Scientific Series

Spacetime fluctuations can invalidate usual notions of distance, time, and even what it means to perform an experiment. This raises a simple question: when spacetime fluctuates, how do we describe what happens? Few detailed ​answers exist, even in the low-energy effective field theory of quantum gravity. In this talk, we present a study of geometric and detector observables in perturbative quantum gravity. Geometric observables—elapsed proper time, proper distance, area, and Shapiro time delay—can be defined in relation to observer worldlines. Proper time observables capture quantum gravity effects in interferometers, leading to predictions for experiments that probe the interface between quantum mechanics and gravity. Several geometric observables display a quantum version of memory. Realistic detector observables—modelled by local operators inserted along worldlines—describe measurements more directly. We compute correlators of worldline-dressed scalars at tree level with an external on-shell graviton. The worldline dressing is hardly innocuous and can, in fact, induce enormous corrections. Gravitational effects can either open or close light cones, as captured by the commutator of dressed scalars. Finally, we discuss the broader landscape of observables now within closer reach in effective field theory and holography.

Gravitational waves (GW) offer a new and independent window onto the Universe. However, as they propagate across cosmological distances, they are inevitably affected by the intervening matter distribution. In this talk, I present a comprehensive overview of my work on the gravitational lensing of GWs, spanning weak lensing by the large-scale structure of the Universe, strong lensing by galaxies, and microlensing by stars and compact objects, and covering lensing effects from the geometric-optics to the wave-optics regimes. I highlight how gravitational lensing can serve not only as a systematic effect in GW observations, but also as a novel probe of cosmic structure and fundamental physics. Finally, I discuss future directions of GW lensing studies, inspired by advances in electromagnetic lensing.

Fault-tolerant logical entangling gates are essential for scalable quantum computing but are limited by the error rates and overheads of physical two-qubit gates and stabilizer measurements. We introduce phantom codes, a class of codes that attain the minimal possible cost: all in-block logical entangling operations reduce to qubit permutations that can be absorbed at the compilation stage, yielding zero physical-gate overhead and perfect logical fidelity. Our first main result is the discovery of such codes via four mechanisms. By exhaustively enumerating all 2.71x1010inequivalent CSS codes up to n=14 we find all codes to this scale and then identify additional instances up to n=21 using SAT methods. We then use quantum Reed-Muller constructions to find higher k instances and a "binzarization-and-concatenation" scheme to achieve higher d. Second, through end-to-end noisy simulations, we demonstrate scalable advantages of phantom codes over the surface code across multiple tasks. This includes a ~207x improvement in logical fidelity for Trotterized many-body quantum simulation at current physical error rates, with comparable qubit counts and a 22% preselection rate; the advantage persists from 8 to $64$ logical qubits. These results establish phantom codes as a viable architectural route to fault-tolerant quantum computation with scalable benefits for workloads with dense local entangling structure.

Over the last few decades, observations of diffuse gamma-ray emission in the Milky Way– in particular, the excess of GeV gamma-rays detected in the Milky Way’s galactic center, and the massive gamma-ray bubbles (the “Fermi bubbles”) centered about the Milky Way’s disk– have challenged astrophysical models. Nearly all past studies of galactic gamma-ray emission make simplifying assumptions about cosmic ray (CR) propagation that may not be valid (e.g., steady-state equilibrium), but recent numerical breakthroughs have enabled fully time-dependent dynamical evolution of CRs in magnetohydrodynamic (MHD) simulations with resolved, multi-phase small-scale structure in the interstellar medium (ISM), allowing self-consistent comparisons to the Milky Way observations. In this talk, I will present new work in which we model diffuse gamma-ray emission in simulations of Milky Way-mass galaxies with fully-resolved, multi-species CR spectra. We find that the gamma-ray spectrum in the galactic center can fluctuate by up to an order of magnitude on million-year timescales due to highly variable star formation and losses from variable structure in the turbulent ISM, with some fluctuations consistent with the Fermi-LAT galactic center excess. I will also show that Fermi bubble-like features arise from stellar feedback in these simulations. Finally, I will present the first results from a new suite of cosmological simulations in which a dark sector with an ultra-light mediator gives rise to a long-range (kiloparsec-scale) self-interaction. The addition of a long-range dark matter self-interaction has dramatic effects on the formation of galaxies and their host halos, and will be testable by current and upcoming astronomical surveys.

Recent work of Bonifacio-Hinterbichler, Bonifacio, and Kravchuk-Mazáč-Pal introduced an analogy at the level of representation theory between conformal field theories and hyperbolic surfaces. In the spectral theory of hyperbolic surfaces, there are analogs of scaling dimensions of local operators and OPE coefficients, and these numbers obey an analog of crossing symmetry. The conformal bootstrap can thus be adapted to constrain these numbers. How strong are the resulting constraints? The main result of this talk is that they are complete constraints: every solution to the crossing equations must come from a hyperbolic surface (this is a rigorous theorem).

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.