Title: Searching for New Physics in the Universe’s Oldest Light with the Atacama Cosmology Telescope

Abstract:
I will discuss recent and ongoing work focused on attempts to restore concordance amongst cosmological data sets, motivated by discrepancies amongst some measurements of the cosmic expansion rate (H_0) and the matter clustering amplitude (S_8).  Particular attention will be paid to models invoking new physics at or prior to recombination, including quasi-accelerating early dark energy (EDE) models and a newly developed extension of this scenario featuring interactions between the EDE field and the dark matter.  In particular, I will discuss constraints on these models derived using the latest data from the Atacama Cosmology Telescope (ACT).  I will conclude with a look ahead to forthcoming CMB measurements from ACT, which will provide a powerful test of these models in the low-noise, high-resolution regime.