# Reconstructing CMB fluctuations and the mean reionization optical depth

## TL;DR

This paper proposes an indirect method to measure the mean reionization optical depth from small-scale CMB polarization data, potentially overcoming current observational challenges and improving constraints on early universe physics.

## Contribution

It introduces a novel approach to reconstruct large-scale polarization and optical depth using small-scale CMB measurements and field maps, bypassing foreground and systematic issues.

## Key findings

- Futuristic CMB surveys can reconstruct large-scale polarization.
- The method enables indirect measurement of optical depth.
- Potential to improve constraints on primordial fluctuations.

## Abstract

The Thomson optical depth from reionization is a limiting factor in measuring the amplitude of primordial fluctuations, and hence in measuring physics that affects the low-redshift amplitude, such as the neutrino masses. Current constraints on the optical depth, based on directly measuring large-scale cosmic microwave background (CMB) polarization, are challenging due to foregrounds and systematic effects. Here, we consider an indirect measurement of large-scale polarization, using observed maps of small-scale polarization together with maps of fields that distort the CMB, such as CMB lensing and patchy reionization. We find that very futuristic CMB surveys will be able to reconstruct large-scale polarization, and thus the mean optical depth, using only measurements on small scales.