Michael Fedderke (Johns Hopkins) : New Directions in Low-Frequency Gravitational-Wave Detection | 9/7/2022
The science case for a broad program of gravitational wave (GW) detection across a wide range of frequencies is exceptionally strong. At present, the GW frequency band lying between the sensitivities of pulsar timing arrays and LISA, roughly 0.01-100 microhertz, is an open frontier in this rapid evolving field. I will discuss recent progress in ideas to access this band, and some associated challenges and opportunities.
I will outline a conceptual mission proposal in which a few carefully chosen asteroids which orbit in the inner Solar System can be employed as naturally occurring gravitational test masses. A GW detector can then be constructed by ranging between these asteroids using optical or radio links and atomic clocks. I will discuss how a newly estimated gravity gradient noise arising from the combined motion of the other ~million asteroids in the inner Solar System sharply cuts off the sensitivity of this proposal below ~microhertz frequencies. Sensitivity in the middle of this band is mostly limited by various solar perturbations to the asteroid test masses, while the high-frequency sensitivity is limited by noise in the ranging link and asteroid rotational motion. A mission of this type promises significant projected GW strain-sensitivity reach for 0.1--10 microhertz frequencies. I will also mention how it could be repurposed for detection of asteroid-mass-scale dark states transiting the Solar System.
Additionally, I will discuss a different proposal that would enable sub-microhertz GW detection by measuring the astrometric GW signal in a novel way. In contrast to previous studies of this signal based on large-scale astrometric survey data, we propose to monitor to extreme precision the angular separations of a small number of hot, distant, photometrically stable white dwarfs. I will discuss why these are good objects to monitor, and sketch out the parameters of the space-based stellar interferometery instrument that would be required to make the required measurements. While this mission would be ambitious, it may be one of the few ways to access this particularly challenging frequency band.
Fei Teng (Penn State): Dynamics of spinning binaries from an EFT perspective | 9/21/2022
In this talk, I will discuss some recent progress on understanding the dynamics of binary astrophysical objects (Kerr black holes or neutron stars) from the EFT perspective. We model the spinning objects by a higher spin field theory with non-minimal couplings to gravity. The order G^2 classical effective Hamiltonian of the binary system is then obtained by matching the gauge invariant scattering amplitudes at one-loop. Up to the fifth order in spin, we find that a unique result can be obtained by imposing a spin shift symmetry and improved high energy behavior in the scattering amplitudes of the binaries. We conjecture that this result corresponds to Kerr black holes. We also find that we need more Wilson coefficients than conventional world line approach to describe a neutron star. I will end the talk by discussing some preliminary attempts to resolve this descrepancy.
Chris Akers (MIT): The black hole interior from non-isometric codes and complexity | 9/28/2022
Quantum error correction has given us a natural language for the emergence of spacetime, but the black hole interior poses a challenge for this framework: at late times the apparent number of interior degrees of freedom in effective field theory can vastly exceed the true number of fundamental degrees of freedom, so there can be no isometric (i.e. inner-product preserving) encoding of the former into the latter. I will explain how quantum error correction nonetheless can be used to explain the emergence of the black hole interior, via the idea of “non-isometric codes protected by computational complexity.
Lorenz Eberhardt (IAS): Unitarity cuts of the worldsheet | 10/12/2022
I will revisit string one-loop amplitudes in this talk. After reviewing the basics, I will explain how Witten’s iepsilon prescription gives a manifestly convergent representation of the amplitude. I will then consider the imaginary part of the amplitude and show directly that it satisfies the standard field theory cutting rules. This leads to an exact representation of the imaginary part of the amplitude. I will also discuss physical properties of the imaginary part such as the singularity structure of the amplitude, its Regge and high energy fixed-angle behaviour and low-spin dominance. Finally, I will tease how Rademacher’s contour can be used to evaluate the full one-loop open string amplitude exactly in terms of a convergent infinite sum.
Reuben Minasian (IPhT ): On consistency of 8D supergravities - anomalies, lattices and counterterms | 10/19/2022
Informal.
Beatrix Muehlmann (McGill): dS in N=2 super Liouville | 10/26/22
In this talk i will discuss the Euclidean gravitational path integral of two-dimensional supersymmetric N=2 (timelike) Liouville theory. We view N= 2 Liouville as a gauge fixed form of a 2d supergravity theory coupled to an N=2 superconformal field theory. N=2 super Liouville admits a positive cosmological constant. I will discuss and contrast the results of supersymmetric localization and the explicit higher-loop evaluation of the path integral around it's dS_2 saddle.
Christina Gao (Illinois): Axion wind detection with the homogeneous precession domain of superfluid helium-3 | 11/2/2022
Axions and axion-like particles may couple to nuclear spins like a weak oscillating effective magnetic field. Existing proposals for detecting this "axion wind" sourced by dark matter exploit analogies to nuclear magnetic resonance (NMR) and aim to detect the small transverse field generated when the axion wind resonantly tips the precessing spins in a polarized sample of material. In this talk, I will describe a new proposal using the homogeneous precession domain of superfluid He-3 as the detection medium, where the effect of the axion wind is a small shift in the precession frequency of a large-amplitude NMR signal. This setup can provide broadband detection of multiple axion masses simultaneously, and has competitive sensitivity to other axion wind experiments such as CASPEr-Wind at masses below 0.1 micro-eV by exploiting precision frequency metrology in the readout stage.
Enrico Sessolo (UM long term visitor): Phenomenology with Asymptotic Safety | 11/9/2022
I will discuss some of the phenomenological aspects of embedding the Standard Model and/or models of New Physics in the framework of trans-Planckian asymptotic safety. In this setting, the presence of an interactive UV fixed point in the renormalization group flow of the gauge and Yukawa couplings imposes a set of boundary conditions at the Planck scale. In the case of New Physics models, the ensuing fixed-point analysis leads to specific predictions for the IR phenomenology. In the Standard Model, it can lead to the dynamical generation of arbitrarily small quantities, for example the Yukawa couplings of (Dirac) neutrinos.
Luca Iliesiu (Stanford): Black hole microstate counting from the gravitational path integral | 11/16/2022
Reproducing the integer count of black hole micro-states from the gravitational path integral is an important problem in quantum gravity. In this paper, we show that, by using supersymmetric localization, the gravitational path integral for 1/16-BPS black holes in N=8 supergravity reproduces the index obtained in the string theory construction of such black holes, including all non-perturbatively suppressed geometries. A more refined argument then shows that, not only the black hole index but also the total number of black hole microstates within an energy window above extremality that is polynomially suppressed in the charges also matches this string theory index. To achieve such a match, we compute the one-loop determinant arising in the localization calculation for all N=2 supergravity supermultiplets in the N=8 gravity supermultiplet. Furthermore, we carefully account for the contribution of boundary zero-modes, which can be seen as arising from the zero-temperature limit of the N=4 super-Schwarzian, and show that performing the exact path integral over such modes provides a critical contribution needed for the match to be achieved.
Sangmin Choi (IP Paris): Holography from Singular Supertranslations on a Black Hole Horizon | 11/30/2022
We investigate the standard and dual BMS supertranslation generators on the black hole horizon and draw some conclusions about black hole physics. Recently, it has been shown that in addition to the conventional BMS supertranslation symmetries, there exists another infinite set of magnetic asymptotic symmetries that are referred to as dual BMS supertranslations. We show that the Dirac bracket between these generators exhibits a central term when the parameter functions have singularities in the complex stereographical coordinates on the sphere. Such singularities are related to Dirac-string-like configurations in the bulk, and should therefore be included in the set of acceptable transformations. We then demonstrate that this anomalous central term can be removed by including an appropriate gravitational Chern-Simons theory on the horizon. This implies that consistency of the asymptotic symmetry algebra requires a new structure on the horizon.
