# Effect of the Early Reionization on the Cosmic Microwave Background and Cosmological Parameter Estimates

## TL;DR

This paper examines how early reionization impacts CMB observations and cosmological parameters, using PCA for model-independent analysis, and finds minimal effects on key parameters with current data.

## Contribution

It introduces a PCA-based method to reconstruct the ionization history during early reionization and assesses its influence on cosmological parameter estimation.

## Key findings

- ERE has negligible impact on tensor-to-scalar ratio r.
- ERE does not significantly affect neutrino mass estimates.
- Improved polarization data can better constrain ERE and cosmological parameters.

## Abstract

The early reionization (ERE) is supposed to be a physical process which happens after recombination, but before the instantaneous reionization caused by the first generation of stars. We investigate the effect of the ERE on the temperature and polarization power spectra of cosmic microwave background (CMB), and adopt principal components analysis (PCA) to model-independently reconstruct the ionization history during the ERE. In addition, we also discuss how the ERE affects the cosmological parameter estimates, and find that the ERE does not impose any significant influences on the tensor-to-scalar ratio $r$ and the neutrino mass at the sensitivities of current experiments. The better CMB polarization data can be used to give a tighter constraint on the ERE and might be important for more precisely constraining cosmological parameters in the future.