General Relativity & Quantum Cosmology

For quantum gravity states associated to open spin network graphs, we study how the boundary (the set of open edges, which carries spin degrees of freedom) is affected by the bulk, specifically by its combinatorial structure and by the quantum correlations among the intertwiners. In particular, we determine under which conditions certain classes of quantum gravity states map bulk degrees of freedom into boundary ones isometrically (which is a necessary condition for holography). We then look at the entanglement entropy of the boundary and recover, for slightly entangled intertwiners, the Ryu-Takayanagi formula with corrections induced by the entanglement entropy of the bulk state. We also show that the presence of a region with highly entangled intertwiners deforms the minimal-area surface, which is prevented from entering that region when the entanglement entropy of the latter exceeds a certain bound, a mechanism which thus leads to the rise of a black hole-like region in the bulk.

Zoom Link: https://pitp.zoom.us/j/96356007543?pwd=U2VrRlhyOThMODdMYllDMnB6VjlZQT09

The era of multi-messenger astronomy is well and truly upon us, with 90 compact binaries observed since the Advanced LIGO detectors saw first light in 2015. Despite our very own cosmic backyard, the Milky Way, being ripe with prospective sources for ground-based gravitational wave detectors, the closest source detected thus far (GW170817, the famed binary neutron star merger) was at a distance of 40 Mpc. In this talk, I will outline a number of prospective Galactic multi-messenger sources, and discuss several ways in which their detection over the next twenty years can be improved through both experimental and analytical techniques. 

Zoom Link: https://pitp.zoom.us/j/93664322641?pwd=VUoxbmFGZE1KczJ1RU1MR09TQ05Ldz09

The main feature of tensor models is their melonic large N limit, leading to applications ranging from random geometry and quantum gravity to  many-body quantum mechanics and conformal field theories. However, this melonic limit is lacking for tensor models with ordinary representations of O(N) or Sp(N). We demonstrate that random tensors with sextic interaction transforming under rank-5 irreducible representations of O(N) have a melonic large N limit. This extends the recent proof obtained for rank-3 models with quartic interaction. After giving an introduction to random tensors, I will present the main ideas of our proof relying on recursive bounds derived from a detailed combinatorial analysis of the Feynman graphs.

Zoom Link: https://pitp.zoom.us/j/94691275506?pwd=RGFaN0NZR0FScFdOTXFzeFVXaXUvUT09

Accurate waveform models are crucial for gravitational-wave (GW) data analysis, and since numerical-relativity waveforms are computationally expensive, it is important to improve the analytical approximations for the binary dynamics. The post-Newtonian (PN) approximation is most suited for describing the inspiral of comparable-mass binaries, which are the main sources for ground-based GW detectors. In this talk, I discuss a method for deriving PN results valid for arbitrary mass ratios from first-order self-force results, by exploiting the simple mass dependence of the scattering angle in the post-Minkowskian expansion. I present results for the spin-orbit dynamics up to the fourth-subleading PN order (5.5PN) and the spin-spin dynamics up to the third-subleading PN order (5PN). I also discuss implications for the first law of binary mechanics.

Zoom Link: https://pitp.zoom.us/j/92861625861?pwd=cHpXUlM1d01pc09mNGhhQVZxRHBiQT09

This talk focuses on the semiclassical behavior of the spinfoam quantum gravity in 4 dimensions. There has been long-standing confusion, known as the flatness problem, about whether the curved geometry exists in the semiclassical regime of the spinfoam amplitude. The confusion is resolved by the present work. By numerical computations, we explicitly find curved Regge geometries from the large-j Lorentzian Engle-Pereira-Rovelli-Livine (EPRL) spinfoam amplitudes on triangulations. These curved geometries are with small deficit angles and relate to the complex critical points of the amplitude. The dominant contribution from the curved geometry to the spinfoam amplitude is proportional to e^{i I}, where I is the Regge action of the geometry plus corrections of higher order in curvature. As a result, the spinfoam amplitude reduces to an integral over Regge geometries weighted by e^{i I} in the semiclassical regime.  As a byproduct, our result also provides a mechanism to relax the cosine problem in the spinfoam model. Our results provide important evidence supporting the semiclassical consistency of the spinfoam quantum gravity.

Zoom Link: https://pitp.zoom.us/j/93699343757?pwd=RnpFeitaTU5qTktzNjlXQW45K1gvQT09

Quantum groups are the proper tool to describe quantum gravity in three dimensions. Several arguments suggest that 2-groups should be used to formulate four dimensional quantum gravity. I will review these motivations and will discuss in particular how 2-groups can be used to extend the definition of a phase space associated to a triangulation or to modify the notion of group field theory to generate topological models. I will also highlight how the kappa Poincaré deformation arises in the 2-group context.

Mergers of compact objects are among the most violent events in the Universe. At close separations, compact binaries emit gravitational waves, releasing enormous amounts of energy until they merge. When matter is present in the system, the event might be accompanied by electromagnetic emission, which can span the entire electromagnetic spectrum. Multi-messenger observations by gravitational-wave detectors and electromagnetic telescopes have opened a new avenue to understand the nature of spacetime and its interaction with matter. In this talk, I will discuss recent efforts to connect first-principle calculations with the electromagnetic radiation emitted in compact binary mergers through general-relativistic magnetohydrodynamic simulations. In particular, I will focus on the relativistic outflows produced in binary neutron star systems and on electromagnetic signatures of supermassive binary black-hole mergers.

Time at the Planck scale is an unexplored physical regime. It is widely believed that probing Planck time will remain for long an impossible task. Yet, we propose an experiment to test the discreteness of time at the Planck scale and show that it is not far removed from current technological capabilities.

The growing catalog of gravitational wave signals from binary black hole mergers has allowed us to probe the properties of these compact objects more precisely than ever before. In particular, binary black hole spin measurements have the potential to reveal the formation channels of these systems. An accurate and precise measurement of the spins of individual merging black holes is required to model their population-level distributions. In this talk, I will discuss recent work on the measurability of individual black hole spins focusing on those in heavy binaries. I will also present novel spin parameterizations that facilitate the study of specific features of binary black hole evolutionary histories. I will contextualize these individual spin measurements within the observed population of binary black holes and conclude by presenting the latest LIGO-Virgo results for the black hole spin distribution.

It has been a long-standing dream of twistor theorists to understand gravity without ever talking about gravitons in space-time. To this end, I will describe the recent discovery of a twistor action formulation of perturbative general relativity. This takes the form of a theory governing complex structure deformations on twistor space. It reduces to Plebanski's formulation of GR on performing the Penrose transform to space-time. Some promising applications include finding on-shell recursion relations like MHV rules for graviton scattering amplitudes, studying the quantum integrability of self-dual GR, etc.

Zoom Link: https://pitp.zoom.us/j/99235001602?pwd=QVN6b2ZPbTM2SkFWNkxYTEhzd0tsdz09

Many cosmological scenarios beyond the Standard Model lead to the formation of a network of cosmic strings. In this talk, I will review how these models lead in a natural way to the production of a stochastic gravitational wave background and how this signal could account for the recently reported results from the NANOGrav collaboration. Finally, we will explain how future observations could allow us to confirm this interpretation of the NANOGrav data.

In this talk I introduce nonlocal (infinite derivative) field theories. First of all, I discuss how and which principles of quantum field theory are affected when higher-order derivative operators are taken into account in a Lagrangian. In particular, I focus on the issue of unitarity and on how to make higher-derivative theories healthy by means of non-polynomial differential operators. I extend the treatment to the gravity sector and consider nonlocal theories whose graviton propagators are ghost-free, and explore the possibility of regularizing singularities. Next, I discuss some recent progress in proving perturbative unitarity for a very general class of nonlocal field theories. Finally, I will make some remarks on nonlocality and quantum gravity.