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.