Scientific Series

Galaxy science and cosmology are rapidly extending to ever higher redshifts, where new observations have motivated a wide range of galaxy formation models. Because these models are all tuned to reproduce observed number densities, clustering provides the critical observable for breaking their degeneracies. Pure-parallel surveys, often underexploited, enable clustering to be measured through field-to-field variance across a broad redshift range. I will show measurements from the JWST PANORAMIC survey, spanning quiescent galaxies at intermediate redshift to luminous Lyman-break galaxies above z∼10. Linear-bias results from COSMOS-Web up to z∼8 reveal a consistent picture: galaxies in the early universe are highly clustered, placing strong constraints on models of early formation. Remarkably, even carefully tuned models struggle to reproduce the observed amplitudes. Cosmic variance also emerges as both a systematic and a tool in number-count statistics, with particular relevance for explaining the presence of ultra-massive quiescent galaxies already in the young universe. I will also provide a brief overview of complementary low-redshift work, including the use of small-scale clustering to reconstruct halo merger trees with graph neural networks, and other efforts to maximize the scientific return of the next generation of galaxy surveys.

Symmetries are one of the most important concepts across many areas of theoretical physics. In this talk, I will explain how to think systematically and generally about the ways in which symmetries act in quantum many-body lattice systems, incorporating locality and the thermodynamic limit. One can think of this as a kind of many-body generalization of representation theory. In particular, this leads to a simple yet precise definition of the concept of an "anomaly" of a symmetry in lattice systems. I will discuss physical interpretations of these "lattice anomalies" and show how they are related to, but distinct from, anomalies in quantum field theory.

The QCD axion may solve the strong CP problem and constitute the dark matter (DM) abundance in our Universe. Peccei-Quinn (PQ) axions may form axion strings if the PQ phase transition occurs after inflation. I will discuss recent advances in the computation of the QCD axion DM mass in this scenario from the most precise and accurate lattice simulations to-date of axion-string networks, leading to a predicted mass range of 40 - 300 µeV. On the other hand, string theory axions, which are compelling solutions to the PQ quality problem, do not generically form strings - except in special inflationary paradigms such as brane inflation - meaning that there is no unique predicted value for the QCD axion DM mass. Nonetheless, if a Grand Unified Theory is assumed, I will explain why in broad regions of the string landscape, the mass of a stringy QCD axion is expected to be ~ 0.01 to 10 neV, independently of its DM fraction.

In the first part of the session I'll give my thoughts on why you should make the effort needed to give great talks, some strategies and advice on how to do so, and some common pitfalls to avoid. In the second part I'll open up to your questions about giving talks, writing papers, or other aspects of scientific communication. 

Elementary quantum reference frames (QRFs) such as clocks have recently led to surprising insights into quantum gravity. But far richer structures should emerge once we consider more full-fledged gravitational QRFs, i.e. quantum coordinate systems. A complete theory of such objects remains elusive, largely due to the subtleties in quantizing diffeomorphism symmetry. Null surfaces provide an ideal simplified setting to explore the properties of quantum coordinates. In this talk, I will show how one may form a useful QRF out of the area/spin 0 degrees of freedom on each light ray: the quantum dressing time. This construction permits relational observables and frame reorientations within an algebraic structure generalizing the crossed product, and consistent with established QRF approaches such as the perspective-neutral formalism. Along the way, I will discuss how diffeomorphism anomalies can be cancelled by the introduction of a suitable classical counterterm; I will describe some implications of the fact that QRFs made from fields (such as the quantum dressing time) must be Kähler-quantised; and I will comment on the consequences of our construction for gravitational entropies. Based on 2510.26589 and WIP with Laurent Freidel.

Primordial black holes (PBHs) are a well-motivated candidate for dark matter that may constitute a sub-fraction of the dark sector in the Earth-mass range. The strongest observational probe of this population is through gravitational microlensing, an effect in which the bending of light by a massive object results in the apparent transient magnification of a distant source. While ground-based observatories have produced tantalizing hints of a PBH population in this mass range, our understanding of this potential signal will be transformed with the launch of the Roman Space Telescope in 2026, whose Galactic Bulge Time Domain Survey will usher in a new era of discovery in this field. In this talk, I will discuss how by leveraging the high-statistics observations that Roman will make, we will be able to discern multiple subpopulations of non-luminous lenses and provide a new window into the existence of macroscopic dark matter.