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| Oct 2 |
Wed |
Mariana Carrillo González (Imperial College London) |
Cosmology, Relativity and Gravitation |
| 15:00 |
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Self-dual Cosmology
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Hicks Seminar Room J11 |
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Abstract:
One of the simplest subsectors of gravitational physics in asymptotically flat spacetimes is the so-called self-dual sector. From a particle physics perspective, this subsector only contains three-point interactions between gravitons of (++—) helicities. This is equivalent to spacetimes with Riemann tensors equal to their Hodge dual. Hidden symmetries of gravitational physics, such as double copy relations that allow us to write gravity as the square of a gauge theory, are explicitly realized in these spacetimes. Additionally, this sector is related to infinite-dimensional algebras arising in celestial holography. In this talk, I will describe an extension of this sector to cosmological spacetimes. This can be seen as a time deformation of the flat space version and gives rise to time-deformed versions of the flat space symmetries. I will show how to build this description and showcase its hidden simplicity. I will also focus on two special cases with additional novel features, including self-dual radiation and self-dual coasting cosmologies.
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| Oct 9 |
Wed |
Tiziano Schiavone (GGI Firenze) |
Cosmology, Relativity and Gravitation |
| 15:15 |
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An effective Hubble constant in f(R) modified gravity |
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Hicks Seminar Room J11 |
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| Oct 16 |
Wed |
Suddhasattwa Brahma (Edinburgh) |
Cosmology, Relativity and Gravitation |
| 15:15 |
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Nonlocal quantum effects in the Early Universe
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Hicks Seminar Room J11 |
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Abstract:
Although inflation is widely regarded as the standard paradigm for the early universe, our understanding of its dynamics is necessarily incomplete. In this talk, I will treat inflation as an open quantum system and derive, in a systematic manner, quantum corrections to cosmological observables due to interactions with unobservable environments. For instance, the "cosmological horizon" is an example of a spacetime boundary that restricts our observable degrees of freedom while still allowing energy and information to flow across it into hidden sectors. Borrowing techniques from Quantum Information Theory, I will show how an Open Effective Field Theory formalism can incorporate non-unitary effects in cosmology and describe dissipation and decoherence of primordial fluctuations. I will emphasize that the out-of-equilibrium nature of gravitational systems necessarily result in non-Markovian dynamics and how this can lead to transient negative growth of entanglement entropy during inflation.
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| Oct 23 |
Wed |
Rishav Roshan (Southampton) |
Cosmology, Relativity and Gravitation |
| 15:15 |
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Using Gravitational Waves to see the Early Universe
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Hicks Seminar Room J11 |
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Abstract:
Gravitational waves are a unique probe of the early Universe, as the Universe is transparent to gravitational radiation right back to the beginning. Here, I will summarise some of the different scopes of primordial events like annihilation of topological defects and evaporation of primordial black holes that could lead to a detectable stochastic gravitational wave background. Any such background would shed light on what (if anything) lies beyond the Standard Model, sometimes at remarkably high scales. We overview the range of strategies for detecting a stochastic gravitational wave background before delving deep into three major primordial events that can source such a background.
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| Oct 30 |
Wed |
Shiladitya Porey (Shiv Nadar U.) |
Cosmology, Relativity and Gravitation |
| 15:00 |
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Primordial Dark Sector Relics from Inflation: An Additional Way to Probe Viable Inflationary Models
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Blackboard Collaborate |
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Abstract:
We investigate the production of non-thermal dark matter (DM) particles, heavy sterile neutrinos from the inflaton, and beyond the Standard Model free-streaming relativistic particles during the reheating era, which is preceded by a slow-roll inflationary epoch. We consider benchmark values for slow-roll single-field inflationary scenarios satisfying current bounds obtained from Planck-BICEP Cosmic Microwave Background (CMB) data. Then, for those benchmark values, we calculate the permissible range of the coupling between the DM particle and the inflaton (y_χ) and the mass of the DM particle (m_χ) for the production of enough DM to explain the total Cold Dark Matter (CDM) mass density of the present universe while satisfying CMB measurements and other cosmological bounds. For slow-roll inflation with quartic potential and non-minimal coupling between inflaton and gravity, we consider the scenario of leptogenesis via the decay of sterile neutrinos produced from the decay of inflaton, and compare the results obtained following both metric and Palatini theory of gravity. Then, we consider inflaton decay as the source of this dark radiation (DR), and use the CMB data from Planck-2018 to constrain the branching fraction for the production of DR from the decay of inflaton, and identify the parameter space involving couplings and mass of the inflaton that will be within the reach of next-generation CMB experiments like spt-3g, CMB-S4, CMB-Bharat, PICO, CMB-HD, etc.
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| Nov 6 |
Wed |
Rita Neves (Sheffield) |
Cosmology, Relativity and Gravitation |
| 15:15 |
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An adiabatic approach to the trans-Planckian problem in loop quantum cosmology
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Hicks Seminar Room J11 |
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Abstract:
One of the criticisms of the inflationary paradigm is that scales that are observable today were trans-Planckian at the onset of inflation. This questions the validity of standard results regarding the primordial power spectrum. Standard cosmology also ignores pre-inflationary dynamics, since it loses predictability close to the initial singularity. Loop Quantum Cosmology (LQC) is an approach to the quantisation of cosmological models. It provides effective pre-inflationary dynamics where the big-bang singularity is resolved in terms of a quantum bounce that connects a contracting epoch of the Universe with an expanding one. In this talk, we investigate the trans-Planckian problem in two models of LQC. We find that one of the models avoids the issue altogether by generating less e-folds of inflation, such that the observable modes never become trans-Planckian. On the other hand, the other model suffers from this problem, as observable modes become trans-Planckian during a time when they lose adiabaticity, making their primordial power spectrum susceptible to changes due to trans-Planckian physics.
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| Nov 13 |
Wed |
Theo Anton (Queen Mary) |
Cosmology, Relativity and Gravitation |
| 15:15 |
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Generalised tests of gravity in cosmology
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Hicks Seminar Room J11 |
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Abstract:
A plethora of modified theories of gravity have been proposed over the last few decades. Testing them all observationally is a considerable challenge, so it is advantageous to develop theory-independent approaches that constrain deviations from General Relativity in a systematic way. Many of the most precise such constraints to date are obtained from astrophysical measurements, for which deviations from GR are described by the parameterised post-Newtonian (PPN) parameters. Some attempts have been made to perform similarly general tests on cosmological scales, but it is not clear that they refer to the same couplings as the PPN formalism does, and so interpreting results from these disparate regimes physically is difficult and potentially misleading. With that problem in mind, I will introduce a framework, called parameterised post-Newtonian cosmology, that allows information from cosmological and astrophysical regimes to be combined consistently. I will present novel constraints on the evolution of the PPN parameters over cosmic history, using data from CMB anisotropies and Solar System experiments concurrently, and I will explain how these ideas can be applied further to test cosmological gravity to high precision.
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| Nov 20 |
Wed |
Enrico Specogna (Sheffield) |
Cosmology, Relativity and Gravitation |
| 15:15 |
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Model-independent Tests for General Relativity |
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Hicks Seminar Room J11 |
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Abstract:
General relativity (GR) is the gravitational framework that underpins the standard model of cosmology, the ΛCDM model; its predictions have been widely tested at astrophysical and cosmological scales, often with remarkable precision. However, our inability to directly observe the constituents of this model's so-called 'dark sector' (i.e., dark matter and dark energy), along with the tensions characterising some of its parameters, prompts us to question the validity of GR at cosmological scales. That implies substituting GR with a modified gravity theory (MG) that can, for instance, explain the observed accelerated expansion of the universe without the need to introduce a cosmological constant (Λ). How can we test if a MG cosmology is sustained by current observations? Because MG theories are numerous, we can turn to general, model-independent parameterizations of gravity, able to capture the phenomenology of several classes of deviations from GR at once. In this talk, I will present two such parameterizations: the μ/Σ framework and the growth index γ. I will show how they can be used to solve a discrepancy within different cosmic microwave background measurements (CMB) known as the 'lensing anomaly', while also explaining the apparent MG detection reported by the Planck CMB telescope as a collateral effect of the same anomaly.
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| Nov 27 |
Wed |
Sergio Sevillano Muñoz (Durham) |
Cosmology, Relativity and Gravitation |
| 15:15 |
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Screening mechanisms in scalar-tensor theories from a particle perspective
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Hicks Seminar Room J11 |
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Abstract:
Scalar-tensor theories are a popular extension of gravity where an extra scalar degree of freedom non-minimally couples to the gravitational sector. Despite existing experimental tests for such modifications from general relativity, there is still no conclusive evidence for or against these theories. A possible reason for this is the presence of screening mechanisms, which can hide the scalar field’s effects (such as long-range forces) in high-density environments, making them undetectable for our local experiments and observations. In this talk, I will use field theory to demonstrate that screening mechanisms can also be expressed as Beyond Standard Model physics. This perspective reveals possible phenomenological implications that don't rely on new long-range forces. In particular, I will focus on how screening mechanisms can lead to spatially dependent masses for elementary particles in the Standard Model.
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| Dec 11 |
Wed |
Cameron Bunney (Nottingham) |
Cosmology, Relativity and Gravitation |
| 15:00 |
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Hicks Seminar Room J11 |
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| Dec 18 |
Wed |
Yuejia Zhai (Sheffield) |
Cosmology, Relativity and Gravitation |
| 15:00 |
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Hicks Seminar Room J11 |
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