Seminars this semester


   Series:

 
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.
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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.
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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.
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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.
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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.
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Mar 6 Wed William Giare (Sheffield) Cosmology, Relativity and Gravitation
15:00
Hicks Seminar Room J11
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Mar 8 Fri Stefano Gariazzo (IFT Madrid) Cosmology, Relativity and Gravitation
15:00
Hicks Seminar Room J11
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Mar 13 Wed Eemeli Tomberg (Lancaster) Cosmology, Relativity and Gravitation
15:00
Hicks Seminar Room J11
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Mar 20 Wed Aindriú Conroy (Charles U Prague) Cosmology, Relativity and Gravitation
15:00
Hicks Seminar Room J11
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Apr 17 Wed Adrià Gómez Valent (Barcelona) Cosmology, Relativity and Gravitation
15:00
Hicks Seminar Room J11
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Apr 24 Wed Elsa Teixeira (Montpellier) Cosmology, Relativity and Gravitation
15:00
Hicks Seminar Room J11
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Apr 26 Fri Thomas Montandon (Montpellier) Cosmology, Relativity and Gravitation
15:00
Hicks Seminar Room J11
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May 1 Wed Suddhasattwa Brahma (Edinburgh) Cosmology, Relativity and Gravitation
15:00
Hicks Seminar Room J11
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May 8 Wed Gabriele Barca (Rome/Sheffield) Cosmology, Relativity and Gravitation
15:00
Hicks Seminar Room J11
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May 22 Wed Eleonora Di Valentino (Sheffield) Cosmology, Relativity and Gravitation
15:00
Hicks Seminar Room J11
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