Fellows & abstracts

Dark matter is usually imagined as passing almost undisturbed through ordinary matter. But if dark matter is an extremely light particle behaving as a coherent wave, its interactions with matter can lead to a very different picture. The Earth itself can distort the dark matter wave around it. This distortion produces a directional pattern and, in some theories, tiny forces that depend on what an object is made of.

In this talk, I will explain how this effect turns the space surrounding the Earth into a new laboratory for dark matter searches. I will focus on MICROSCOPE, a satellite designed to test whether different materials fall in the same way. Its precision measurements can also be used to search for the characteristic forces produced by a distorted dark matter field. Existing MICROSCOPE data already probe this possibility, and a dedicated analysis could significantly extend its reach. More broadly, this provides an example of how the environment around a precision experiment can become part of the dark matter detector itself.

Compact astrophysical objects host extreme conditions, including high temperatures, densities, and electromagnetic fields, that make them ideal laboratories for searching for particles and forces beyond the Standard Model of particle physics. Axions are among the most well-motivated candidates for such new physics: they could explain the seemingly unnatural smallness of the neutron’s electric dipole moment and account for dark matter, which makes up about 85% of the matter in the universe. Axions convert into detectable photons in the presence of a strong magnetic field, an effect that underlies a significant experimental effort to detect them. In this talk, I will discuss the production and conversion of axions around nature’s most powerful magnets, neutron stars, and the prospects for detecting axions using existing and forthcoming radio observations of these stars.

Thermal misalignment provides an alternative to the standard misalignment mechanism for the cosmological production of scalar dark matter. Feeble couplings to particles in the thermal bath generate a finite-temperature potential that drives the scalar towards large field values early in the radiation era, dynamically inducing misalignment before scalar oscillations begin. The relic abundance is therefore controlled primarily by particle masses and couplings rather than the initial field value. As a light spectator field, the scalar acquires inflationary fluctuations that generically source isocurvature perturbations. Unlike standard misalignment, where light scalars are strongly constrained by cosmic microwave background bounds for high-scale inflation, thermal misalignment can naturally suppress the isocurvature signal.

The quantum tunneling effect is one of the most counterintuitive phenomena in quantum mechanics: a quantum particle can tunnel through a barrier that would be impenetrable in classical physics. Although our macroscopic world consists of inherently quantum objects, observing clear signatures of tunneling often requires microscopic probes or specially engineered quantum systems, such as superconducting circuits. In this talk, I will explain how the tunneling effect can manifest in the electrical transport of macroscopic solids. We consider small-gap semiconductors that are insulating within the linear response regime, but in which strong electric fields can generate charge carriers through tunneling. I will show that these carriers give rise to highly singular current–voltage characteristics and, when an AC electric field is applied, generate strong high harmonics. We further show a scaling law for the current amplitude at frequency nω　produced by the applied electric field E at fundamental frequency ω.

Geometry has shaped our understanding of the physical world since antiquity. In today’s quantum era, advances in experimental probes are revealing a new notion of “shape” encoded in the quantum state of matter. In this talk, I will discuss some recent advancements showcasing how mathematical notions of topology and geometry help characterize quantum materials, particularly metals, and give rise to experimentally accessible signatures in both solid-state and ultra-cold atomic platforms.

A major goal in theoretical condensed matter physics is the classification of phases of matter and the transitions between them. The physics of a system as it passes through a continuous phase transition is strongly constrained by symmetry: it turns out that such systems exhibit self-similarity at different scales and are described by models known as conformal field theories (CFTs). These constraints are often so stringent that it is reasonable to try to produce a complete menu of the possible CFTs that could describe a phase transition. In my talk, I will describe some recent progress in carrying out this program.

A standard approach to understanding complex quantum many-body systems is to introduce probes and study how the systems respond. For example, correlation functions of local operators encode information about the spectrum and dynamics of excitations. Extended probes, such as line and surface operators, can reveal nonperturbative features of these systems and host interesting physics of their own.

In this talk, we will explore two examples. First, we will discuss effective field theories for boundaries in gauge theories and explain how boundary dynamics can be used to diagnose confinement and Higgsing. Second, we will examine vortex strings in quantum electrodynamics and study how electron and photon fields respond to their presence.

Extremal black holes lie at the zero-temperature limit of black-hole physics: they carry the maximum charge or angular momentum allowed at fixed mass. In general relativity, their horizons are smooth. I will describe how remarkably fragile this smoothness is. In a broad class of physically motivated extensions of general relativity, including theories with non-minimally coupled fields such as axions and theories with higher-curvature corrections, any nontrivial such modification can make an extremal horizon singular, producing infinite tidal forces. Near-extremal black holes avoid the true singularity but can retain parametrically large tidal effects. I will explain the geometric mechanism behind this phenomenon, its universality, and whether these otherwise subtle signatures of physics beyond general relativity might be observable

The success of modern theoretical physics rests on two fundamental pillars of gravity and quantum mechanics. Whilst for many purposes they can be thought of as describing different regimes, fundamental questions such as the nature of black hole singularity and beginning of our universe require the two theories to be unified – finding a theory of quantum gravity. Before discovering such a theory in our own universe, one has to first gain a good theoretical understanding of quantum gravity in simpler settings. In this context, one of the main approaches to quantum gravity follows from extending the idea of quantum mechanical “sum over all possible paths” to “sum over all possible spacetimes”. In this talk, I will discuss examples of tractable settings where precise questions about quantum gravity can be answered, leading to universal lessons that can extend far beyond these setups

Black holes are remarkable because an enormous number of microscopic quantum states can be described by just a few macroscopic quantities, such as energy, charge, and angular momentum. This suggests a simple picture: when gravity has enough energy to make a black hole, the black hole should dominate the story. I will describe a series of examples where this intuition is not complete. Near special, highly quantum regimes, the preferred state can instead split into multiple components: a central black hole that carries most of the entropy, while surrounding objects or a dilute gas carry a substantial fraction of the charge or angular momentum. These configurations arise as endpoints of black-hole instabilities and as phases of supersymmetric ground states. Using holography, we can study the same phenomenon from several complementary viewpoints: gravitational thermodynamics, exact quantum-state counting in the dual field theory, and the Euclidean gravitational path integral. I will explain how these perspectives reveal a surprisingly rich phase structure.

I will discuss how Liouville theory provides new tools for studying de Sitter quantum gravity in two and three dimensions, connecting exact conformal field theory data to cosmological wavefunctions and gravitational path integrals. I will outline how supersymmetric extensions, solvable string theories and matrix-integral descriptions offer routes to understanding the microscopic origin of de Sitter horizon entropy

Effective field theory (EFT) provides a systematic framework for understanding how the unknown high-energy physics can leave observable imprints at low energies. These imprints are encoded in a set of parameters known as Wilson coefficients, whose values are constrained by fundamental principles such as symmetry, unitarity, and causality. In this talk, I will focus on four-dimensional EFTs with maximal supersymmetry (SUSY), whose low-energy limits are N=4 super Yang–Mills and N=8 supergravity for gauge and gravity theories, respectively. I will show that SUSY and tree-level factorization of the scattering amplitudes impose novel nonlinear constraints on the Wilson coefficients. When these constraints are combined with positivity bounds, the allowed values of the Wilson coefficients converge towards a unique theory, finding open and closed superstring theory as the unique high-energy completions of the gauge and gravity theories, respectively.

What particles are allowed in a consistent quantum theory? To answer this question, I will use tools from the S-matrix bootstrap program. The S-matrix, or scattering matrix, encodes the quantum mechanical probabilities for particle scattering processes. The bootstrap program attempts to study the S-matrix using fundamental physical principles like Lorentz invariance, analyticity, unitarity, and crossing symmetry. Using these principles, I will sketch a proof which bounds the allowed masses and spins of particles in any consistent quantum theory. I will also describe some other exciting bootstrap results from recent years.

I will present an overview of recent developments in the numerical conformal bootstrap in two distinct directions. First, focusing on just one theory, I will present the latest high-precision determinations of the CFT data of the 3D Ising CFT, including the results from spectrum extraction. After that, I will move on to studies of the full space of local, unitary CFTs by considering bounds from correlators of conserved currents. I will highlight remarkable and as-yet unexplained topographic features in the "world atlas of 3D CFTs," motivating a program of CFT cartography.

The gravitational waves emitted during the final stages of black hole mergers provide a unique probe of strong-field gravity and a precision test of General Relativity and the black hole paradigm. In this talk, I will review our theoretical understanding of how perturbed black holes relax to equilibrium, and discuss the challenges this picture poses for current and future gravitational-wave observations. I will focus in particular on nonlinear effects in black hole ringdown, including the excitation of nonlinear harmonics, and discuss how their observation could open a new window onto the nonlinear dynamics of gravity.

The distribution of galaxies preserves a fossil imprint of sound waves that traveled through the early Universe. This pattern, known as baryon acoustic oscillations (BAO), provides a cosmic ruler for tracing the expansion history of the Universe and investigating the physics behind its accelerated expansion. But as measurements become more precise, how do we check that they are not being biased by observational effects, modeling assumptions, or our own expectations? Drawing on the second data release of the Dark Energy Spectroscopic Instrument (DESI) and the validation effort I led, I will explain how we put cosmological measurements to the test using simulated universes, changes to the analysis, and procedures that hide the cosmological answer until key checks are complete. I will discuss how these tests build confidence in the BAO measurements and provide a robust foundation for investigating cosmic expansion and the nature of dark energy.

Cosmological findings from the combination of DESI and Planck data have placed very tight constraints on the total neutrino mass within the ΛCDM model, leading to tension with particle physics experiments. Based on full-shape analyses of (e)BOSS data, we show that the neutrino mass signal is highly sensitive to background effects, clouding the reliability of its measurement. However, by disregarding the background and focusing on the effect of structure suppression, we discover that most of the neutrino information is derived from the amplitude of the wiggles, rather than the broadband suppression of the power spectrum, as has been believed for many years. This insight offers a promising pathway for robustly extracting the neutrino signal. This work is mainly based on arxiv: 2407.06117, and 2503.14744, 2504.18464

Most of the matter and structure in the Universe is invisible to us directly. So how can we map it, study its history, or use it to understand the underlying physics of the cosmos?

One surprisingly powerful probe begins with the light of distant, bright quasars, powered by supermassive black holes. As these photons travel billions of years toward us, intervening neutral hydrogen absorbs some of their light, leaving behind a rich pattern of shadows known as the "Lyman-alpha forest." Hidden among this forest are much larger absorbers, the “mountains” among the trees, associated with dense reservoirs of neutral gas and the environments of galaxies. These systems can obscure the subtle patterns that cosmologists hope to measure precisely, but they also carry a story of their own, allowing astronomers to trace galaxies and gas in the early Universe.

In this talk, I will present a visual journey through these Lyman-alpha forests and mountains: learning to read their shadows, imagining the three-dimensional structures of gas and galaxies behind them, and comparing those inferences with simulated universes. Along the way, we will see how features that first appear to be messy contaminants can become tools for studying galaxies, intergalactic gas, and the Universe on its smallest accessible scales.

When the gauge group of a physical system is non-compact, defining physical quantum states—and their inner product—becomes quite subtle. I’ll explain what goes wrong in quantum mechanical theories, as well as a proposal to overcome the obstacles, and use it obtain the first complete canonical quantization of de Sitter-JT gravity in closed universes. Based on 2512.03030.

What happened at the beginning of time? What would we measure close to the big-bang? What is even the mathematical framework for describing measurements that an observer such as ourselves, would make in a universe with a positive cosmological constant, such as our own universe? To address these questions, we need to develop a fundamental understanding of gravity at the quantum level. One way to make progress on quantum gravity is to uncover a holographic description of gravity from an observer's perspective. I will explain why a holographic description could be useful, and summarize the state of the art on one proposal of a holographic description of a universe with a positive cosmological constant (like our own), based on the (so-called) SYK model.

It was recently mathematically shown that extremal black holes can form in General Relativity, thereby falsifying the “third law”. The proof relies on a technique known as characteristic gluing, which can be used to construct spacetimes with certain properties. I will describe how characteristic gluing can be implemented numerically, and will give a brief overview of the type of problems to which it can be applied.