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

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

Problem Statement
The standard $\Lambda$CDM cosmological model faces several tensions, particularly regarding the Hubble constant, negative neutrino masses, and dynamic dark energy. Recent analyses suggest that resolving these tensions may require a higher total Thomson scattering optical depth ($\tau \approx 0.09$) than the value inferred from Planck's low-$\ell$ EE polarization power spectrum ($\tau \approx 0.054 - 0.06$). However, increasing $\tau$ using standard late-phase reionization models (driven by galaxies at $z \lesssim 10$) typically produces low-$\ell$ EE mode amplitudes that are inconsistent with Planck observations.

This paper investigates an alternative scenario: the "Pop III.1 Flash." This theory posits that supermassive black hole (SMBH) progenitors formed as supermassive stars ($\sim 10^5 M_\odot$) in pristine, metal-free minihalos at very high redshifts ($z \sim 20-25$). These stars produce a transient, early phase of cosmic reionization before the universe recombines and remains neutral until standard galaxy-driven reionization begins at $z \sim 10$. The authors aim to determine if this early flash can contribute significantly to the total $\tau$ without violating current low-$\ell$ CMB polarization constraints.

Methodology
The authors model the reionization history of the universe as a superposition of two components:

1. Late-phase Reionization: Modeled using a standard $\tanh(z)$ function to describe the ionization fraction of hydrogen and helium, parameterized by a midpoint redshift ($z_{re}$) and transition width ($\Delta z$).
2. Pop III.1 Flash: Modeled following J. C. Tan (2025), assuming supermassive stars form at $z_{form}$, creating HII regions that peak in ionization at $z_{flash}$ (considering cases of $z_{flash} = 20$ and $25$). The ionization fraction rises over a timescale ($t_{rise} \approx 30$ Myr) and decays exponentially based on the recombination timescale in overdense regions.

Using the linear Boltzmann solver CLASS, the authors compute the CMB EE power spectra for various reionization histories. They assume a flat $\Lambda$CDM cosmology with parameters derived from the Planck "Plik best fit" and a minimal neutrino mass. They compare models with elevated total $\tau$ (specifically $\tau \approx 0.079$ and $\tau \approx 0.091$) achieved either through standard late-phase reionization alone or through a combination of late-phase reionization ($z_{re}=7$) and the Pop III.1 Flash.

Key Results
The study finds that the redshift dependence of reionization significantly alters the shape of the CMB EE power spectrum:

• Suppression of Low-$\ell$ Modes: Because the Pop III.1 Flash occurs at very high redshift ($z \sim 20-25$), the contribution to the lowest multipole moments ($\ell \lesssim 6$) is dramatically reduced compared to standard low-$z$ reionization models with the same total $\tau$.
• Boost at Intermediate $\ell$: The power at $\ell \gtrsim 6$ is boosted. For the Pop III.1 models, the "reionization bump" shifts to higher multipoles ($\ell \approx 11$ for $z=20$) due to the smaller comoving distance to the scattering surface.
• Statistical Fit: For a target $\tau \approx 0.079$, the Pop III.1 Flash model yields a $\chi^2 = 6.7$ for the $\ell \le 6$ modes, whereas a standard $\tanh(z)$ model (requiring $z_{re} \approx 10.1$) yields $\chi^2 = 13.6$. Similarly, for $\tau \approx 0.091$, the Pop III.1 model maintains lower power at $\ell \lesssim 6$ than the standard model.
• Tension at Intermediate Scales: While the Pop III.1 models fit the lowest $\ell$ modes better, the authors note that at intermediate scales ($7 \le \ell \le 18$), the boosted power in the Pop III.1 models appears to be in tension with the latest Planck NPIPE data.

Significance and Claims
The paper claims that the Pop III.1 Flash scenario provides a "natural way" to increase the total Thomson scattering optical depth to values ($\tau \sim 0.08-0.09$) that could alleviate cosmological tensions (such as the Hubble tension and preferences for negative neutrino masses) while remaining closer to Planck's low-$\ell$ polarization observations than standard models.

The authors emphasize that this scenario relies on the existence of WIMP dark matter annihilation in the first generation of stars to allow them to grow to supermassive scales. Consequently, the model predicts the existence of WIMPs with masses $\sim 100-1,000$ GeV. The paper suggests that future CMB polarization experiments (e.g., LiteBIRD, CLASS) with improved sensitivity at $10 \lesssim \ell \lesssim 30$ could distinguish between these models. Additionally, the scenario offers a potential explanation for the anomalous depth of the EDGES 21-cm absorption signal via an enhanced radio background from free-free emission during the Pop III.1 Flash, and predicts specific signatures in the patchy kinetic Sunyaev-Zel'dovich (pkSZ) effect.