Reionization of the universe
After recombination (redshift z~1200) the gas which fills the universe
is made of neutral atoms (mainly hydrogen and helium). However, observations
show that the gas seen along the lines of sight from the Earth up to distant
quasars (z~6) is ionized. Therefore, the universe must have been reionized
at some intermediate redshift. In usual scenarios this is due to the radiation
emitted by the first stars. One of the tests which models of structure
formation must satisfy is thus to be able to reionize the universe while
matching obervations at low redshifts (galaxy luminosity function,..).
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