Tom Hartman (Cornell): Averaged null energy and the renormalization group | 1/19/2024
Averaged null energy, defined by integrating the null energy over a light ray, is known to be closely tied to causality in AdS/CFT, to deformations of the modular Hamiltonian in quantum field theory, and to the Lorentzian inversion formula in CFT. I will discuss a new connection between averaged null energy and the monotonicity of the renormalization group, and use the averaged null energy condition (ANEC) to derive the c-theorem in two dimensions and the a-theorem in four dimensions. The derivation is based on contact terms that appear in correlation functions of the light-ray operator. Combined with previous results, this also gives a new derivation of the c- and a-theorems from the monotonicity of relative entropy, and hints at a more general role for Lorentzian inversion in non-conformal QFTs.
Oliver DeWolfe (CU Boulder): The backwards-forwards holographic map for the black hole interior | 1/26/2024
The black hole information problem describes a tension between the point of view of an observer exterior to a black hole, who should see it evaporate according to the laws of quantum mechanics, and an observer who falls in, who should see nothing special about spacetime as they pass the event horizon. The experience of the infalling observer must be encoded in the fundamental description of the black hole; such a “holographic map” must be inherently non-isometric. I describe the “backwards-forwards map”, a candidate holographic map involving the time evolution of both the interior and exterior descriptions as well as post-selection, in a toy model for a black hole made out of qubits.
Masha Baryakhtar (Washington): Coherence in the sky: precision astrometry and new particles | 2/02/24
In the era of a wealth of data from the sky, new perspectives can lead to parametric improvements in discovery reach. I will discuss two ideas that make use of surprising properties of coherent radiation to open new directions for detection
First is intensity interferometry, which relies on the second-order coherence of light. By recording photon counts rather than electromagnetic fields at a telescope, intensity interferometry admits longer baselines in the optical and thus greater precision than traditional interferometry. I will describe the Extended-Path Intensity Correlator (EPIC): a proposed telescope array that extends the scope of intensity interferometry. Combined with advances in spectroscopy and single-photon detection, EPIC can achieve unprecedented precision in astrometry with applications including exoplanet detection and black hole measurements.
Second is superradiance: stimulated emission of radiation from an absorbing body. I will discuss how rotating black holes, through the process of superradiance, become laboratories in the sky for ultralight bosons including the elusive QCD axion. When a boson's Compton wavelength is comparable to the horizon size of a black hole, the black hole spins down and converts energy into an exponentially growing cloud of bosons. Depending on the bosons' interactions, the resulting systems can be visible across the spectra: emitting gravitational wave radiation, populating the galaxy with axion waves, or appearing as novel pulsar-like objects in the sky.
Gustavo Joaquin Turiaci (Washington): Gravitational index of the heterotic string | 2/09/24
Black holes with two charges in string theory are singular due to vanishing horizon area at extremality. Two seemingly contradictory resolutions are available in the literature. On one hand, it has been argued that higher-derivative effects create a string-sized extremal horizon. On the other hand, it has been argued that before such a small black hole even forms, there is a transition to a winding condensate and eventually a gas of strings. We show that, with some modifications, these two perspectives are compatible, but correspond to different observables. A rotating non-extremal black hole solution with higher-derivative corrections contributes to the gravitational path integral for the index, while the transition to the gas of strings happens for the thermal partition function. Along the way, we extend the recently developed ``new attractor mechanism" by incorporating higher-derivative corrections. We also use effective theory to rule out the possibility of a string-size black hole as the correct description of the near-extremal microstates.
Asher Berlin (Fermilab): Electromagnetic Signals of High-Frequency Gravitational Waves | 2/16/24
There is strong motivation to extend the observable frequency range of gravitational waves (GWs) beyond the Hz - kHz regime already probed by LIGO and Virgo. In particular, higher-frequency GWs can give rise to new classes of electromagnetic signals that can be searched for with small-scale detectors. A gauge-invariant description shows that existing experiments designed for the detection of axion dark matter only need to reanalyze existing data to search for such signals. I will also discuss how electromagnetic cavities can operate as exquisite mechanical to electromagnetic converters, enabling a broader search across orders of magnitude of unexplored parameter space.
Cliff Burgess (McMaster/Perimeter): The Gravity of Light Scalars (Naturally) | 3/08/24
We live in remarkable times: the recent advent of gravitational-wave observations allows testing gravity in a strongly relativistic regime. We also have plausible candidates for UV physics that reconciles General Relativity with Quantum Mechanics. But there is also Bad News: Decoupling - which beautifully explains why low-energy measurements are largely insensitive to UV details - seems a central organizing feature of Nature that thwarts the extraction of fundamental insights about UV physics from astrophysical or cosmological observations. This talk argues that all is not lost because some UV features can penetrate the decoupling barrier in interesting ways. In particular generic accidental symmetries can robustly point to the existence of scalars in the low-energy effective theory (and these are not just axions). Normally we are taught that naturalness arguments preclude these scalars from being light enough or too weakly coupled to be important for tests of gravity, but I argue that the additional information that the observed Dark Energy is so small puts us in a regime where some scalars are pseudo-dilatons (ie naturally light with Brans-Dicke couplings to matter). The question of why these scalars are not already detected motivates more detailed studies of whether screening mechanisms exist that could have hidden them from present-day tests of gravity. Crucially they must do so in a way consistent with other properties of UV completions of gravity (in a way that standard screening mechanisms - like Chameleons - are not). The talk describes new proposals for such UV-consistent screening mechanisms and why they thread a blind spot in current theoretical approaches to testing gravity. If time permits I will also explore other implications these models might have, including possible relevance to other problems like the Hubble tension.
Thomas Baumgarte (Bowdoin): Critical Phenomena in Gravitational Collapse | 3/22/24
Critical Phenomena, including the appearance of universal scaling laws and critical exponents in the vicinity of phase transitions, appear in different fields of physics and beyond. Critical phenomena in gravitational collapse to black holes were first observed by Matt Choptuik 30 years ago - a seminal discovery that launched an entire new field of research. While these phenomena are well understood in spherical symmetry, critical collapse of gravitational waves has remained elusive. In this talk I will review the appearance of scaling laws and self-similarity close to the onset of black hole formation, and will then present simulations of gravitational-wave collapse with three independent numerical codes. These results strongly suggest that the threshold solution for vacuum collapse is not universal, and that our understanding of critical collapse in the absence of spherical symmetry will have to be broadened.
Shu-Heng Shao (Stony Brook): What's Done Cannot Be Undone: Non-Invertible Symmetries | 3/29/24
In massless QED, we find that the classical U(1) chiral symmetry is not completely broken by the Adler-Bell-Jackiw anomaly. Rather, it is resurrected as a generalized global symmetry labeled by the rational numbers. Intuitively, this new global symmetry in QED is a composition of the naive axial rotation and a fractional quantum Hall state. The conserved symmetry operators do not obey a group multiplication law, but a non-invertible fusion algebra. We further generalize our construction to QCD, and show that the neutral pion decay can be derived from a matching condition of the non-invertible global symmetry.
Anastasia Volovich (Brown): Recent developments in N=4 Yang-Mills Amplitudes | 4/05/24
The most important experimental probes of fundamental physics involve the scattering of elementary particles. Over the years we have seen significant progress in understanding the properties of scattering amplitudes and in our ability to carry out new computations both for theoretical and phenomenological purposes. I will overview some recent developments in N=4 Yang-Mills amplitudes.
Rachel Rosen (Carnegie Mellon): On Causality Conditions in de Sitter Spacetime | 4/12/24
Causality conditions provide powerful constraints on low energy theories. In this talk, I will discuss how standard causality conditions of AdS and flat spacetimes can be extended to de Sitter spacetime. In particular, I will consider the Shapiro time delay experienced by a particle in a black hole or shockwave background and discuss how "fastest null geodesics" can be defined using spatial shifts on the boundary of de Sitter and the relevance of the "stretching" of the de Sitter Penrose diagram. I will discuss a few illustrative examples.
