The Impact of Population III.1 Flash Reionization for Cosmic Microwave Background Polarization and Thomson Scattering Optical Depth

Journal article, 2026

The Population III.1 theory for supermassive black hole (SMBH) formation predicts a very early (z ∼ 20–25) transient phase, the “Pop III.1 Flash,” of cosmic reionization powered by supermassive stars that are SMBH progenitors. The Universe then quickly recombines to become mostly neutral, with this state persisting until galaxies begin to reionize intergalactic gas again at z ∼ 10. The overall Thomson scattering optical depth, τ, from the Flash has been shown to be τPopIII.1 ∼ 0.03, leading to a total τ ∼ 0.08–0.09. Such a value, while significantly larger than that previously inferred from Planck observations of the low-l EE polarization power spectrum of the cosmic microwave background (CMB), can help relieve several “tensions” faced by the standard ΛCDM cosmological model, especially the preference for negative neutrino masses and dynamic dark energy. Here we compute EE power spectra of example models of the Flash. We find that, because of its very high redshift, the contribution to l ≲ 6 modes is dramatically reduced compared to usual low-z reionization models for the same value of τ, while the power at l ≳ 6 is boosted. Thus the Pop III.1 reionization scenario provides a natural way to increase τ, while remaining closer to the latest CMB low-l polarization observations.