Cosmology and Gravitation

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

The standard models of particle physics and cosmology predict the existence of a cosmological magnetic field. Estimates of the magnetic field strength and coherence scale are derived from MHD simulations and new MHD invariants. Such magnetic fields are consistent with upper and lower bounds obtained from various cosmological observations.

Quasars - accreting super-massive black holes - are the most luminous non-transient sources known and can be seen at redshifts of z > 7, when the Universe was just ~5% of its current age.  This implies that black holes with masses of up to ~10^9 M_Sun formed less than 800 Myr after the Big Bang, impossible under the default paradigm of Eddington-limited accretion onto stellar mass black holes.  The greatest barrier to understanding the formation and growth of these objects is the lack of data: quasars are very rare at these distances/times with fewer than ten known with z > 7 at present.  I will report on recent observational developments in this field, with a particular focus on the early results from the Euclid mission, for which the Wide Survey has the necessary combination of area, wavelength coverage and depth to increase the number of known quasars by an order of magnitude and to push to redshifts z > 8 and beyond.

We are now firmly within the ALMA-inspired era of star formation studies. We have abundant high sensitivity and resolution observations of star-disk-outflow systems as well as the ability to perform complex high-resolution plasma astrophysics simulations of their formation. I review recent three-dimensional nonideal MHD simulations that reveal surprising new insights into the role of magnetic fields and magnetic field dissipation in creating outflows, arcs, and cavities on scales of hundreds to thousands of au. Magnetic fields are responsible for a complex inflow-outflow structure around a protostar. I end the talk with results that may answer the longstanding big question in star formation studies: what stops the mass infall on to a protostar?

The cosmic neutrino background is like the cosmic microwave background, but less photon-y and more neutrino-ey. The CNB is also less talked about than the CMB, mostly because it's nearly impossible to detect directly. But if it could be detected, it would be interesting in several ways that are discussed.

Quasi-Einstein equations are generalizations of the Einstein equation. They arise from warped product Einstein metrics (Kaluza-Klein reductions), Ricci solitons, cosmology, near-horizon geometries, and smooth measured Lorentzian length spaces. Despite their apparent generality, they often have a surprising rigidity. I will review some recent developments in the area, focusing on near-horizon geometries, including Dunajski and Lucietti's near-horizon version of the Hawking rigidity theorem. I will discuss an application to 5-dimensional extreme (Myers-Perry type) black holes whose horizons admit the structure of the group SU(2).

The 21-cm line from neutral hydrogen holds immense potential as a probe of early astrophysics and cosmic evolution. Realizing the potential of this observational probe, however, is limited by our ability to control systematic effects in the data and model the cosmological 21-cm signal. In this talk, I will provide an overview of the observational prospects made possible through the cosmic 21-cm signal and discuss the challenges confronting interferometric experiments that are targeting a high-redshift detection. I will focus on recent developments from the Hydrogen Epoch of Reionization Array (HERA), covering our latest upper limits on the 21-cm power spectrum and their implications for early X-ray heating of the intergalactic medium. I will additionally discuss our latest efforts to understand and mitigate mutual coupling, a complex systematic effect in the data that threatens to thwart our efforts to detect the cosmological 21-cm signal. I will conclude by discussing the path forward with HERA, with a preliminary view of what to expect from forthcoming analyses.

Gravitational forces from the largest structures in the Universe leave a detectable imprint on galaxies and their local environment. I will present a new approach to tracing the tidal field using these correlations: the intrinsic alignment of small groups of galaxies, or "multipelts". Multiplets mostly consist of 2-4 galaxies within 1 Mpc/h of each other, and we measure their orientations relative to the galaxy-traced tidal field. Using spectroscopic redshfits from the DESI Y1 survey, we detect intrinsic alignment out to projected separations of 100 Mpc/h and beyond redshift 1. We find a simillar signal regardless of galaxy luminosity or color, which could make multiplet alignment a useful tool for mapping the direction of the tidal field and any cosmological effects which impact it. Our detection demonstrates that galaxy clustering in the non-linear regime of structure formation preserves an interpretable memory of the large-scale tidal field.