Constraints on Patchy Reionization from Planck CMB Temperature Trispectrum

TL;DR

This study uses Planck 2015 CMB temperature data to constrain patchy reionization through trispectrum analysis, finding no evidence of patchiness but significantly improving existing limits on bubble sizes and contamination of primordial B-modes.

Contribution

First to constrain patchy reionization using CMB trispectrum data, setting new limits on bubble sizes and impact on B-mode polarization detection.

Findings

No significant evidence of patchy reionization in Planck data.

Constraints exclude bubble sizes smaller than approximately 10 Mpc at 50% ionization.

Patchy reionization is unlikely to significantly contaminate primordial B-mode signals for r ≥ 0.001.

Abstract

We present constraints on the patchy reionization by measuring the trispectrum of the Planck 2015 cosmic microwave background (CMB) temperature anisotropies. The patchy reionization leads to anisotropies in the CMB optical depth, and the statistics of the observed CMB anisotropies is altered. We estimate the trispectrum of the CMB temperature anisotropies to constrain spatial variation of the optical depth. We show that the measured trispectrum is consistent with that from the standard lensed CMB simulation at $2\sigma$. While no evidence of the patchy reionization is found in the Planck 2015 temperature trispectrum, the CMB constraint on the patchy reionization is significantly improved from previous works. Assuming the analytic bubble-halo model of Wang and Hu (2006), the constraint obtained in this work rules out the typical bubble size at the ionization fraction of $\sim0.5$ as $R\gtrsim 10$ Mpc. Further, our constraint implies that large-scale $B$-modes from the patchy reionization are not a significant contamination in detecting the primordial gravitational waves of $r\gtrsim0.001$ if the $B$ mode induced by the patchy reionization is described by Dvorkin et al. (2009). The CMB trispectrum data starts to provide meaningful constraints on the patchy reionization.