Astrophysics Colloquium The H0 Puzzle: Early vs. Late Time Resolutions Presented by Bhuvnesh Jain Department of Physics and Astronomy, University of Pennsylvania Thursday, February 6, 2020 11:00 A.M. in 169-336 and on Webex Abstract The expansion rate H0 measured directly is significantly higher than inferred from cosmic microwave background measurements at high redshift. Possible resolutions to this puzzle generally involve new physics in the pre-recombination era. I will describe a low redshift resolution based on the Cepheid calibration of Supernova distances. Fifth forces due to unscreened scalar fields can modify the period-luminosity relation of Cepheids and alleviate the H0 tension. I will also discuss empirical progress on the sigma8 tension. I'll conclude with some highlights from cluster scale tests of dark matter and galaxy evolution. JPL Contact: Olivier Dore (4-0690)   Site Manager: Webmaster: CL 08-3220