The 21cm probe of Reionization

I discuss our current understanding of the reionization process, including the ionized bubble and neutral hydrogen island distribution based on the excursion set models, the stages of reionization, and the corresponding 21cm signal. We show that the 21cm signal is well correlated with the matter density on large scales, but such correlation breaks down on small scales. The transition from neutral to ionized proceeds in  five stages according to the complicated geometry of the reionization.