Constraining the CMB Optical Depth Through the Dispersion Measure of Cosmological Radio Transients

TL;DR

This paper proposes using dispersion measures of extragalactic radio transients like FRBs during the Epoch of Reionization to precisely constrain the CMB optical depth, offering a new method for cosmological parameter measurement.

Contribution

It introduces a novel approach linking radio transient dispersion measures to the CMB optical depth, enabling high-precision reionization history constraints.

Findings

Radio transients can probe $ au_{CMB}$ during EoR.

FRBs can be detected with high significance by SKA.

Method achieves sub-percent accuracy in measuring $ au_{CMB}$.

Abstract

The dispersion measure of extragalactic radio transients, such as of recently discovered Fast Radio Burst FRB150418, can be used to measure the column density of free electrons in the intergalactic medium. The same electrons also scatter the Cosmic Microwave Background (CMB) photons, affecting precision measurements of cosmological parameters. We explore the connection between the dispersion measure of radio transients existing during the Epoch of Reionization (EoR) and the total optical depth for the CMB, $\tau_{CMB}$, showing that the existence of such transients would provide a new sensitive probe of $\tau_{CMB}$. As an example, we consider the population of FRBs. Assuming they exist during the EoR, we show that: (i) such sources can probe the reionization history by measuring $\tau_{CMB}$ to sub-percent accuracy, and (ii) they can be detected with high significance by an instrument such as the Square Kilometer Array.