Constraining Planck scale physics with CMB and Reionization Optical Depth

We present proof of principle for a two way interplay between physics at very early Universe and late time observations. We find a relation between primordial mechanisms responsible for large scale power suppression in the primordial power spectrum and the value of reionization optical depth $\tau$. Such mechanisms can affect the estimation of $\tau$. We show that using future measurements of $\tau$, one can obtain constraints on the pre-inflationary dynamics, providing a new window on the physics of the very early Universe. Furthermore, the new, re-estimated $\tau$ can potentially resolve moderate discrepancy between high and low-$\ell$ measurements of $\tau$, hence providing empirical support for the power suppression anomaly and its primordial origin.