Talks

Whether quantum gravitational effects can be enhanced beyond the Planck scale is an important question for testing quantum gravity. In this talk, I will present how the fluid/gravity correspondence provides insight into the enhancement of quantum gravity fluctuations in a finite causal diamond. I will begin by reviewing the fluid/gravity correspondence in Rindler space. Based on this setup, I will show a dual fluid description of shockwave geometry using the Petrov classification. Then, I will discuss ongoing progress toward understanding connections with Carrollian fluids living on the boundary of causal diamonds. I will conclude by clarifying the assumptions under which an observable scale could emerge.

Overtones are known to improve the performance of fits to the ringdown, both in numerical-relativity simulations and gravitational-wave observations. Although the overtone frequencies are a concrete prediction of general relativity, it remains an open question whether they are excited to the extent that fits to data would suggest. In this talk, after a brief introduction to the ringdown, overtones, and black-hole spectroscopy, I will present work where we take a pragmatic approach and investigate the practical utility of each additional overtone in extracting information from the ringdown. We look at the dependence of the ringdown start time on the number of overtones, and the feasibility of detecting deviations from general relativity in the ringdown frequencies. We suggest that there is no clear highest-excited overtone, but rather the utility of each additional overtone decreases compared to the one before. Finally, we perform Bayesian parameter estimation (as opposed to least-squares fits) to obtain posterior distributions on the overtone amplitudes and phases, allowing us to investigate their correlation structure. Due to strong correlations it becomes increasingly hard to measure individual amplitudes and phases for the highest overtones. However, we find that the joint measurement of overtone amplitudes (i.e., the correlation structure itself) is sensitive to the frequencies and decay times of even the highest overtones, possibly offering an avenue to perform consistency tests with general relativity.

Weak lensing of galaxies and of the CMB provide direct probes of the cosmic matter density field, but are sensitive to different redshift ranges and different survey systematics. The combination of these probes provides a way to calibrate systematics and test LCDM across cosmic time. I will talk about the cross-correlation of cosmic shear and CMB lensing using DES Y3 and new lensing reconstructions from SPT-3G. The main result of this analysis is a first high-significance measurement of the lensing-shear cross-correlation using polarization-only lensing maps, allowing us to sidestep the issue of extragalactic foregrounds without losing much constraining power on large scales. Additionally, using a pure blue shear sample and a variety of foreground-mitigated CMB lensing reconstructions we are able to conduct data-driven tests to ensure our results are robust to both galaxy intrinsic alignments and foregrounds in the CMB.

When performing the gravitational path integral, multiple saddles can contribute to a given computation. In black hole thermodynamics, one encounters infinitely many critical points of the action, most of which are complex. Surprisingly, the sum over these saddles diverges in spacetime dimensions greater than three, and the on-shell action is not even bounded from below. Starting from the Lorentzian gravitational path integral, we show that only a finite number of saddles actually contribute. Interestingly, as the temperature decreases, the number of relevant saddles increases, reproducing known results from JT gravity coupled to gauge fields in the extremal limit. We also discuss the implications of this approach for the computation of supersymmetric indices.

Entanglement provides a powerful framework for characterizing quantum phases. The reduced density matrix (RDM) of a local region is known to encode non-trivial information about the phase of matter, as demonstrated by topological entanglement entropy and the Li-Haldane conjecture. We argue that this RDM harbors a much richer structure, which is revealed by examining the genuine multipartite entanglement (GME) shared among different partitions of the subregion. This talk introduces a comprehensive toolkit based on bipartite and multipartite entanglement witnesses to rigorously study how entanglement is distributed in quantum matter. Using these tools, we will explore a range of quantum phases such as the quantum Ising model, quantum spin liquids as well as measurement-induced phase transitions. In the end, we will move beyond GME and present a more collective form of entanglement, Genuine Network Multiparty entanglement (GNME), recently developed in the field of quantum information and foundations, and see how it serves as a sharper tool for detecting and characterizing novel phases of matter.

Recently there has been significant interest in energy-energy correlators which measure the energy flux created at the point of collision by calorimeter-like detectors at asymptotically large distances. The primary building blocks of these observables are Average Null Energy operators (ANECs) which bear a strong resemblance to part of the charges associated to the symmetries of asymptotically flat spacetimes. In this talk, I will elucidate that relationship and show that calculable quantities that are more familiar in discussions of asymptotic symmetries and flat-space holography can be written in terms of energy-energy correlators, thereby allowing for natural connections to observables at particle experiments.

Gravitational wave signals provide a unique insight into the dynamics of the early universe. Notably, secondary gravitational waves sourced by curvature perturbations from inflation are a generic feature. I will outline the details of this mechanism and discuss several scenarios where a significant signal may arise.

Weak gravitational lensing of the CMB has been established as a robust and powerful observable for precision cosmology. By mapping the matter distribution, it provides a clean window to measure structure growth and key cosmological parameters such as the neutrino mass. In this talk, I will present the major improvements to the Atacama Cosmology Telescope lensing measurement following the latest public data release (DR6). In particular, I will discuss both the inclusion of DR6 data collected during the day (which has never before been used in any lensing analysis) and the prospect of accessing wider sky areas thanks to Galactic foreground mitigation strategies. These improvements will dramatically increase the forecasted power of CMB lensing measurements with the Simons Observatory.

In this talk, I will describe recent developments in our understanding of edge modes associated to subregions in gauge theories, leveraging their realization as reference frames. I will begin with the picture in classical Maxwell theory, where I will introduce the concept of subregional Goldstone mode as a relational observable parametrizing the corner symmetry group. With this as a guiding principle, I will then move on to non-Abelian lattice gauge theory, where we can carry out the construction directly at the quantum level. I will characterize a novel hierarchy of relational subregional algebras, which encompasses the so-called electric and magnetic center algebras usually considered in the literature, for which we provide a new general definition. This leads to corresponding entropy hierarchies. Interestingly, some of the relational algebras can be factors, and so the physical Hilbert space factorizes. Except in the Abelian case, the subregional Goldstone mode is generically only defined on a subspace of the Hilbert space, stemming from the incompleteness of certain edge mode frames. I will conclude with on-going efforts to have a quantum description of edge modes in the continuum, employing algebraic QFT methods. An overarching theme will be the relation between the subregional Goldstone mode and the asymptotic soft sector of the theory.