Cosmology and Gravitation

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.

The Dark Energy Spectroscopic Instrument (DESI) is the largest galaxy redshift survey to date, aiming to catalog ~63 million galaxies over 17000 deg^2 of the sky by the end of 8 years of observation. The DR1 analyses were completed in 2024 with exciting new constraints on cosmological parameters within the standard LCDM model as well as extensions such as evolving dark energy. I will discuss the cosmological results from the DR1 fullshape and BAO analysis which was presented in the Fall of 2024 and the theory systematic tests/validation that went into it. I will then discuss the advantages of including cross correlations of DESI galaxies with CMB lensing and some key results from my joint analysis of 3D clustering with CMB lensing using the DESI DR1 galaxy sample and ACT DR6 and Planck PR4 lensing maps. In particular we find that just including galaxy-lensing cross-correlations on top of the fullshape and BAO analysis tightens amplitude constraints by ~30%. The second data release (DR2) spans three years of observation with analyses expected to be presented in Spring of 2026. I will briefly discuss the expected improvements in constraining power from DR2 and summarize the types of analyses that DESI will perform and their cosmological relevance.

The interstellar medium (ISM) plays an important role in sculpting the structure of our Galaxy: in addition to being the birthplaces of stars, ISM substructures have the capacity to significantly perturb stellar orbits. However, conventional stellar-dynamical studies often rely on idealized toy models or omit these gas “fluctuations” entirely, leaving their impact on the evolution of stellar systems poorly understood. In this talk, I will present a model for ISM fluctuations that is both theoretically tractable and easily incorporated into traditional N-body simulations, while retaining the essential features of a realistic ISM. The model is derived from a characterization of the ISM in the state-of-the-art TIGRESS magnetohydrodynamics simulations, which include self-consistent, first-principles gas microphysics and resolve scales down to 2 parsecs. I will then highlight several key differences between the dynamical effects of these realistic fluctuations and those assumed in prevailing models, focusing on orbital heating and radial migration in galactic disks. Finally, I will discuss the importance of accounting for the ISM’s influence in drawing robust conclusions about the Milky Way’s dynamical history and dark matter substructure.

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.

Weak lensing of galaxies and of the CMB provide direct probes of the cosmic matter density field, but are sensitive to different redshift ranges and different survey systematics. The combination of these probes provides a way to calibrate systematics and test LCDM across cosmic time. I will talk about the cross-correlation of cosmic shear and CMB lensing using DES Y3 and new lensing reconstructions from SPT-3G. The main result of this analysis is a first high-significance measurement of the lensing-shear cross-correlation using polarization-only lensing maps, allowing us to sidestep the issue of extragalactic foregrounds without losing much constraining power on large scales. Additionally, using a pure blue shear sample and a variety of foreground-mitigated CMB lensing reconstructions we are able to conduct data-driven tests to ensure our results are robust to both galaxy intrinsic alignments and foregrounds in the CMB.