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| Jan 24 |
Wed |
Matteo Forconi (Rome) |
Cosmology, Relativity and Gravitation |
| 15:00 |
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JWST’s Revelations and the Super-LCDM’s Promise |
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Hicks Seminar Room J11 |
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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.
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| Jan 31 |
Wed |
Sebastian Schuster (Sheffield) |
Cosmology, Relativity and Gravitation |
| 15:00 |
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What's Physical? A Space-Time Koan |
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Hicks Seminar Room J11 |
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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.
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| Feb 7 |
Wed |
Luca Marchetti (New Brunswick) |
Cosmology, Relativity and Gravitation |
| 16:00 |
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Scalar cosmological perturbations from quantum-gravitational entanglement |
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Hicks Seminar Room J11 |
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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.
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| Feb 15 |
Thu |
Cora Uhlemann (Newcastle) |
Cosmology, Relativity and Gravitation |
| 11:00 |
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Making dark matter waves - the cosmic web and wavelike dark matter
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Hicks Seminar Room J11 |
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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.
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| Feb 21 |
Wed |
João Paulo M Pitelli (Campinas State) |
Cosmology, Relativity and Gravitation |
| 15:00 |
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Thermal effects on a global monopole with Robin boundary conditions |
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Hicks Seminar Room J11 |
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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.
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| Mar 6 |
Wed |
William Giare (Sheffield) |
Cosmology, Relativity and Gravitation |
| 15:00 |
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Do we need to rethink inflation?
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Hicks Seminar Room J11 |
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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.
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| Mar 8 |
Fri |
Stefano Gariazzo (IFT Madrid) |
Cosmology, Relativity and Gravitation |
| 15:00 |
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Relic neutrinos: decoupling and direct detection perspectives
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Hicks Seminar Room J11 |
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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.
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| Mar 13 |
Wed |
Eemeli Tomberg (Lancaster) |
Cosmology, Relativity and Gravitation |
| 15:00 |
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Primordial black holes and stochastic inflation |
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Hicks Seminar Room J11 |
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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.
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| Mar 20 |
Wed |
Aindriú Conroy (Charles U Prague) |
Cosmology, Relativity and Gravitation |
| 15:00 |
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Unruh-DeWitt Particle Detectors in Bouncing Cosmologies
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Hicks Seminar Room J11 |
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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.
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| Apr 19 |
Fri |
Adrià Gómez Valent (Barcelona) |
Cosmology, Relativity and Gravitation |
| 14:00 |
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Is there still room for low-z solutions to the Hubble tension?
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Hicks Seminar Room J11 |
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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.
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| Apr 24 |
Wed |
Elsa Teixeira (Montpellier) |
Cosmology, Relativity and Gravitation |
| 15:00 |
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Exploring Signatures of the Dark Sector through Fluid Approximations
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Hicks Seminar Room J11 |
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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.
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| Apr 26 |
Fri |
Thomas Montandon (Montpellier) |
Cosmology, Relativity and Gravitation |
| 15:00 |
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Relativistic matter bispectrum of cosmic structure
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Hicks Seminar Room J11 |
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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.
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| May 8 |
Wed |
Gabriele Barca (Rome/Sheffield) |
Cosmology, Relativity and Gravitation |
| 15:15 |
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Cut-Off Physics Effects on the Primordial Universe
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Hicks Seminar Room J11 |
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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.
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| May 15 |
Wed |
Álvaro Álvarez Domínguez (Madrid) |
Cosmology, Relativity and Gravitation |
| 15:15 |
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Black holes from light? |
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Hicks Seminar Room J11 |
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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
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| May 29 |
Wed |
Lucia Menendez-Pidal (Madrid) |
Cosmology, Relativity and Gravitation |
| 15:00 |
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Hicks Seminar Room J11 |
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| Jun 5 |
Wed |
Ethan James German (Sheffield) |
Cosmology, Relativity and Gravitation |
| 15:00 |
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Hicks Seminar Room J11 |
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