explaining the effect of the decreasing $H_0$. Furthermore, our analysis gives suggestions on how a cosmological model can be tested taking into account a parametrized evolution of the Hubble constant. A new analysis with SNe Ia, BAO, quasars and GRBs has been performed with new likelihood showing a less pronounced tension with the SNe Ia.
Oct 18	Wed	Ruchika (INFN Rome)	Cosmology, Relativity and Gravitation
15:00		Reconciling JWST and HST with Planck
		Hicks Seminar Room J11
	Abstract: The recent observations from the James Webb Space Telescope have led to a surprising discovery of a significant density of massive galaxies with masses of $M \ge 10^{10.5} M_{\odot}$ at redshifts of approximately $z\sim 10$. This corresponds to a stellar mass density of roughly $\rho_*\sim 10^6 M_{\odot} Mpc^{-3}$. Despite making conservative assumptions regarding galaxy formation, this finding may not be compatible with the standard $\Lambda$CDM cosmology that is favored by observations of CMB Anisotropies from the Planck satellite. Parallely, SH0ES 2022 results confirmed more than 5 sigma deviation in determining the value of the Hubble Constant from the local distance ladder (using HST) and inverse distance ladder (utilizing Planck). Assuming both SH0ES and the Planck team are not making any errors, I will try to convince them that we need to look for new physics or new theoretical models to alleviate the discrepancy/cosmological crisis. We propose the G-Transition hypothesis, a negative cosmological constant model at low redshifts or Interacting Dark Energy/Early Dark Energy to come to the rescue. But before saying anything concrete, we need to see the similar effects in all cosmological probes.
Oct 25	Wed	Syed Naqvi (Jagellonian U, Krakow)	Cosmology, Relativity and Gravitation
15:00		Chaos and Einstein-Rosen gravitational waves
		Blackboard Collaborate
	Abstract: In this study, we examine the Einstein-Rosen solution to investigate cylindrical standing gravitational waves. Similar to how standing mechanical waves reveal captivating features such as Chladni patterns and non-linear Faraday waves, these standing gravitational waves also provide valuable insights into non-linear aspects of general relativity. Our investigation reveals the existence of chaotic geodesics within the Einstein-Rosen spacetime, highlighting their sensitivity to initial conditions. This sensitivity is confirmed through the observation of an underlying fractal structure. We elucidate the source of this chaotic behaviour by examining the homoclinic and heteroclinic network. Furthermore, we attribute the intricate dynamics of test particles in this spacetime to the complex interplay between stable and unstable manifolds around hyperbolic points.
Nov 15	Wed	Emre Özülker (Istanbul Tech U.)	Cosmology, Relativity and Gravitation
15:00		Dark energy phenomenology, negative dark energy density, and the sign-switching $\Lambda$
		Hicks Seminar Room J11
	Abstract: A dark energy density that attained negative values in the past is phenomenologically motivated by the presence of this feature in parametric and nonparametric reconstructions of the cosmological functions based on the observational data, and also by the success of cosmological models that feature such a dark energy density in addressing the observational tensions. I show how a negative dark energy density can alleviate the tensions by focusing on the first peak in the cosmic microwave background power spectrum, and what happens to the equation of state parameter of such a (potentially effective) dark energy source when local energy-momentum conservation holds. I also argue a negative energy density is not theoretically problematic but even abundant in theoretical physics when treated as an effective source. In the second half, I focus on a specific dark energy model that features a negative density in the past, namely, the sign-switching cosmological constant model. I briefly describe how and why the model was introduced, describe its phenomena, show the latest constraints on its parameters, and discuss its extensions and underlying mechanisms.
Nov 22	Wed	Markus Fröb (Leipzig)	Cosmology, Relativity and Gravitation
15:00		Invariant observables in quantum gravity and graviton loop corrections to the Hubble rate and the Newtonian potential
		Blackboard Collaborate
	Abstract: I present work done in the last years on the construction of dynamical coordinate systems for highly symmetric backgrounds, such as Minkowski, de Sitter, and FLRW cosmologies, and which are needed in the relational approach to construct gauge-invariant observables in gravity. I show that it is possible to restrict the inevitable non-local contributions to the past light cone such that the obtained observables are causal. Lastly, I present some applications, namely the leading quantum gravitational corrections to the local expansion rate of our universe (the Hubble rate) and the Newtonian gravitational potential. Based on arXiv:1711.08470, arXiv:1806.11124, arXiv:2108.11960, arXiv:2109.09753, and arXiv:2303.16218
Nov 29	Wed	Manuel Reichert (Sussex)	Cosmology, Relativity and Gravitation
15:00		From fluctuating gravitons to Lorentzian quantum gravity
		Hicks Seminar Room J11
	Abstract: I will review recent progress in the asymptotic safety approach to quantum gravity. This includes the computation of momentum-dependent graviton correlation functions, the phase structure of the Standard Model of Particle Physics with asymptotically safe gravity, and the first computation directly in space-times with Lorentzian signatures via the spectral function of the graviton.
Dec 13	Wed	Ana Alonso Serrano (AEI Potsdam)	Cosmology, Relativity and Gravitation
15:00		Thermodynamics as a tool for (quantum) gravitational dynamics
		Blackboard Collaborate
	Abstract: I present a review of concepts of thermodynamics of spacetime and the gravitational dynamics encoding in it, discussing also the recovery of Weyl transverse gravity instead of General Relativity. Then, I present a formalism to analyse low-energy quantum gravity modifications in a completely general framework based on the thermodynamics of spacetime. For that purpose, I consider quantum gravity effects via a parametrized modification of entropy by an extra logarithmic term in the area, predicted in most of the different approaches to quantum gravity. These results provide a general expression of quantum phenomenological equations of gravitational dynamics. Furthermore, I outline the application of the modified dynamics to particular models, such as cosmology and its predictions.
Jan 24	Wed	Matteo Forconi (Rome)	Cosmology, Relativity and Gravitation
15:00		JWST’s Revelations and the Super-LCDM’s Promise
		Hicks Seminar Room J11
	Abstract: The recent observations by the James Webb Space Telescope (JWST) of massive galaxies at high redshifts (z ∼ 10) significantly challenge the Lambda Cold Dark Matter (ΛCDM) cosmological model. These observations suggest a higher stellar mass density than previously predicted, and raise questions about galaxy formation and matter distribution in the early universe. To reconcile these findings with standard predictions, an investigation one can look into potential systematics. If systematic errors are ruled out, one might also wonder whether this new anomaly is somehow originated from the same underlying issue as the Hubble tension, suggesting the need for a beyond-ΛCDM phenomenological explanation. One potential avenue is exploring the Dark Energy Sector. Another challenge to the standard ΛCDM model arises from allowing non-Gaussian fluctuations. Using the super sample signal, it is possible to promote the standard ΛCDM model to a more comprehensive Super-ΛCDM model. This model allows to study non-Gaussianity traces using only the power spectrum. The implications of this model extend to the field of neutrino physics, indicating that the traditional constraints on neutrino masses might need revision.
Jan 31	Wed	Sebastian Schuster (Sheffield)	Cosmology, Relativity and Gravitation
15:00		What's Physical? A Space-Time Koan
		Hicks Seminar Room J11
	Abstract: Evaluating the physicality of a given space-time can prove difficult. Often, this is relegated to easy-to-check concepts: Absence of closed, time-like curves (vulgo: no time travel); validity of energy conditions (vulgo: mass/energy should be positive); geodesic completeness (vulgo: we shan't disappear); the related hole-freeness (vulgo: again, we shan't disappear); and more. The problem is that these are not necessarily mutually compatible with each other. Worse, as in the case of energy conditions, not all such concepts are either easy to justify or even fulfilled in known, physical situations. This talk will serve two purposes. The first is to make everyone queasy about the push-me-pull-you nature of physicality, as this allows us to critically examine which type of physicality may be more or less important in any given situation. Here, reverse-engineered metrics like warp drives and tractor beams will be in the spotlight. The second is to hone in on one particularly befuddling concept: Time travel. Fascinating as it is, in general relativity the space-time will either contain it or not. General relativity cannot explain why it might be there, or whether the confusion and contractions arising from it are due to the concept itself or from being in the wrong physical framework. Many arguments have to, at some point, wave their hands and allude to an unknown theory beyond it. To actually make this step beyond general relativity, I will present a very simple, quantum toy model of time travel with an emergent notion of time. While this first toy model will turn out to be not particularly illuminating, it still serves as a good starting point for more complicated toy models with richer structure.
Feb 7	Wed	Luca Marchetti (New Brunswick)	Cosmology, Relativity and Gravitation
16:00		Scalar cosmological perturbations from quantum-gravitational entanglement
		Hicks Seminar Room J11
	Abstract: A major challenge at the interface between quantum gravity and cosmology is to understand how cosmological structures can emerge from physics at the Planck scale. In this talk, I will discuss the main challenges associated with the understanding of such an emergence process and provide a concrete example of how they can be addressed by extracting the physics of scalar and isotropic cosmological perturbations from full quantum gravity, as described by a causally complete Barrett-Crane group field theory model. From the perspective of the underlying quantum gravity theory, cosmological perturbations will be associated with (relational) nearest-neighbor two-body entanglement, providing crucial insights into the potentially purely quantum-gravitational nature of cosmological perturbations. I will also show that at low energies the emergent relational dynamics of these perturbations are perfectly consistent with those of general relativity, while at trans-Planckian scales quantum effects become important. Finally, I will comment on the implications of these quantum effects for the physics of the early universe and outline future research directions.
Feb 15	Thu	Cora Uhlemann (Newcastle)	Cosmology, Relativity and Gravitation
11:00		Making dark matter waves - the cosmic web and wavelike dark matter
		Hicks Seminar Room J11
	Abstract: Despite the astonishing success of cosmological probes in constraining the LCDM model, the dark matter mass remains one of the least constrained physical parameters. Wavelike dark matter is an intriguing alternative to standard cold dark matter with key particle physics motivations (like the QCD axion or ultralight axion-like particles) and distinct astrophysical signatures. With a simple dynamical model for the evolution of the dark matter wavefunction, I will demonstrate how to predict the formation of destructive and constructive wave interference leading to topological defects and granules dressing the cosmic web of large-scale structure. Our wave-based formalism is a versatile tool to describe the complex phase-space dynamics of cold dark matter in position space; and the fundamental description for wavelike dark matter such as ultralight particles, leading to exciting and varied probing mechanisms bridging cosmology and astroparticle physics.
Feb 21	Wed	João Paulo M Pitelli (Campinas State)	Cosmology, Relativity and Gravitation
15:00		Thermal effects on a global monopole with Robin boundary conditions
		Hicks Seminar Room J11
	Abstract: The quantum theory of a scalar field propagating on a spacetime with a naked singularity is not determined until we specify a boundary condition at the boundary. When this choice is not unique, any physical observable will depend on the particular choice of boundary condition. In this work we illustrate this explicit dependence by analyzing the transition rate of an Unruh-DeWitt detector coupled to a thermal state in the singular scenario of a global monopole. We show that the naked singularity manifests thermal effects with a non-trivial behavior with respect to the admissible boundary conditions. In particular, we show that the transition rate is finite at the singularity only for the Dirichlet boundary condition and that the divergence for the other possible (Robin) boundary conditions is consistent with the divergence of the thermal fluctuations.
Mar 6	Wed	William Giare (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Do we need to rethink inflation?
		Hicks Seminar Room J11
	Abstract: I will discuss some arguments that have led me to question whether we need to reassess our understanding of cosmic inflation. Large-scale CMB temperature and polarization measurements from the Planck satellite and the BICEP/Keck collaboration have established stringent constraints on the amplitude of primordial gravitational waves (r < 0.036) and the spectral index of scalar modes (ns = 0.9649 ± 0.0044). In contrast, small-scale CMB data from the Atacama Cosmology Telescope yield divergent predictions, pointing towards a scale-invariant spectrum (ns = 1.008 ± 0.015). This leads to an overall disagreement regarding the inflationary potential as inferred by CMB experiments probing different angular scales in the sky. The well-known Hubble tension further compounds the challenge. Solutions involving new physics at early times may reshape the predictions for inflation based on large-scale measurements to align with trends observed in small-scale data. As a result, with inflation, we find ourselves between the known and the unknown.
Mar 8	Fri	Stefano Gariazzo (IFT Madrid)	Cosmology, Relativity and Gravitation
15:00		Relic neutrinos: decoupling and direct detection perspectives
		Hicks Seminar Room J11
	Abstract: A background radiation of relic neutrinos, originated during the early phases of the Universe expansion, is predicted by the standard cosmological model, but has never been confirmed by a direct measurement. In this seminar, I will review some of the theoretical and phenomenological aspects of relic neutrino decoupling, present indirect evidence of their existence and discuss proposed techniques and ongoing experimental efforts for attempting the first direct detection of the neutrino background.
Mar 13	Wed	Eemeli Tomberg (Lancaster)	Cosmology, Relativity and Gravitation
15:00		Primordial black holes and stochastic inflation
		Hicks Seminar Room J11
	Abstract: Quantum fluctuations from cosmic inflation give rise to the macroscopic structures of the universe. The strongest fluctuations collapse into primordial black holes, a dark matter candidate and a possible source of gravitational waves. Stochastic inflation is a tool to compute the fluctuation statistics non-perturbatively, needed for accurate black hole predictions. I discuss recent progress in these computations, their numerical implementation and analytical approximations, and the implications for black hole abundance in single-field models of inflation.
Mar 20	Wed	Aindriú Conroy (Charles U Prague)	Cosmology, Relativity and Gravitation
15:00		Unruh-DeWitt Particle Detectors in Bouncing Cosmologies
		Hicks Seminar Room J11
	Abstract: There is no well-defined notion of a particle in quantum field theory in curved spacetime due to the lack of global symmetries. The standard procedure in quantum field theory is to treat fields rather than particles as the fundamental object of interest. Nevertheless, in a seminal 1976 paper by W. G. Unruh, an operational meaning was given to the particle concept by examining the absorption and emission of field quanta by a two-level atom. This is the so-called Unruh-DeWitt detector and, in this operational sense, we say a particle is what a particle detector detects! In this talk, we begin by formulating an analytic model of a non-singular bouncing cosmology, the bounce phase of which receives a correction inspired by loop quantum cosmology. We then study the semi-classical particle production associated with spacetime within the Unruh-DeWitt particle detector framework, analysing the rate of particle detection with the aim of (a) understanding quantum effects at early times; (b) identify relics of pre-bounce physics; and (c) highlighting signatures of non-singular theories.
Apr 19	Fri	Adrià Gómez Valent (Barcelona)	Cosmology, Relativity and Gravitation
14:00		Is there still room for low-z solutions to the Hubble tension?
		Hicks Seminar Room J11
	Abstract: The ∼5\sigma mismatch between the value of the Hubble parameter measured by SH0ES and the one inferred from the inverse distance ladder (IDL) constitutes the biggest tension afflicting the standard model of cosmology, which could be pointing to the need of physics beyond LCDM. In this talk I will review the background history required to solve the H0 tension if we consider standard prerecombination physics, paying special attention to the role played by the data on baryon acoustic oscillations (BAO) employed to build the IDL. I will show that the anisotropic BAO data favor an ultra-late-time (phantom-like) enhancement of H(z) at z<0.2, accompanied by a transition in the absolute magnitude of supernovae of Type Ia M(z) in the same redshift range. The effective dark energy (DE) density must be smaller than in the standard model at higher redshifts. Instead, when angular BAO data (claimed to be less subject to model dependencies) is employed in the analysis, the increase of H(z) must start at much higher redshifts, typically in the range z= 0.5-0.8. In this case, M(z) could experience also a transition (although much smoother) and the effective DE density becomes negative at z\sim 2. Both scenarios require a violation of the weak energy condition, but leave an imprint on completely different redshift ranges and might also have a different impact on the perturbed observables. They allow for the effective crossing of the phantom divide. I will put the accent on the utmost importance of the choice of the BAO data set in the study of the possible solutions to the H0 tension.
Apr 24	Wed	Elsa Teixeira (Montpellier)	Cosmology, Relativity and Gravitation
15:00		Exploring Signatures of the Dark Sector through Fluid Approximations
		Hicks Seminar Room J11
	Abstract: The persistent discrepancy between theoretical predictions of the standard cosmological model and precision measurements from diverse observational probes remains a pressing challenge in modern cosmology. Over the past decade, mounting evidence for persistent discrepancies in the inferred values of cosmological parameters derived from both model-dependent and -independent methodologies has motivated the proposal of alternatives to the standard paradigm. In this seminar, I will focus on the exploration of potential missing physics within the standard model, focusing on the enigmatic dark sector comprising dark matter and dark energy, and any potential interactions between them. Leveraging on fluid approximations for the physical nature of the dark sector and its underlying dynamics, we assess the viability of various models in reconciling the observed cosmological tensions.
Apr 26	Fri	Thomas Montandon (Montpellier)	Cosmology, Relativity and Gravitation
15:00		Relativistic matter bispectrum of cosmic structure
		Hicks Seminar Room J11
	Abstract: Upcoming surveys of cosmic structures will probe scales ranging from the nonlinear regime to scales close to the cosmological horizon. This opens up the possibility of probing the ΛCDM model, as well as early universe scenarios with non-Gaussianity. Modeling the galaxy bispectrum is challenging, as it involves general relativity, radiation, and large nonlinearities. In this talk, I will present the latest developments we have achieved in the numerical and theoretical modeling of the matter angular bispectrum on the light cone, including relativistic and radiation effects. This is a crucial step towards modeling the observable bispectra, i.e., the galaxy number count bispectrum and the weak lensing bispectrum.
May 8	Wed	Gabriele Barca (Rome/Sheffield)	Cosmology, Relativity and Gravitation
15:15		Cut-Off Physics Effects on the Primordial Universe
		Hicks Seminar Room J11
	Abstract: Cosmological singularities represent the limit of predictability of General Relativity, but in the high-energy regimes close to the singularity quantum effects are expected to play an important role. I will present some alternative quantization procedures constructed to introduce different kinds of cut-offs. They will be implemented on various cosmological models, both in an effective semiclassical description and on a pure quantum level, with the aim of studying the fate of singularities. These alternative quantization procedures can be powerful for their easy implementation to various systems and for the possible derivation of phenomenological signatures.
May 15	Wed	Álvaro Álvarez Domínguez (Madrid)	Cosmology, Relativity and Gravitation
15:15		Black holes from light?
		Hicks Seminar Room J11
	Abstract: General Relativity theoretically allows the formation of black holes through the gravitational collapse of purely electromagnetic radiation. However, this scenario would involve electromagnetic strengths surpassing the critical Schwinger limit, resulting in the generation of electron-positron pairs. This quantum phenomenon counteracts the collapse, with the created particles scattering out of the collapsing region, carrying their energy. Here, we show that this dissipative effect alone is enough to prevent the formation of black holes from light in the non-classical regime. arXiv:2405.02389
May 29	Wed	Lucia Menendez-Pidal (Madrid)	Cosmology, Relativity and Gravitation
15:00		The measurement problem and Quantum Cosmology
		Hicks Seminar Room J11
	Abstract: If, according to most theories of Quantum Gravity, the Universe started as a quantum system, there must have been a quantum-to-classical transition fairly early in its history. However, explaining the emergence of Classical Mechanics from Quantum Mechanics is not an easy task, and one is faced with the so called "measurement problem". Intuitively, the measurement problem can be understood as the difficulties in defining what exactly is a measurement in Quantum Mechanics. How does one measure if everything is quantum? How do we distinguish between measuring device and system? These are some of the questions that appear when talking about measurements. If one wants to consider the entire Universe as a quantum system, this problem is exacerbated. There are several proposals to solve this conundrum, but all of them change our perspective about Quantum Mechanics. This shift in perspective has consequences, in particular regarding the canonical interpretation of Quantum Gravity. In this talk, I will first give an accessible introduction to the measurement problem and present two modifications to Quantum Mechanics that have been proposed to resolve it, namely Bohmian Mechanics and Spontaneous Collapse Models. I will then dive deeper in Spontaneous Collapse Models, introducing the principal choices of Collapse Operators. Additionally, I will show how Spontaneous Collapse terms can be implemented in Quantum Cosmology and how these new terms allow the Universe to transition from a fully quantum state to a classical state using a simple toy model as an example. Finally, I will briefly present the shortcomings and prospects of the application of Spontaneous Collapse Models in Quantum Gravity and Quantum Cosmology.
Jun 5	Wed	Ethan James German (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Adiabatic inspirals under electromagnetic radiation reaction on Kerr spacetime
		Hicks Seminar Room J11
	Abstract: Consider an electrically charged particle orbiting a spinning black hole, described by the Kerr spacetime. As the charged particle moves, it undergoes a radiation-reaction process, driven by the electromagnetic force which causes it to lose orbital energy and angular momentum. Consequently, it inspirals towards the black hole until the final plunge. In this work we calculate how the orbital parameters for eccentric orbits change along the inspiral, by applying flux-balance laws. We make comparisons to inspirals driven by gravitational waves, and to non-relativistic Keplerian approximations based on the Abraham-Lorentz force law. We find that the electromagnetic inspirals circularize less efficiently than gravitational inspirals, and we quantify the effect of the black hole spin. I will also describe work in progress on calculating the local self-force acting on the particle, which includes a conservative part that cannot be found through the flux-balance approach.
Jun 12	Wed	Luis Escamilla (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Reconstructing the Dark Energy
		Hicks Seminar Room J11
	Abstract: One of the biggest challenges in Cosmology is the lack of understanding of the underlying nature of the Dark Sector, which includes Dark Matter and Dark Energy. To address this problem, several approaches can be considered: assuming that the standard model is incomplete or incorrect, questioning the data, etc. An alternative approach to explore possible solutions or at least give weight to favorable ones involves the so-called "reconstructions". In this talk, I will review my research on using the reconstruction method to study Dark Energy. I will discuss different types of reconstructions, their mechanisms, and the results I obtained during my PhD. The focus will be on "model-independent reconstructions" and their applications to the equation of state, the density parameter of Dark Energy, and the interaction kernel in an Interacting Dark Sector scenario.
Jun 19	Wed	Eleonora Di Valentino (Sheffield)	Cosmology, Relativity and Gravitation
15:00		Anomalies and Tensions in Cosmological Data: Challenges and Insights
		Hicks Seminar Room J11
	Abstract: The standard Lambda Cold Dark Matter cosmological model has been incredibly successful in explaining a wide range of observational data, from the cosmic microwave background radiation to the large-scale structure of the universe. However, recent observations have revealed a number of inconsistencies among the model's key cosmological parameters, which have different levels of statistical significance. These include discrepancies in measurements of the Hubble constant, the S8 tension, and the CMB tension. While some of these inconsistencies could be due to systematic errors, the persistence of such tensions across various probes suggests a potential failure of the canonical LCDM model. I will examine these inconsistencies and discuss possible explanations, including modifications to the standard model, that could potentially alleviate them. However, I will also discuss the limitations of these proposed solutions and note that none of them have successfully resolved the discrepancies.
Jul 17	Wed	Nils Albin Nilsson (IBS Daejeon)	Cosmology, Relativity and Gravitation
15:00		Radiation fields and gravitational-wave observables with spacetime-symmetry breaking
		Hicks Seminar Room J11
	Abstract: Presently, interest in tests of the underlying principles of fundamental physics is high, both in theory and experiment. This is motivated by a search for a unifying theory encompassing both GR and QFT, for example quantum gravity. In the literature, it has been suggested that the underlying spacetime symmetries could be broken in small but detectable ways in some approaches to quantum gravity. To this end, a generic effective-field theory has been in use for decades, and many strong constraints exist. In this talk, I will introduce the state of the art in the search for spacetime-symmetry breaking in gravity, with special focus on gravitational-wave solutions and observables. I will show how extra polarisations appear at the generation stage and how the propagation properties are altered, as well as constraints available from LVK data. Finally, I will discuss plans for the future with next-generation detectors such as LISA.
Oct 2	Wed	Mariana Carrillo González (Imperial College London)	Cosmology, Relativity and Gravitation
15:30
		Hicks Seminar Room J11
Oct 9	Wed	Tiziano Schiavone (GGI Firenze)	Cosmology, Relativity and Gravitation
15:00
		Hicks Seminar Room J11
Oct 16	Wed	Enrico Specogna (Sheffield)	Cosmology, Relativity and Gravitation
15:00
		Hicks Seminar Room J11
Oct 23	Wed	Rishav Roshan (Southampton)	Cosmology, Relativity and Gravitation
15:00
		Hicks Seminar Room J11
Oct 30	Wed	Shiladitya Porey (Novosibirsk State U.)	Cosmology, Relativity and Gravitation
15:00
		Blackboard Collaborate