Reionization

Reionization

Reionization is a process that occurs after the epoch of galaxies and is the second of the two major phase changes in the universe. The first is recombination in which protons and electrons combined to form neutral atoms. However, it is known from the lack of absorption lines out to quasars of redshift z=4 that most of the current intergalactic medium consists not of neutral atoms (which would form absorption lines) but rather of ionized plasma with only small pockets of neutral atoms forming the Lyman-alpha forest.

As a result, there must have been a period in which the hydrogen between the galaxies reionized. This period is a topic of major astrophysical interest as of the 2004. It is believed that reionization has been completed by z=4 because more distant quasars have absorption lines corresponding to neutral hydrogen between us and the quasar while closer quasars do not.

The standard explanation for reionization is that star light from hot blue population III stars ionizes the intergalactic medium. However, this explanation has seriously been called into question by observations from WMAP in 2003 which show that reionization has begun at z=15. Right now, astronomers are trying to figure out what is going on.