General Relativity & Quantum Cosmology

In this talk, I will start by motivating the introduction of new- (quantum) physics effects in black hole spacetimes through a principled-parameterized approach. I will then discuss how these new-physics effect can affect the formation of non-singular black holes by applying the principled-parameterized approach to gravitational collapse. This will serve as a motivation to study non-singular black hole parameterizations beyond circularity. Finally, I will present how quantum gravity effects linked to these guiding principles can be traced back to observational imprints in synthetic (ng)EHT images, in particular in photon rings.

Zoom Link: https://pitp.zoom.us/j/93325159463?pwd=aGlQUHJzWWg1d3hBRzAwOURGY3NFdz09

Quantum field theories in two dimensions (2d) with an underlying Bondi-van der Burg-Metzner-Sachs (BMS) symmetry augmented by u(1) currents are expected to holographically capture features of charged versions of cosmological solutions in asymptotically flat 3d spacetimes called Flat Space Cosmologies (FSCs). I will present a study of the modular properties of these field theories and the corresponding partition function. Furthermore, I will derive the density of (primary) states and find the entropy and asymptotic values of the structure constants exploiting the modular properties of the partition function and the torus one-point function. The expression for the asymptotic structure constants shows shifts in the weights and one of the central terms and an extra phase compared to earlier results in the literature for BMS invariant theories without u(1) currents present. The field theory results for the structure constants can be reproduced holographically by a bulk computation involving a scalar probe in the background of a charged FSC.

Zoom Link: https://pitp.zoom.us/j/99205444635?pwd=Tk02UlgvcjJCU3JSWWphY1JQSlhFQT09

"I describe a candidate for a fundamental physical theory called the causal theory of views. This describes a world constructed by a continual creation of events; where an event is a transition at which a small portion of the possible becomes actual. I first recall older results which includes the emergence of space and, with space, a non-relativistic N-body quantum dynamics. I next describe recent progress on this model including, in a different limit, a formulation of a cut off quantum field theory, which we describe in terms of an S-Matrix formulation of amplitudes. The dynamics is specified by an action principle consisting of a kinetic energy and potential energy term. The former are based on measures of how quickly components of causal change do so with respect to averaged notions. The potential energy terms measure how much local moves alter an observer's ""view"" of the universe, as seen from their perspective. These results show that quantum dynamics is restored in an N to infinity limit. Measurable non-linear corrections to quantum dynamics emerge to higher order in 1/sqrt{N}. "

Analogical reasoning has a long and distinguished history as a method for making discoveries in physics. I will discuss novel uses of formal analogies in twentieth century particle physics and condensed matter physics. I will then offer some reflections on how methodological lessons from these cases could inform the use of analogies in discoveries related to quantum gravity.

In modern cosmology there is an agreement that the seeds of structure formations resides in the quantum fluctuation of the geometry in the early universe, but there is no agreement about how these could be derived from a quantum theory of gravity. In this talk I present a proposal based on the covariant formulation of Loop Quantum Gravity. I describe how to define a wavefunction of the universe in this context, and a how we can study fluctuations and correlations between spacial regions. The results obtained so far has been made possible by recent progress in numerical computations. I discuss the current state of this research program and the possible implications for modeling the early universe.

Quantum gravitational phenomena dominate the physics of a black hole in the past of a high curvature spacelike surface. Because of the backreaction of the evaporation, this surface crosses the horizon. I describe a recent line of investigation on the possible evolution compatible with nonperturbative quantum gravity, and in particular I illustrate what can be predicted using loop quantum gravity.

I will discuss a number of theoretical, observational, and conceptual aspects of the Everpresent Λ cosmological model arising from fundamental principles in causal set theory and unimodular gravity. In this framework the value of the cosmological constant (Λ) fluctuates, in magnitude and in sign, over cosmic history. At each epoch, Λ stays statistically close to the inverse square root of the spacetime volume. Since the latter is of the order of H^2 today, this provides a way out of the cosmological constant puzzle without fine tuning. I will review the theoretical background of this idea. I will also describe a phenomenological implementation of this model, and discuss recent results on the statistics of its simulations and observational tests of it.

I will discuss a recently observed connection between Gaussian thermal partition functions in d=2L+1 dimensions and L-loop conformal graphs in D=4. While the origin of such a connection is still unclear, I will argue that it offers some interesting perspectives in quantum field theory and in mathematics.

Zoom link:  https://pitp.zoom.us/j/98795029694?pwd=aGdSdWtDQ1VqbzFvSWE4SkM5c3lJUT09

The nonlocality of causal sets gives us hope of solving the cosmological constant puzzle (``why is the universe so smooth, big and old if there is only one scale---the discreteness scale---in the theory?’’) On the other hand locality, , GR and local QFT, must be recovered from quantum gravity in the continuum approximation at large scales, which is a challenge. If we are lucky though (like Goldilocks, the universe gets the nonlocality ``just right’’) nonlocality may be a rich source of phenomenology. Yasaman Yazdi’s talk will be on cosmological models based on the nonlocality of causal sets. I will give a couple of examples of more astrophysical phenomenological models based on simple assumptions---randomness due to spacetime uncertainty and Lorentz invariance---and pose a research question: is there a model of ``quantum swerves’’?