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

TL;DR

The paper addresses the tension between large Thomson optical depth values and Planck low-$l$ $EE$ constraints by proposing an early Population III.1 flash reionization scenario at $z\sim20$–$25$, contributing $\tau_{\rm PopIII.1}\sim0.03$ to a total $\tau\sim0.08$–$0.09$. It adopts a two-component reionization model (late-time tanh plus an early flash) and computes the corresponding CMB $EE$ spectra using the CLASS code with Planck cosmology, exploring $z_{\rm flash}=20$ and $25$ alongside plausible ionization parameters. The results show that the Pop III.1 component can decrease the low-$l$ ($l\lesssim8$) $EE$ power while boosting power at higher $l$ for the same $\tau$, offering a path to reconcile a higher $\tau$ with Planck data. Forecasts indicate that future missions like LiteBIRD could distinguish these scenarios with ~20% precision in the $10\lesssim l\lesssim30$ range, and pkSZ constraints from the Simons Observatory will further test the nonmonotonic history, with broader implications for high-$z$ 21-cm signals and the role of dark matter annihilation in the first stars.

Abstract

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, $τ$, from The Flash has been shown to be $τ_{\rm PopIII.1}\sim0.03$, leading to a total $τ\sim0.08-0.09$. 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 $Λ$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 $τ$, while the power at $l\gtrsim8$ 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.

Figures (2)

• Figure 1: (a) Top: Ionization fraction history of the universe, shown as a function of redshift, with $x_{i,{\rm H}}$ measuring the fraction of H atoms that are ionized. The green dashed line shows a standard reionization history described with a ${\rm tanh} (z)$ function with $z_{\rm re}=8$ [Eq. \ref{['eq:rion']}], from fully ionized conditions at low redshift to fully neutral conditions at high redshift. The dashed blue line is an equivalent model, but with $z_{\rm re}=7$. Note, these models include a standard treatment of He reionization (see text). The red lines show fiducial ionization histories resulting from Pop III.1 supermassive stars, peaking at $z_{\rm flash}=20$ (dotted) and 25 (dashed). (b) Bottom: Thomson optical depth to electron scattering, $\tau$, integrated out to redshift, $z$. The green and blue dashed lines show the ${\rm tanh}(z)$ models with $z_{\rm re}=8$ and 7, respectively, representing the contribution from "standard" galaxies and AGN. The red dotted and dashed lines show the contribution from Pop III.1 supermassive stars with epoch of peak flash ionization at $z_{\rm flash}=20$ and 25, respectively, which have been combined with the ${\rm tanh} (z)$ with $z_{\rm re}=7$ model.
• Figure 2: CMB $EE$ power spectra for various reionization models. The green dashed line shows the ${\rm tanh}(z)$ model with $z_{\rm re}=8$, yielding a total $\tau=0.057$. The red and blue dashed lines show equivalent ${\rm tanh}(z)$ models with $\tau$ of 0.079 and 0.091, respectively. The red and blue solid lines show fiducial ${\rm tanh}(z)$ + Pop III.1 reionization models with $z_{\rm flash}=20$ and 25 that also yield $\tau$ of 0.079 and 0.091, respectively. The shaded region around the red solid line shows a cosmic-variance-limited error range to the $z_{\rm flash}=20$ model, assuming 70% sky coverage, i.e., a reasonable approximation for expected LiteBIRD uncertainties. The grey circles and error bars show the Planck NPIPE data Planck:2020oloTristram:2020wbi.