First constraints on the high redshift 21-cm power spectrum from HERA

Abstract

21-cm emission from neutral hydrogen atoms provides a unique window into galaxy formation and cosmology in the first billion years of cosmic history. As the first galaxies form after the Big Bang, they generate intense ultraviolet and X-ray radiation fields, which ionize and heat their otherwise neutral surroundings. The resulting modulations in the brightness of 21-cm emission relative to the background can be detected in principle by a single, well-calibrated dipole receiver, as features in the sky-averaged radio spectrum below ~200 MHz. Spatial fluctuations in the 21-cm background are expected also, and can in principle be detected statistically with the current generation of interferometers. In just the last few years, enormous progress has been made on both fronts. The sky-averaged 78 MHz feature reported by the EDGES collaboration in 2018 caused a flurry of activity, largely aimed at explaining its anomalous amplitude. Since then, the MWA, LOFAR, and HERA have all reported upper limits on the 21-cm power spectrum. In this talk, I will focus in particular on the first limits from HERA -- the most stringent limits reported to date -- and describe their implications for galaxy formation and cosmology. I will also discuss the ongoing EDGES controversy, and how JWST and SPHEREx can provide independent tests of astrophysical scenarios that produce EDGES-like 21-cm absorption troughs at frequencies below ~100 MHz.