Cosmology and Gravitation

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.

Two puzzling periods of accelerated expansion are thought to bookend the timeline of our Universe: cosmic inflation in the beginning, and dark energy domination in recent times. New observations from experiments such as DESI and the Simons Observatory are enabling unprcedented insights into both of these epochs. In this talk, I will highlight the critical and cross-cutting role of gravitational lensing in realizing this scientific promise, focusing on recent joint analyses of CMB lensing and galaxy clustering in redshift space, as well as delensing of CMB B-mode polarization. Along the way, I will discuss several enabling methodological developments, including a new understanding of extragalactic CMB foregrounds, fingers-of-god, pixel-free estimators, and more.

The Vera C. Rubin Observatory’s LSST promises unprecedented cosmological constraints, but achieving them requires more than just statistical power—it demands forecasting pipelines that can account for complex astrophysical systematics and modeling challenges on small scales. In this talk, I present work within the LSST Dark Energy Science Collaboration (DESC) to develop a modular forecasting framework that connects realistic data modeling with infrastructure built for extensibility and validation. Drawing on my role as forecasting group lead, I will outline key challenges in pipeline design, the role of validation in maintaining forecast credibility, and the use of good coding practices—such as modularization and model registries—to ensure long-term adaptability. I’ll close with a look at new directions, including forecasts at high redshift and multi-probe combinations, underscoring how thoughtful infrastructure enables reliable science in the LSST era.

The advent of gravitational wave (GW) detections has opened a new window into our understanding of stellar-mass black holes and neutron stars. Ongoing advancements and upcoming third-generation GW detectors are expected to provide increasingly detailed insights into the properties of compact object binaries across cosmic time. In this context, the merger rate density inferred from GW detections can be interpreted as the convolution of the cosmic star formation history, chemical evolution, and binary stellar evolution.
Studying this connection from the perspective of the host galaxies of binary compact objects offers a complementary approach, not only shedding light on the physical conditions that favor different formation channels but also enhancing our ability to interpret GW observations. In this talk, I will discuss two different approaches to modeling the host galaxies of binary compact objects: one based on population synthesis models combined with galaxy catalogs from cosmological simulations, and another based on empirical galaxy scaling relations. I will highlight some of the key results from these models and discuss the next steps where host galaxy properties can provide critical constraints on the origin and evolution of compact object binaries.

Zoom link: https://pitp.zoom.us/j/98089871850