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

The Population III.1 theory for supermassive black hole (SMBH) formation predicts a very early ($z\sim20-25$), transient phase, ``The Flash'', of cosmic reionization powered by supermassive stars that are SMBH progenitors. The universe then quickly recombined to become mostly neutral, with this state persisting until galaxies begin to reionize intergalactic gas again at $z\sim 10$. The overall Thomson scattering optical depth, $\tau$, from The Flash has been shown to be $\tau_{\rm PopIII.1}\sim0.03$, leading to a total $\tau\sim0.08-0.09$. Such a value, while significantly larger than that previously inferred from {\it Planck} observations of the low-$l$ $EE$ polarization power spectrum of the CMB, can help relieve several ``tensions'' faced by the standard $\Lambda$CDM 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 $l\lesssim8$ modes is dramatically reduced compared to usual low-$z$ reionization models for the same value of $\tau$, while the power at $l\gtrsim8$ is boosted. Thus the Pop III.1 reionization scenario provides a natural way to increase $\tau$, while remaining closer to the latest CMB low-$l$ polarization observations.