Institute for Advanced Study / Princeton University Joint Astrophysics Colloquium

I will discuss recent and ongoing work focused on attempts to restore concordance amongst cosmological data sets, motivated by discrepancies amongst some inferences of the cosmic expansion rate (H_0) and the matter clustering amplitude (S_8).  I will explain why the most viable models to resolve the H_0 problem invoke new physics at or prior to the last scattering epoch.  Such models include modified recombination scenarios, quasi-accelerating early dark energy (EDE) models (and extensions thereof, featuring EDE-dark matter interactions), or scenarios featuring new light particles with non-trivial interactions.  I will present constraints on such scenarios derived using data from the Atacama Cosmology Telescope (ACT), the Planck satellite, and large-scale structure surveys.  I will highlight newly obtained constraints on EDE models derived from Lyman-alpha forest data, which severely hinder the ability of this scenario to resolve the H_0 problem.  I will conclude with a look ahead to forthcoming CMB analyses from ACT, which will provide a powerful test of these scenarios in the low-noise, high-resolution regime.