Search Talks

A basic calculation in QFT is the construction of the Yukawa potential from a tree-level scattering amplitude. In the massless limit, this reproduces the 1/r potential. For gravity, scattering mediated by a massless graviton is thus consistent with the Newtonian potential.

In de Sitter spacetime, the cosmological constant gives rise to a mass-like term in the graviton propagator. This raises the question what the classical potential looks like when taking into account curvature effects.

In this talk, I will introduce an operator-based formalism to compute scattering amplitudes in curved spacetime, and I will show how to construct the Newtonian potential in a dS background. Remarkably, the potential gives rise to an additional repulsive force, and encodes the de Sitter horizon in a novel and non-trivial way.

With the release of the third gravitational wave transient catalog (GWTC-3), the LIGO and Virgo detectors have reported nearly 100 gravitational waves from colliding black holes and neutron stars. Among these detections there have been numerous surprises, such as the heavy GW190521, the confidently asymmetric GW190412, and the exceptionally small secondary of GW190814. In addition to analyses of each individual sources' properties, such as their masses and spins, one can also summarize the collective properties of the colliding objects as population probability distributions over these parameters. As catalog sizes continue to grow, it enables both finer grained investigations into the population properties of merging compact objects, and robustly testing GR in the strong gravity regime. In this talk I will present data driven statistical models to look for deviations to underlying theoretical expectations, both for individual gravitational waveform models and population models describing the astrophysical distributions of merging compact binaries. I will present the results of an analysis using this novel data-driven model on the 11 compact binary mergers in GWTC-1, then move towards hierarchical models, inferring the binary black hole mass distribution with similar data-driven methods. I will conclude with showing new results from the LVK population analyses of GWTC-3 and motivate the need towards developing more data-driven statistical models for the incoming swath of observations expected in the fourth observing run that, as we have seen, will likely continue to further challenge theoretical expectations.
 

Sociologists have interesting things to say about the practice of natural science. I will discuss the sociological phenomenon of multiples in scientific discoveries, with examples drawn from how the ΛCDM cosmology grew, and examples of possible multiple discoveries to come from issues arising in our present well-tested but certainly incomplete cosmology.

A salient feature of black holes near extremality is the appearance of an AdS$_2$ throat in their near-horizon geometry. Depending on the underlying theory, these AdS$_2$ throats may be unstable to fragmentation, wherein a single throat is instead replaced by a tree-like structure of branched AdS$_2$ throats. For Einstein-Maxwell theory, the underlying reason behind this instability is the existence of multi-centered configuration in the moduli space of black hole solutions at fixed total charge. Given the success of the Schwarzian/SYK paradigm for understanding a single AdS$_2$ it is time to revisit the fragmentation story. To build up intuition, I will present a model, studied in the statistical mechanics literature, that shares many features with SYK, including exact solvability at large-N and an emergent conformal symmetry that gets weakly broken in the UV. The novel feature of this model is the appearance of a spin glass phase at O(1) temperatures, which I will try to relate to the fragmentation story.

A quantum error-correcting code depends on a classical decoding algorithm that uses the outcomes of stabilizer measurements to determine the error that needs to be repaired. Likewise, the design of a decoding algorithm depends on the underlying physics of the quantum error-correcting code that it needs to decode. The surface code, for instance, can make use of the minimum-weight perfect-matching decoding algorithm to pair the defects that are measured by its stabilizers due to its underlying charge parity conservation symmetry. In this talk I will argue that this perspective on decoding gives us a unifying principle to design decoding algorithms for exotic codes, as well as new decoding algorithms that are specialised to the noise that a code will experience. I will describe new decoders for exotic fracton codes we have designed using these principles. I will also discuss how the symmetries of a code change if we focus on restricted noise models, and how we have leveraged this observation to design high-threshold decoders for biased noise models. In addition to these examples, this talk will focus on recent work on decoding the color code, where we found a high-performance decoder by investigating the defect conservation laws at the boundaries of the color code. Remarkably, our results show that we obtain an advantage by decoding this planar quantum error-correcting code by matching defects on a manifold that has the topology of a Moebius strip.

Zoom Link: https://pitp.zoom.us/j/91540245974?pwd=RDkzaVJZZ2tkTldxM2pkdXU5VHlIZz09

Complex and poorly understood astrophysics impacts our ability to constrain cosmological and astrophysical models from the large scale structure. Two major sources of systematic errors are galaxy biasing (non-linear mapping between dark matter and galaxies) and baryonic feedback (impact of supernovae or AGN on LSS). In the first part of my talk, I will describe a hybrid perturbation theory model of galaxy biasing and show its validation at sub-percent accuracy. I will then describe the cosmological constraints obtained using this model on the measurements from the first three years of observations of the Dark Energy Survey (DES). In the second part of my talk, I will describe tomographic measurements and analysis of the cross-correlations between thermal Sunyaev-Zel'dovich (tSZ) effect and gravitational lensing. Using data from ACT, Planck, and DES, we obtain the highest significance (20 sigma) measurements to-date and use them to constrain models for the pressure profiles of halos across a wide range of halo mass and redshift. We find evidence for reduced pressure in low mass halos, consistent with predictions for the effects of increased feedback from AGN. Lastly, we also comment on application of this study to the sigma8 tension and hydrostatic mass bias as inferred from the cluster count analysis.