Joint Astrophysics Colloquium

Joint Astrophysics Seminar

Mapping Cosmic Hydrogen In the Infant Universe

Avi Loeb

Harvard/CFA

The re-ionization history of cosmic hydrogen, left over from the big bang, provides crucial fossil evidence for when the first stars and black holes formed in the infant universe. Current observations provide a mixed message. The large-scale polarization anisotropies of the cosmic microwave background measured by WMAP imply a reionization redshift of 10-20. However, the extent of the ionized regions around the highest redshift quasars indicate a significantly neutral universe at a redshift of 6.4. I will summarize the status of current observational and theoretical studies, and address the possibility that the time evolution of the mean ionization fraction might have been non-monotonic. The truth will likely be revealed over the next decade through observations of the Lyman-alpha spectra of galaxies, quasars and gamma-ray bursts, and most importantly the detection of the 21 cm transition from diffuse hydrogen at redshifts above 6.