Cosmology and Gravitation

Lambda Cold Dark Matter (LCDM) is the most successful theory for the formation of structure in the Universe. Although its predictions have been verified on large scales, they are still contested on the scale of dwarf galaxies, whose dynamical properties are often cited as evidence for the need to revise some of LCDM’s basic tenets. In this context, I will discuss the recent discovery of the faintest galaxies known to date, and how their properties may be used to place constraints on the clustering of dark matter on the smallest galactic and sub-galactic scales, as well as on the viability of some of the proposed alternatives to LCDM.

Modern telescopes such as Hubble and JWST deliver exquisitely detailed images that encode rich information about galaxies, stars, and dark matter. Extracting this information requires solving high-dimensional inverse problems in which each pixel represents a parameter of the sky. In this talk, I will present a Bayesian framework that uses diffusion-based generative models to perform inference directly in these high-dimensional spaces. By learning expressive priors over galaxy morphologies and likelihoods over complex, non-Gaussian noise distributions, this approach enables the reconstruction of astronomical images at unprecedented fidelity. This talk will cover a wide range of applications where we have applied these techniques, from the recovery of detailed images of protoplanetary discs using ALMA data and high redshift galaxies using Hubble data and strong gravitational lensing to a discussion about the misscalibration of the generative model. The generative model often needs to be trained on low redshift galaxies or using simulations, all of which could introduce biases in our inference. To address this, I will present a framework which can update the parameters of the generative model using corrupted data only. Together, these results illustrate how generative modeling can be used in a principled statistical framework for astronomy.

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.

Galaxy formation in the first billion years marks a time of great upheaval in our cosmic history: the first sources of light in the Universe, these galaxies ended the 'cosmic dark ages' and produced the first photons that could break apart the hydrogen atoms suffusing all of space starting the process of 'cosmic reionization'. The past few years have seen cutting-edge instruments such as the James Webb Space Telescope (JWST) provide tantalising glimpses of such galaxies assembling in an infant Universe. Puzzlingly, these observations are also yielding a sample of unexpectedly numerous and large black holes (up to a 100 million solar masses) within the first 600 million years, posing an enormous challenge for galaxy formation models. I will show how this data is providing an unprecedented opportunity to pin down the reionization state of the Universe in addition to providing an unrivalled resource for understanding the reionization topology in the forthcoming era of 21cm cosmology. I will also show how these early systems provide a powerful testbed for Dark Matter models beyond "Cold Dark Matter". Finally, I will try to give a flavour of the gravitational wave event rates expected from such early black holes in the Laser Interferometer Space Antenna Array (LISA) era.

  Supermassive black holes (SMBHs) exist in many galaxies. Their growth is accompanied by strong energy output, which is capable of regulating host galaxy evolution. Understanding SMBH growth is thus critical for studying galaxy formation and evolution. However, it has been difficult to quantify SMBH growth in different galaxies and cosmic epochs. In this talk, I will present Trinity, an empirical technique to determine the typical SMBH mass and growth rate in different galaxies and dark matter halos from z=0-10. I will discuss how the galaxy—SMBH connection from Trinity will help observational astronomers extract more information from data, as well as theoretical astronomers create better simulations of galaxy evolution. In addition, I will give an overview of the Trinity predictions that match latest JWST data, as well as those that do not match observations. Finally, I will talk about how observations with next-generation telescopes will enable a better understanding of the galaxy—SMBH connection, and how Trinity will be helpful for quantifying the constraining power of future observations.

With increasingly precise measurements of the small-scale cosmic microwave background (CMB), the kinetic Sunyaev–Zel’dovich (kSZ) effect has emerged as a powerful probe of cosmology and astrophysics through cross-correlations with galaxy surveys. In this talk, I will review established applications, focusing on large-scale velocity reconstruction and its ability to test signatures of the early universe, such as primordial non-Gaussianity and compensated isocurvature perturbations. I will then introduce a new way to combine these data sets, which provides unique constraints on the distribution of ‘missing baryons’ at low redshifts, thereby opening a new window on baryonic feedback and cosmological inferences from the small-scale distribution of matter. Finally, I will explore the sensitivity of this statistic to the epoch of helium reionization, and compare its prospects to other proposed probes such as optical-depth fluctuations in the observed CMB.

Unimodular Henneaux-Teitelboim (HT) Gravity provides a fully diffeomorphism-invariant route to unimodular gravity by promoting the cosmological constant to a conjugate variable with an associated “unimodular time”. I will present two complementary applications. In 4D minisuperspace, starting from the connection representation, unimodular Hartle–Hawking wave packets yield a unitary inner product and normalizable states whose peaks track classical FRW dynamics. This work reframes “creation from nothing” as the emergence of a semiclassical Universe from interference of incident/reflected packets near the bounce, without invoking Vilenkin’s contour. In parallel, I will introduce a 2D cousin obtained via a centrally extended JT/KSY construction. In de Sitter quantum cosmology, superposing Lambda-eigenstates produces unitary evolution in unimodular time, controlled interference near T=0, and sharply separated WKB branches at late times. The same mechanism naturally accommodates transient quantum deformations of global dS and suggests a topology change interpretation in which contracting/expanding branches play the role of Universe/anti-Universe pairs.

Compact objects like neutron stars and black holes host extreme conditions that cannot be replicated in terrestrial laboratories or even by the most ambitious future particle colliders. These conditions make them unparalleled natural laboratories for exploring new physics. Neutron stars stand out due to their incredibly strong electromagnetic fields, some of the most intense in the universe. This makes them ideal for testing the limits of electromagnetism and for probing physics beyond the Standard Model, including axions and dark photons, which are among the leading dark matter candidates. In this talk, I will highlight recent advancements in using pulsars to detect axions and discuss ongoing efforts to probe axions using magnetars, ultra-magnetic neutron stars that are the most magnetic objects in the Universe.

Galaxy clusters are visible across the electromagnetic spectrum. Observations of their structure, abundance, and evolution provide constraints to cosmology. We are in a golden age of statistical power for galaxy clusters, where observations will provide multiwavelength data for tens of thousands of galaxy clusters. However, our ability to maximize the use of clusters as cosmology probes is limited by how well we measure cluster masses and quantify systematic effects in galaxy cluster detection and characterization. I will discuss modeling efforts that enable us to test for systematics that arise from both astrophysical and observational effects.

Perturbative calculations with light and polynomially self-interacting scalar fields in de Sitter spacetime are commonly plagued by infrared divergences as the scalar fluctuations become strongly coupled on large scales. Using techniques familiar from stochastic inflation, we treat super-Hubble fluctuations non-perturbatively through a probability distribution featuring composite operators of the light scalar. We find that these composite operators behave like late-time conformal primaries and admit a power-law four-point function in position space that is strikingly simple. We attribute physical meaning to these composite operators and conjecture that light interacting scalars in de Sitter in the stochastic picture hadronize into a tower of composite scalars on superhorizon scales. We further investigate whether these composite scalars can be described by a weakly-coupled effective field theory.