The first large sample of absorption lines in the spectra of pairs
of QSOs that are close in the sky contains some surprises. The
absorbers in the paired QSOs have very similar redshifts, as if the
absorption is from blue and not red galaxies. The absorption is
not from winds flowing out of galaxies at high velocities,
suggesting that the metals entered the intergalactic medium at
earlier times.

We see no change in the absorption from neutral hydrogen in the
background QSO as a sight line passes by a foreground QSO.
Either the foreground QSO UV is emitted into a narrow beam
toward us, or QSOs emit for < 1 Myr at the luminosity we seen.

We use large hydrodynamic numerical simulations to interpret these
data, but we can not find a simulation that matches even the
intergalactic hydrogen absorption in single QSOs. We may have a
problem with the simulations (we need self-shielding and
radiative transfer), with the astrophysics (we need a less energetic
photoionizing spectrum) or with cosmology (we need a higher amplitude
for the clustering of matter on small scales, sigma_8 > 0.9).