Later, about 300,000 years after the Big Bang, things were cool enough for electrons and protons to form neutral hydrogen, making the universe transparent to visible light but opaque to higher-energy light, which (if there were much of it about, and there wasn't) would be absorbed by these same H atoms.

Later still, the first stars and quasars started to pump out UV light. This radiation was avidly absorbed by surrounding reservoirs of neutral H, sometimes ionizing the atoms in the process.

As time passed, more stars/galaxies/quasars formed, more UV was produced, and more of the neutral H was being turned back into ions. At a certain point, the great majority of H would be re-ionized.

Since bare electrons and protons cannot absorb light, UV photons could thereafter proceed largely unhindered through the cosmos.

Now, evidence of a reionization era in the early universe has been glimpsed in the form of a quasar spectrum exhibiting a paucity of radiation at UV and shorter wavelengths.

A new study of quasars (Becker et al., Los Alamos preprint at this URL) made with the Sloan Digital Survey telescope looks at this process happening.

The study shows that quasar spectra out to a redshift of about 6 feature UV emission, but that the furthest-out quasar yet glimpsed, at a redshift of 6.28, does not.

This suggests that this earliest-after-the-Big-Bang quasar was active in an epoch when neutral H was still plentiful enough to choke off high energy radiation.

Thus a re-ionization era would seem to have occurred around Z=6, at a time corresponding roughly to 800 million years after the Big Bang.

(Editor's Note: This story is based on PHYSICS NEWS UPDATE, the American Institute of Physics Bulletin of Physics News Number 555, September 6, 2001, by Phillip F. Schewe, Ben Stein, and James Riordon.)

18-Sep-2001