21cm Emission and Absorption During Reionization

After the universe recombined but before the first stars and quasars formed and reionized the universe, an epoch known as the cosmic "Dark Ages", neutral hydrogen in the intergalactic medium should have emitted and absorbed 21cm-wavelength radiation at an intensity determined by its spin temperature. If its spin temperature differed from that of the local cosmic microwave background (CMB), the redshifted 21cm radiation from such gas should be observable today either in absorption or emission relative to the CMB. We will review the two processes that can decouple the spin temperature of hydrogen from the CMB, Lyman-alpha pumping and collisional excitation. We will then present recent work that uses N-body/hydrodynamical numerical supercomputer simulations of cosmological structure formation to predict the signal as seen from Earth. Such predictions are relevant to observations to be carried out with a planned new generation of radio telescopes, such as the Low Frequency Array (LOFAR) and the Square Kilometer Array (SKA), for which a major goal is the direct observation of these unseen Dark Ages and the formation of the first stars, quasars, and galaxies.

Abstract Author(s): Marcelo Alvarez<br />Paul Shapiro<br />Kyungjin Ahn