A Short Introduction to Reionization Physics

The epoch of reionization probes the state of our universe when the very first stars formed and ionized the hydrogen atoms in the surrounding medium. Since the epoch has not yet been probed observationally, it is often called the "final frontier" of observational cosmology. This final frontier is attracting a lot of attention because of the availability of a large number of telescopes in a wide variety of wavebands. This review article summarizes some of the concepts required to understand the interesting physics of reionization and how to analyze the high-redshift universe using related observations.Comment: Review article published in a special issue of General Relativity and Gravitation in memory of Professor T. Padmanabha

Warm dark Matter constraints from the joint analysis of CMB, Lyman-\alpha, and global 21 cm data

With the help of our previously built MCMC-based parameter estimation package \texttt{CosmoReionMC}, we investigate in detail the potential of 21~cm global signal, when combined with CMB and observations related to the QSO absorption spectra, to constraint the mass of Warm Dark Matter (WDM) particle. For the first time, we simultaneously vary all the free parameters (mass of WDM particle, cosmological parameters, and astrophysical parameters) to address the long-overlooked issue of the possible degeneracies between the Dark Matter particle mass $m_X$ and cosmological/astrophysical parameters. From the existing CMB and QSO absorption spectra data, we can rule out $m_X < 2.8$~keV at 95\% confidence level. Including the mock 21~cm global signal data expected in the future, the forecasted constraint is found to be much tighter $m_X > 7.7$~keV, assuming that the true dark matter model is the usual cold dark matter. In case the mock 21~cm signal is constructed for dark matter particles having $m_X = 7$~keV, our forecasts indicate that $\left(m_X / \text{keV}\right)^{-1}$ is in the range $[0.1, 0.2]$ ($95\%$ confidence level). This implies that the future 21~cm data should allow detection of the WDM particle mass if $m_X \sim 7$~keVComment: Submitted to MNRA