Session H07: Crisis in Cosmology

Crisis in Cosmology: Measuring The Local Value of the Hubble Constant

I will review the latest measurements of the local value of the Hubble constant by the SH0ES team as well as new studies addressing each rung of the distance ladder. I will also go over prospects of new probes like strong lenses and kilonovae to measure the Hubble constant.   

Clarifying the Hubble Constant Tension

The Hubble constant estimated from the local Universe (via the Cepheid distance ladder) is in three-sigma tension with the value extrapolated from cosmic microwave background data assuming the standard cosmology. Whether this discrepancy represents physics beyond the Standard Model or deficiencies in our understanding of the data is the subject of intense debate. In this talk, I will review the community's attempts to explain and interpret the Hubble constant tension, clarifying the current picture using Bayesian probability theory. Finally, I will consider the potential for independent datasets - measurements of gravitational wave standard sirens in particular - to arbitrate the dispute.   

The Hubble Constant from the Cosmic Microwave Background

In this talk, I will review constraints on the Hubble constant from measurements of the cosmic microwave background (CMB). I will explain how the CMB can be used to constrain the Hubble constant, and how it is complementary with other local probes of expansion. I will review recent results from satellite and ground-based experiments, including from WMAP, Planck, the Atacama Cosmology Telescope (ACT), and the South Pole Telescope (SPT). I will contrast these results, discussing differences in the measurements, in particular where each measurement constraints the CMB angular power spectrum, and the overall consistency of the results. I will discuss degeneracies between CMB-derived constraints on the Hubble constant with other cosmological parameters, in a standard 6-parameter LCDM cosmology and other model extensions. Finally, I will discuss future constraints from next-generation CMB experiments, including the ground-based CMB-S4 experiment.