Everything about Gunn-peterson Trough totally explained
In
astronomical spectroscopy, the Gunn-Peterson trough is a feature of the
spectra of
quasars due to the presence of neutral hydrogen in the
Intergalactic Medium (IGM). The trough is characterized by suppression of electromagnetic emission from the quasar at wavelengths less than that of the
Lyman Alpha line at the
redshift of the emitted light. This effect was originally predicted in
1965 by
James E. Gunn and Bruce Peterson.
First Detection
For over three decades after the prediction, no objects had been found distant enough to show the Gunn-Peterson trough. It wasn't until
2001, with the discovery of a quasar with a redshift
by Robert Becker using data from the
Sloan Digital Sky Survey, that a Gunn-Peterson trough was finally observed. The paper also included quasars at redshifts of
and
, and, while each of these exhibited absorption at wavelengths on the blue side of the Lyman Alpha transition, there were numerous spikes in flux as well. The flux of the quasar at
, however, was effectively zero beyond the Lyman Alpha limit, meaning the IGM must have still been neutral when the light from the quasar was emitted.
Evidence for Reionization
The discovery of the trough in a
quasar, and the absence of the trough in quasars detected at redshifts just below
presented strong evidence for the hydrogen in the universe having undergone a transition from neutral to ionized around
. After
recombination, the universe was expected to be neutral, until the first objects in the universe started emitting light and energy which would
reionize the surrounding IGM. However, as the scattering cross section of photons with energies near that of the Lyman Alpha limit with neutral hydrogen is very high, even a small fraction of neutral hydrogen will make the
optical depth of the IGM high enough to cause the suppression of emission observed. Despite the fact that the ratio of neutral hydrogen to ionized hydrogen may not have been particularly high, the low flux observed past the Lyman Alpha limit indicates that the universe was in the final stages of reionization.
Following the first release of data from the
WMAP satellite in
2003, the determination by Becker that the end of reionization occurred at
appeared to conflict with estimates made from the WMAP measurement of the electron column density. However, the WMAP III data released in
2006 now seems to be in much better agreement with the limits on reionization placed by observation of the Gunn-Peterson trough.
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