A measure of the average depression between Lyman absorption lines in the spectrum of the faint quasar BR1202-0725 ($z_{em}=4.695$) is presented. The relatively high resolution of the spectrum ($sim 40$ km s$^{-1}$) allows the selection of regions free of strong absorption lines in the Lyman alpha forest. A reliable evaluation of the continuum shape is based on the careful flux calibration and on the large wavelength interval covered (5000--9300 ~AA). A best estimate of $tau _{_{GP}}leq 0.02pm 0.03$ has been found for the Gunn-Peterson optical depth at the highest absorption redshift observed at this resolution, $z simeq 4.3$. The derived baryon density of the intergalactic medium is $Omega _{IGM}mincir 0.01$ if the observed quasars are the major contributor to the ionizing UV background flux. This limit, when compared with the total baryon density deduced from the nucleosynthesis, could imply that most of the baryons are already in bound systems at $zsim 5$.

Quasar spectrums show a Gunn-Peterson effect - a trough which corresponds to the absorbtion of light by a neutral hydrogen atom. It's a trough rather than the usual line because the quasar is so far away its spectrum is continually shifted by the expansion of the universe as the light is absorbed by intergalactic hydrogen. Nearer objects do not show such a trough. Newer/nearer hydrogen has since been ionized (into a proton and an electron). This ionization appears to be somewhat of a mystery. Curently the best bet seems to be hypothetical very large short-lived pure stars in the early universe which shone very brightly and whose radiation ionized the hydrogen of the universe.

As soon as astronomers found distant quasars, they began to think of ways they could use them to understand the early universe. In 1965, Jim Gunn (who went on to work for SDSS) and Bruce Peterson of Caltech predicted that distant quasars should show evidence of the end of the cosmic dark ages. But until recently, no one had ever seen an object distant enough to check their prediction. About a million years after the big bang, the universe was full of a thick gas of hydrogen atoms. Hydrogen atoms absorb ultraviolet light well, so any light traveling through early the universe was quickly absorbed by a hydrogen atom. The universe was dark. Over time, the gas clumped together to form the first stars, which began to emit light - but this light too was quickly absorbed. Eventually, the stars became bright enough that their light had enough energy to break the hydrogen atoms into protons and electrons. After this happened, light could pass freely through the universe. The cosmic dark age was over. Gunn and Peterson realized that even a small amount of remaining hydrogen atoms - as little as 1 remaining atom for every 100,000 broken - should have enough of an effect to be noticed in the spectrum of a distant object. Gunn and Peterson predicted that astronomers should see a "trough" in the ultraviolet part of an object's spectrum - less light than expected - because of the remaining hydrogen atoms. This effect was called the "Gunn-Peterson trough," and astronomers began to look for it. In the summer of 2001, Robert Becker from the Lawrence Livermore National Laboratory in California led a team of astronomers that examined the spectrum of the distant quasar shown above. Becker's team found an unmistakable Gunn-Peterson trough in the quasar's spectrum. Because the quasar was so far away, its trough was shifted from the ultraviolet into the infrared. The team's discovery ended a nearly 40-year search. SDSS astronomers will now look for Gunn-Peterson troughs in other distant quasars to try to gain a better understanding of the effect.