General Relativity & Quantum Cosmology

Merging neutron stars have long been speculated to produce Gamma-Ray Bursts (GRBs). While this has been the standard assumption for short GRBs for decades, this was only officially confirmed with the still relatively recent detection of gravitational wave counterpart GRB 170817A. In this presentation, I will discuss how we model short GRBs and their afterglows produced by the relativistic jets launched during the merger. A recent development of interest includes the apparent confirmation of some 'long' GRBs to also follow from mergers rather than collapsing massive stars, which has implication for the prospects of future multi-messenger detections. Joint modelling of GRB / afterglow data and GW signals makes it possible to further constrain the shared model parameters between the two, most notably the orientation of the system.

Zoom link: https://pitp.zoom.us/j/95551884959?pwd=bUVGVzhXK3pLb3MxMGV4ejZzUmRYdz09

66 - It has been proposed that the exponential decay and subsequent power law saturation of out-of-time-order correlation functions can be universally described by collective 'scramblon' modes. We develop this idea from a path integral perspective in several examples, thereby establishing a general formalism. After reformulating previous work on the Schwarzian theory and identity conformal blocks in two-dimensional CFTs relevant for systems in the infinite coupling limit with maximal quantum Lyapunov exponent, we focus on theories with sub-maximal chaos: we study the large-q limit of the SYK quantum dot and chain, both of which are amenable to analytical treatment at finite coupling. In both cases we identify the relevant scramblon modes, derive their effective action, and find bilocal vertex functions, thus constructing an effective description of chaos. The final results can be matched in detail to stringy corrections to the gravitational eikonal S-matrix in holographic CFTs, including a stringy Regge trajectory, bulk to boundary propagators, and multi-string effects that are unexplored holographically.

Event Horizon Telescope (EHT) has produced images of the black hole in the galaxy M87 and in the center of our galaxy, and I will briefly review what we have learned from the images.  In the near future we expect that repeated visits to M87, or high fidelity images of the galactic center, will produce movies of the turbulent plasma in these sources.  The statistical properties of the movies can be predicted using state-of-the-art numerical simulations.  I will explore what information can be extracted from the movies, especially information about source inclination and black hole spin.

Zoom link TBA

I will argue that a fruitful strategy to make progress in quantum gravity is to connect distinct approaches and transfer methods and ideas from one approach to another. As a concrete example, I will explain recent results in causal-set quantum gravity. The first, namely the construction of a higher-order curvature operator in causal sets, is motivated by the idea to use causal sets as a Lorentzian regularization of the gravitational path integral, in which one can search for asymptotic safety. The second, namely an upper bound on the mass of scalars is inspired by the "matter matters" program in asymptotically safe quantum gravity, in which observational tests of quantum gravity are based on gravity's interplay with matter. I will argue that a similar program for causal sets can provide new, observationally motivated constraints on causal set quantum gravity.To provide background and motivation for these results, I'll provide brief and pedagogical introductions into the key features and key open challenges of both asymptotically safe quantum gravity as well as causal set quantum gravity.

Zoom Link: https://pitp.zoom.us/j/99855484685?pwd=M0IvOFk1OWNCdVlZVDdkUFQ4MzZKUT09

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.