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