View Full Version : When Did the Universe Reionize Itself?

2005-Jun-21, 04:21 PM
The answer may lie surprisingly close to home: in the stellar halo of our own Galaxy. (http://KenCroswell.com/ChargingUpTheUniverse.html)

The timing of the universe's reionization left its mark on the Milky Way's stellar halo, say astronomers in Australia and Japan. The sooner this reionization occurred, the more concentrated our Galaxy’s stellar halo should be.

For most of its early life, the universe was not ionized: it had cooled off from the big bang, and its hydrogen and helium had their full complement of electrons. Then the first stars and quasars began to shine, ripping electrons from the hydrogen and helium. However, the exact timing of this reionization is unknown. It may have occurred as early as redshift 30, corresponding to a time just 100 million years after the big bang. Or it may have occurred as late as redshift 6, when the universe was 900 million years old.

Now Kenji Bekki of the University of New South Wales in Sydney, Australia, and Masashi Chiba of Tohoku University in Sendai, Japan, have proposed a way to determine when reionization occurred: observe how concentrated or spread out a galaxy's stellar halo is.