This is the second installement of 'a talk that I just attended' (the first was the new theory about black holes thread). ((it helps keep me awake at talks if I have to summarize them afterwards))

This one was by Robert Becker (UC Davis/IGPP) in which he talked of an observable evidence for the identification of the time of the reionization of the Universe. At first it was ionized then became neutral some 300 000 years after the BB and is now mostly ionized again (I'm talking of intergalactic matter here).

The idea is:
- look at quasars, the more distant the better
- in spectra of quasars one can see the so-called Lyman alpha forest which is absorption of the quasars light by intervening clouds of hydrogen at redshifts smaller than the quasar (naturally). The higher the redshift the denser the clouds become.
- If one sees a quasar past the reionization limit than there would be no Lyman alpha forest because all the light from the quasar would be blocked and there would be simply no flux at redshifts smaller than the quasars position but larger than the limit of reionization.

So they observed a quasar at 6.3 and they claim to see a large range of redshift (approximately between 6 and 6.3 where there is no light flux from the quasar. (They use data from Sloan Digital Sky Survey and Keck)

The implications being that reionization occured around redshift 6 which puts it around 900 million years after the big bang.

According to surveys there are not enough quasars observed (the density of quasar is too small) for them to be the cause of the reionization. Therefore it would be caused by starlight.

caveats: based on only one quasar and uncertainties are still large.

<insert standard disclaimers>

Yep...that is called the Gunn-Peterson trough first predicted back in 1965 and seen at a red shift of z = 6.28.

You can find the paper on the topic at http://xxx.lanl.gov/abs/astro-ph/0108097

This paper has pictures of the spectrum so you can see the GP Trough. You can also find a picture of it on SkyServer, the SDSS EPO web site at http://skyserver.pha.jhu.edu/en/sdss...iscoveries.asp

Almost as interesting is the process that led to the discovery of these high red shift quasars. You can read about that at http://xxx.lanl.gov/abs/astro-ph/0108063

Rob