Abstract

Our understanding of cosmology remains plagued by the persistent tension between early- and late-universe inferences of the Hubble constant, H0.  Amongst recent developments, new results from the Atacama Cosmology Telescope (ACT) have further reinforced the early-universe picture from Planck and other probes, clearly demonstrating that systematic errors are not responsible for the "low" values of H0 derived from these observations.  I will begin by reviewing the recent ACT results and placing these within an updated context for the H0 tension.  I will then focus on a particular well-motivated theoretical scenario aiming to resolve the tension, the "early dark energy" (EDE) model.  This scenario invokes a new component in the early universe that acts to decrease the physical size of the sound horizon imprinted in the cosmic microwave background (CMB) and thereby increase the inferred H0 value.  However, as I will discuss, EDE models that can successfully fit the CMB and local H0 data run into trouble when confronted with high-precision large-scale structure (LSS) data.  I will show that the inclusion of LSS data from the Dark Energy Survey and other large photometric surveys weakens evidence for the existence of EDE, and that further inclusion of data from the Baryon Oscillation Spectroscopic Survey further tightens the bound.  A joint analysis of cosmological data sets without local H0 data (e.g., SH0ES) shows no evidence for EDE, and yields an H0 value that is in significant tension with local H0 measurements.  Thus, the EDE scenario as currently formulated is unlikely to restore cosmological concordance.  I will conclude with a look ahead to forthcoming CMB measurements from ACT with substantially higher precision, which could provide important clues toward an H0 tension resolution.