Tangled τ: Planck CMB Constraints on Flash Reionization
Abstract
Recent cosmological tensions have renewed interest in the role of reionization in parameter inference, and flash reionization has been proposed as a transient high-redshift ionization episode that can increase the Thomson optical depth while remaining compatible with CMB polarization data. In this work, we present the first Markov Chain Monte Carlo analysis of the flash reionization model fit to Planck cosmic microwave background temperature, polarization, and lensing data. We perform this analysis using Planck PR4 data in ΛCDM with flash reionization. We find that CMB data allow a wide range for the redshift of the flash, zflash, but with constraints that are tightly correlated with the peak ionization fraction xflash. We find constraints xflash=0.24 +0.13 −0.07 and xflash= 0.15 +0.07 − 0.05 for the benchmark flash reionization scenarios with fixed zflash=20 and $25$ respectively. The standard ΛCDM parameters do not exhibit any significant shifts, and in particular, the total optical depth to reionization is not appreciably changed, with τ=0.061 ± 0.007 in flash reionization as compared to τ=0.059± 0.006 in the standard tanh parametrization. We repeat this analysis for Planck PR3 data in place of PR4, and find that the mild preference for a flash is replaced by 95% CL upper bounds, given by xflash<0.28 and xflash<0.18 for zflash=20 and $25$, respectively, from Planck PR3 data.
Related
- Constraints on Reionization from Planck Low-ℓ EE Polarization and the Lyman-Series Forests
- Inflationary models in a minimally coupled f(R,T) gravity: Constraints from Planck, BICEP/Keck, and ACT
- Lyman-α forest 1D flux power spectrum constraints on QSO-assisted reionization models
- Fast Lensing And Sub-minute High-accuracy photometry (FLASH) with JWST: Pipeline and Primordial Black Hole Microlensing Constraints from M31
- Resonantly Scattered CIV Emission in Local Star-forming Galaxies: Radiative Transfer Constraints on High-ionization Gas in Reionization Analogs
BibTeX
@misc{upadhyay2026tangled,
title = {{Tangled $τ$: Planck CMB Constraints on Flash Reionization}},
author = {Param Upadhyay and Elisa G. M. Ferreira and Evan McDonough},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2609.24989},
eprint = {2609.24989},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.24989},
}
RIS
TY - UNPB
TI - Tangled τ: Planck CMB Constraints on Flash Reionization
AU - Param Upadhyay
AU - Elisa G. M. Ferreira
AU - Evan McDonough
PY - 2026
DA - 2026/10/28
PB - arXiv
DO - 10.48550/arxiv.2609.24989
UR - https://arxiv.org/abs/2609.24989
AB - Recent cosmological tensions have renewed interest in the role of reionization in parameter inference, and flash reionization has been proposed as a transient high-redshift ionization episode that can increase the Thomson optical depth while remaining compatible with CMB polarization data. In this work, we present the first Markov Chain Monte Carlo analysis of the flash reionization model fit to Planck cosmic microwave background temperature, polarization, and lensing data. We perform this analysis using Planck PR4 data in ΛCDM with flash reionization. We find that CMB data allow a wide range for the redshift of the flash, z_flash, but with constraints that are tightly correlated with the peak ionization fraction x_flash. We find constraints x_flash=0.24 ^+0.13 _−0.07 and x_flash= 0.15 ^+0.07 _ − 0.05 for the benchmark flash reionization scenarios with fixed z_flash=20 and $25$ respectively. The standard ΛCDM parameters do not exhibit any significant shifts, and in particular, the total optical depth to reionization is not appreciably changed, with τ=0.061 ± 0.007 in flash reionization as compared to τ=0.059± 0.006 in the standard tanh parametrization. We repeat this analysis for Planck PR3 data in place of PR4, and find that the mild preference for a flash is replaced by 95% CL upper bounds, given by x_flash<0.28 and x_flash<0.18 for z_flash=20 and $25$, respectively, from Planck PR3 data.
ER -