Cosmology

Understanding galaxy formation is an outstanding problem in Astrophysics. The feedback processes that drive it, exploding stars and accretion onto supermassive black holes, are poorly understood. This results in an order unity uncertainty in the distribution of the gas inside halos, the ``missing baryon problem''. Because baryons are 15% of the total mass in the universe, this baryonic uncertainty is the largest theoretical systematics for percent precision weak lensing surveys like DES, HSC, Rubin Observatory, Roman Observatory and Euclid.
By measuring the kinematic and thermal Sunyaev-Zel'dovich effects (kSZ and tSZ), high resolution and high sensitivity CMB experiments can solve these issues by measuring the gas thermodynamics in galaxy groups and clusters, at high redshift and out to the outskirts of the halo. I will present joint tSZ, kSZ and dust measurement of BOSS (CMASS) galaxy groups, for which clustering and lensing data is also available. Using data from the Atacama Cosmology Telescope (ACT), we produced the highest significance kSZ measurement to date. This measurement shows with high statistical confidence that the gas is more spread out than the dark matter. It informs the modeling of the CMASS galaxy-galaxy lensing data, and shows that the small-scale ``lensing is low'' tension is not entirely caused by baryonic effects. Finally, comparing the observed kSZ and tSZ to hydrodynamical simulations reveals insight about the modalities of feedback.

Measurements of gravitational lensing in the cosmic microwave background (CMB) allow the dark matter distribution to be mapped out to uniquely high redshifts. After giving a brief overview of current and upcoming CMB lensing measurements, I will focus on two new ways of using CMB lensing, in combination with galaxy surveys, to constrain the early universe. First, I will explore how CMB lensing and galaxy surveys could provide insights into current discrepancies in measurements of the Hubble constant. Second, I will explain why new approaches to de-lensing – removing the lensing effect to reveal the primordial polarization sky – will be important for probing the early universe with the Simons Observatory CMB experiment.

 

On Monday July 20th, we announced the final results from extended Baryon Oscillation Spectroscopic Survey (eBOSS), the last large-scale structure galaxy survey to be undertaken within the umbrella of the Sloan Digital Sky Survey (SDSS). This marks the culmination of 20 years of galaxy surveys undertaken using the Sloan Foundation Telescope. In this seminar I will summarise the measurements presented in the 23 scientific papers that were released last Monday, and present the impact of the measurements from eBOSS and previous SDSS galaxy surveys on the development of the standard cosmological model.

I will present a brief review of large-N tensor models and their applications in quantum gravity. On the one hand, they provide a general platform to investigate random geometry in an arbitrary number of dimensions, in analogy with the matrix models approach to two-dimensional quantum gravity. Previously known universality classes of random geometries have been identified in this context, with continuous random trees acting as strong attractors. On the other hand, the same combinatorial structure supports a generic family of large-N quantum theories, collectively known as melonic theories. Being largely solvable, they have opened a new window into strongly-coupled quantum theory, and via holography, into quantum gravity. Prime examples are provided by the SYK model and generalizations, which capture essential features of Jackiw-Teitelboim gravity.