Much information about the IGM has been obtained through its absorption of light from distant quasars. Quasars are not only bright, so that they can be observed out to large distances, but also have well-behaved continua, against which absorption can be analyzed relatively easily. One of the most important tests of the presence of intergalactic neutral hydrogen was proposed by Gunn & Peterson (1965). The Gunn-Peterson test makes use of the fact that the Lyα absorption of neutral hydrogen at 
has a very large cross-section. When the ultraviolet continuum of a distant quasar (assumed to have redshift 
) is shifted to 
at some redshift 
, the radiation would be absorbed at this redshift if there were even a small amount of neutral hydrogen. Thus, if the Universe were filled with a diffuse distribution of neutral hydrogen, photons bluer than Lyα would be significantly absorbed, causing a significant decrement of flux in the observed quasar spectrum at wavelengths shorter than 
. Using the hydrogen Lyα cross-section and the definition of optical depth (see Chapter 16 for details), one obtains that the proper number density of 
atoms obeys
where 
is Hubble's constant at redshift 
, and 
is the absorption optical depth out to that can be determined from the flux decrements in quasar spectra. Observations show that the 
absorption optical depth is much smaller than unity out to 
. The implied density of neutral hydrogen in the diffuse IGM is thus much lower than the mean gas density in the Universe (which is about 
). This suggests that the IGM must be highly ionized at redshifts .
As we will show in Chapter 3, the IGM is expected to be highly neutral after recombination, which occurs at a redshift 
. Therefore, the fact that the IGM is highly ionized at 
indicates that the Universe must have undergone some phase transition, from being largely neutral to being highly ionized, a process called re-ionization. It is generally believed that photoionization due to energetic photons (with energies above the Lyman limit) are responsible for the re-ionization. This requires the presence of effective emitters of UV photons at high redshifts. Possible candidates include quasars, star-forming galaxies and the first generation of stars. But to this date the actual ionizing sources have not yet been identified, nor is it clear at what redshift re-ionization occurred. The highest redshift quasars discovered to date, which are close to z = 6.5, show almost no detectable flux at wavelengths shorter than 
(Fan et al., 2006).
Although this seems to suggest that the mass density of neutral hydrogen increases rapidly at around this redshift, it is not straightforward to convert such flux decrements into an absorption optical depth or a neutral hydrogen fraction, mainly because any 
can result in an almost complete absorption of the flux. Therefore it is currently still unclear whether the Universe became (re-)ionized at a redshift just above 6 or at a significantly higher redshift. At the time of writing, several facilities are being constructed that will attempt to detect 21cm line emission from neutral hydrogen at high redshifts. It is anticipated that these experiments will shed important light on the detailed re-ionization history of the Universe.
