Session I04: Cosmology I

On the Origin of Supermassive Black Holes

One of the outstanding open questions of astrophysics is how the supermassive black holes (SMBHs) that reside in the centers of large galaxies come into being. After a short review of different ideas, I present a model that aims to explain the birth of all SMBHs via a single mechanism and involves them being the very first objects to form after the Big Bang. The model relies on special conditions that occur when the very first stars are forming co-located with the central cusps of dark matter minihalos, with these conditions enabling the growth of supermassive ($\sim$100,000 solar mass) stars, which then collapse to SMBHs. Key features of this model are that all SMBHs have formed by redshift 20 and show relatively weak clustering properties. Such features are testable by observations of the high z universe, e.g., with the HST and JWST, and by surveys of the local SMBH population. The model also makes predictions for the frequency of SMBH mergers, which may be tested by gravitational wave experiments, such as NANOGrav.   

Cosmology constraints from CMB lensing and the large scale structure of the Universe

Current and upcoming galaxy surveys will enable precision measurements of various observables of large-scale structures, such as galaxy clustering, abundance of galaxy clusters, and the weak gravitational lensing (WL). In the context of precision cosmology, cross-correlation analyses between independent tracers have become a powerful tool to infer valuable information of the Universe while providing extra safety checks on systematics affecting the datasets. As the matter density perturbations are mostly dominated by non-luminous dark matter, and consequently are not directly observable, we can probe them by cross-correlating the tracers of the same underlying matter density. One of the important tracers arises from the deflections of the Cosmic Microwave Background (CMB) photon's path caused by the WL effect. In this talk, I will discuss how the cross-correlation between the CMB lensing and the LSS data can probe different aspects of cosmic structure formation and shed light on current tensions in Cosmology. In particular, I will present the measurements of the cross-correlation between the CMB lensing from Planck and galaxy lensing from the deepest Stage-III galaxy WL survey, the Subaru Hyper Suprime-Cam (HSC). This cross-correlation signal is measured at a significance level of $3.1 \sigma$. The amplitude of our best-fit model with respect to the best-fit 2018 Planck cosmology is $A = 0.81\pm 0.25$, consistent with A = 1. Our result is also consistent with previous CMB lensing and galaxy WL cross-correlation studies using different surveys.   

Fundamental Physics, Cosmology, and Astrophysics with the Simons Observatory and CMB-S4

The Cosmic Microwave Background (CMB), the afterglow of Big Bang, gives us a snapshot of the early universe. Because temperature and polarization fluctuations record so much information about cosmological parameters, the CMB has become a cornerstone of modern cosmology, and future observatories will provide a wealth of new data. Curl-type (or "B-mode") polarization is particularly interesting because it probes gravitational waves and the energy scale of inflation. However, polarized emission from the Milky Way is a serious contaminant to this signal. Meanwhile, large area, high-resolution surveys constrain the number of light relics permitted by beyond-the-standard-model scenarios. The same data give us a comprehensive view of the microwave sky. We can use the CMB as a backlight to learn about structure and astrophysical objects. For example, Sunyaev-Zeldovich scattering of CMB photons allows us to detect distant galaxy clusters, while gravitational lensing of the CMB by nearby structure is a powerful tool to probe the expansion history of the universe and even measure the mass of the neutrino. We see emission from supermassive black holes, dusty star-forming galaxies, and even flaring stars in our own stellar neighborhood.   

Particle Detectors on Causal Sets

Causal set theory is an approach to quantum gravity which replaces Lorentzian manifolds with a discrete set of points and a partial order collectively called a causal set. In this talk, a simple model for a particle detector will be used for causal sets embedded in both two and four-dimensional Minkowski space with free quantum scalar fields propagating on them. Evidence will be presented that even along geodesics, the detector will register particles, and these detections do not vanish in the infinite density limit in the case of four dimensions providing a potential avenue to test the validity of causal set theory.