Cosmology and Gravitation

Next-generation spectroscopic surveys will push beyond the reach of current experiments, targeting galaxies at z > 2. This high-redshift regime is attractive for several reasons, among them the opportunity to cross-correlate galaxy samples with CMB lensing near the peak of the lensing kernel. Lyman alpha emitters (LAEs) are one of the key tracers that will enable next-generation experiments to probe these redshifts, and with imaging preparation for such surveys already under way, we can begin targeting and observing LAEs now. In this talk, I will demonstrate LAE target selection using medium-band imaging from the Intermediate Band Imaging Survey (IBIS). Using spectroscopic redshifts from Dark Energy Spectroscopic Instrument (DESI) follow-up observations, I will then present measurements of their 3D clustering, jointly fitting the LAE and QSO auto- and cross-spectra to constrain the LAE bias, Fingers-of-God effect, and the impact of radiative transfer. Our results demonstrate that LAEs can be reliably selected and modeled for next-generation surveys, marking necessary progress in the modeling preparation these experiments require.

The discovery by JWST of a large population of active black holes at z > 5, together with the evidence from Pulsar Timing Arrays for a gravitational-wave background, likely produced by supermassive black hole binaries, offers new constraints on the early growth and evolution of massive black holes. Both observations challenge standard expectations. JWST points to more numerous or more rapidly growing black holes at early times than many models predict, while the estimated PTA amplitude requires a larger than expected population of very massive black holes at low redshift. At the same time, tidal disruption events open a window onto the elusive population of intermediate-mass black holes. In this talk, I will present L-GalaxiesBH, a model that follows the formation, growth and dynamics of massive black holes and their host galaxies from their earliest emergence to the local Universe. I will then show how we jointly confront it with JWST, PTA and tidal disruption event data to constrain the model's parameters and piece together the evolutionary history of massive black holes.

One of the most fundamental questions that can be asked when studying any physical theory is: What are its true degrees of freedom? These modes are the building blocks of theories, which can evolve and be observed, shaping how a theory connects to the physical Universe. Finding them, however, is not always as simple as it sounds. In this talk, I will outline the standard methods to identify the propagating modes in general theories, reflect on their strengths and limitations, and present an alternative recipe that leads to insights about the dynamical nature of theory in a quick, simple, and straight-forward way. 

Current pulsar timing array (PTA) analyses do not take full advantage of pulsar distance information, thereby missing out on improved angular resolution and on a potential factor-of-two gain in detection sensitivity for individual gravitational-wave (GW) sources. In this work, we investigate the impact of precise pulsar distance measurements on angular resolution as an extension to previous work measuring the angular resolution of a dense, isotropic PTA [Jow et al., 2025]. We present a coherent map-making technique that utilizes precise pulsar distance measurements to reach the diffraction-limited resolution of an individual source: δθdiff ∼ (1/SNR)(λGW/r) ≈ 2 arcmin, where the SNR refers to the detection strength of the source. With this level of angular resolution, identifying an EM counterpart may become feasible, enabling multi-messenger follow-up. We show that for SNR = 10, which may be the current sensitivity level using a coherent analysis, the diffraction limit is reached with roughly 9 pulsars. Moreover, angular resolution scales sharply with the number of known pulsar distances as ∼ (1/SNR)Ndist/2 . Thus, each additional pulsar with high signal-to-noise timing and precise distance measurement can improve PTA resolution by an order of magnitude. The distance to the best-timed millisecond pulsar (PSR J0437−4715) is already constrained to subparsec levels. We argue, therefore, that a coherent analysis of PTA data, fully incorporating pulsar distance information, is timely.

Although we will never get the same level of details for exoplanets as we do for Solar System bodies, the large diversity of exoplanets revealed by exoplanet hunting missions, e.g. Kepler and TESS, provide thousands of study cases to refine formation and evolution pathways as well as theories of how their climate is shaped by their environment. Particularly amenable for atmospheric characterization, short-period exoplanets with dayside blasted with stellar radiation are some of the best-characterized exoplanets to this day. Due to their synchronous rotation, they exhibit large day-to night difference, and their observation can be difficult to interpret without a full understanding of their “3D-ness”. In the past 2 decades, a suite of observational techniques along with new space-based (e.g. JWST) and ground-based observatories with exquisite precision that now allows us to reveal the inhomogeneous nature of these exoplanets and provide a more comprehensive view into their atmosphere, or lack thereof. In this talk, I will present what we have learned from observations of a variety of close-in planets ranging from scorching hot exoplanets and how these observational techniques are now used to study temperate rocky planets to investigate their habitability.

Studying low-redshift analogues of phenomena in the early Universe is an established method for getting a closer look at intriguing physics. However, determining when a local analogue functions well to represent distant systems, and what limitations need to be kept in mind, can be challenging. Notably, low redshift examples often reveal complexity that gets hidden when looking at high redshift objects. For distant objects, limited spatial and spectral resolution and reduced sensitivity can lead to extraordinary claims that do not hold up over time. I'll discuss specific examples of local analogues that reveal complexity that should be considered when studying distant objects, focussing on local compact galaxy groups and super Eddington-accreting active galactic nuclei.

Galaxy clusters assemble hierarchically, and their present-day dynamical state encodes information about cluster formation timescales and mass distribution. The diffuse intracluster light (ICL), a fossil record of tidal stripping and accretion, offers a complementary probe that is sensitive to a cluster’s assembly history. In this talk I will present recent works based on photometric surveys (Euclid, UNIONS, DESI Legacy, KiDS) and cosmological hydrodynamic simulations (e.g. IllustrisTNG, Hydrangea), addressing two questions: (i) how observational tracers (e.g. magnitude gaps, galaxy-stellar-mass ratios) can be used to identify cluster dynamical state and its imprint on cluster properties; and (ii) what the ICL fraction and morphology reveal about the underlying mass and the stage of assembly. If time allows, I will briefly describe ongoing work with the FLAMINGO simulations that connects high-redshift protoclusters to their descendant clusters using observational indicators of environment and assembly.