Scientific Series

We construct the phase space of a spherically symmetric causal diamond in (d+2)-dimensional Minkowski spacetime. Utilizing the covariant phase space formalism, we identify the relevant degrees of freedom that localize to the d-dimensional bifurcate horizon and, upon canonical quantization, determine their commutators. On this phase space, we find two Iyer-Wald charges. The first of these charges, proportional to the area of the causal diamond, is responsible for shifting the null time along the horizon and has been well-documented in the literature. The second charge is much less understood, being integrable for d ≥ 2 only if we allow for field-dependent diffeomorphisms and is responsible for changing the size of the causal diamond. 

Big Bang Nucleosynthesis (BBN) is a powerful tool for probing both new physics and LCDM, and complements analyses utilizing the Cosmic Microwave Background (CMB) and results from particle experiment.  I will provide two examples of BBN probes of BSM models.  I will then discuss new kinds of analyses that can be performed with the recently-released fast and differentiable BBN code LINX.  In particular, LINX can be used to perform full BBN+CMB joint analyses at a level of sophistication that has never been achieved before, even in LCDM analyses.

Recent advances in General Relativity point toward unanticipated, and dynamic, relations between ultracompact objects and the universe they inhabit. The possibility for strongly gravitating systems, like astrophysical black holes (BHs) and their embedding cosmology, to directly interact has been dubbed "cosmological coupling." We focus on recent results from the DOE Stage IV Dark Energy (DE) Spectroscopic Instrument (DESI), which strongly suggest that DE is dynamical. Using typical empirical models for the cosmic star-formation rate density as a proxy to BH production, we show that the DESI-inferred time-evolution of DE is consistent with cosmologically coupled stellar-collapse BHs as the source of DE. The predicted cosmological expansion rate today, H_0 = 69.94 +/- 0.81 km/Mpc/s, is in excellent agreement with H_0 = 69.58 +/- 1.58 km/Mpc/s recently reported by the Chicago-Carnegie Hubble Program using Cepheid, Tip of the Red Giant Branch, and J-Region Asymptotic Giant Branch stellar distance-ladder calibrations. With DESI Redshift Space Distortions and Year 3 datasets on the horizon, we highlight exciting prospects for further observational confrontation in the near term.

Dark matter and neutrinos are elusive neutral matter filling up our Universe. The PandaX (Particle and Astrophysical Xenon) experiment, located in the China Jinping Underground Laboratory, is dedicated to searching for dark matter particles and studying the fundamental properties of neutrinos. The current running experiment, PandaX-4T, contains a sensitive time-projection-chamber with a 3.7-ton liquid xenon target. In this talk, after an overview, I will present recent results from PandaX-4T in dark matter direct detection, double beta decay of 136Xe, as well as solar neutrinos. I will also present a concrete plan for the next-generation xenon observatory in CJPL, PandaX-xT.

We study the infrared on-shell action of Einstein gravity in asymptotically flat spacetimes, obtaining an effective, gauge-invariant boundary action for memory and shockwave spacetimes. We show that the phase space is in both cases parameterized by the leading soft variables in asymptotically flat spacetimes, thereby obtaining an equivalence between shockwave and soft commutators. We then demonstrate that our on-shell action is equal to three quantities studied separately in the literature: (i) the soft supertranslation charge; (ii) the shockwave effective action; and (iii) the soft effective action.

In this talk I will do three things. First, I will outline the conditions under which the interaction rate of inelastic processes with a system consisting of N targets scales as N^2. Second, I will present computations of interaction rates for several weakly interacting particles, including the Cosmic Neutrino Background and QCD axion dark matter, and will explain the underlying physics. Third, I will introduce new quantum observables that do not rely on net energy transfer, but can still extract these N^2 effects. This talk will not address a concrete experimental proposal, but the effects presented may point to a new class of table-top and ultra-low threshold particle detectors.

In this seminar, I will discuss recent progress towards developing robust methods to constrain PNG in the non-linear regime based on the LSS consistency relations — non-perturbative statements about the structure of LSS correlation functions derived from symmetries of the LSS equations of motion. Specifically, I will present non-perturbative models for the squeezed matter bispectrum and collapsed matter trispectrum in the presence of local PNG, as well as in the presence of a more general “Cosmological Collider” signal sourced by inflationary massive particle exchange. Using N-body simulations with modified initial conditions, I will demonstrate that these models yield unbiased constraints on the amplitude of PNG deep into the non-linear regime (k~2 h/Mpc at z=0). Finally, I will discuss how these non-perturbative methods can provide insight into the scale-dependent bias signature associated with the Cosmological Collider scenario.

As endpoints of massive stellar evolution, showcases for the densest matter in the universe, and sites for heavy element nucleosynthesis, neutron star mergers are superb laboratories for astrophysics, strong gravity and nuclear physics. Gravitational-wave observations of these mergers are beginning to reveal neutron stars’ internal structure, provide insight into the astrophysical processes that form them, and expose their role in the chemical evolution of the Galaxy. I will survey some of my recent work in these areas and describe how our theoretical understanding of neutron stars is being shaped by gravitational-wave discoveries.

In quantum field theory (QFT) the spin-statistics theorem says that in a unitary QFT, a particle has half-integer spin if and only if it is a fermion. I show how to phrase this statement in the language of functorial field theories. More precisely, I explain when a functorial field theory "has fermions" and "has spinors" and when they are "related". I will then restrict to topological field theories (TFTs) and define unitary TFTs. There are counterexamples of the spin-statistics theorem for non-unitary TFTs. I will prove that every unitary TFT satisfies the spin-statistics theorem.

I will discuss recent progress on the initial conditions for inflation and how this can potentially enhance gravitational wave non-Gaussianity (NG). In particular, there is a significant additional NG contribution in the flattened configuration, offering a straightforward way to boost parity-violating NG from Chern-Simons gravity, which typically suffers from graviton ghost-production. Furthermore, I will explore how this shape – at the level of the trispectrum – can enable direct measurement of NG in the stochastic gravitational wave background (SGWB) using pulsar timing arrays (PTA). This is particularly intriguing as it allows us to investigate NG at scales complementary to those observed in the CMB, which could help us distinguish between various early universe models and physical mechanisms.