Flash Ionization of the Early Universe by Population III.1 Supermassive Stars

Journal article, 2025

The Population III.1 theory for supermassive black hole formation predicts that a substantial fraction of the early Universe was ionized by supermassive stars at redshifts z ∼ 20-30, an era we refer to as “The Flash.” This is followed by recombination to a mainly neutral state within a few tens of Myr. Here we discuss the implications of this ionization for the scattering optical depth of the cosmic microwave background (CMB), τ. We find a fiducial contribution of τPopIII.1 ∼ 0.04. Combining this with the contribution to reionization by standard galaxy populations at z ≲ 10 with τgal ≃ 0.06, yields a total of τ ≃ 0.10. As noted recently by several authors, such a value may help resolve apparent “problems” faced by ΛCDM of discrepant CMB-based measures of the Hubble constant (“Hubble tension”), as well as negative neutrino masses and dynamical dark energy that have been implied by recent baryonic acoustic oscillation results from the Dark Energy Spectroscopic Instrument. In addition, free-free emission from The Flash boosts the cosmic radio background, which could help explain the large 21 cm absorption depth reported by the Experiment to Detect the Global EoR Signature.