F. Distribution of quasars and Ly- clouds
The spectra of quasars are populated by narrow absorption lines from intervening gas clouds along the line of sight (the Ly- forest). Owing to the great redshift of most quasars these absorption clouds provide an important probe of clustering at large distances and at times long in our past.
Wu et al. (1999) used the large-scale uniformity of the Ly- forest to argue against fractal distribution of matter. Recently, Croft et al. (2002) showed that it is possible to estimate the full 3-D power spectrum of density fluctuations P(k) from the (one-dimensional) Ly- flux power spectrum. This is extremely important, as it allows us to check for theoretical predictions at large redshifts (z 2-4). It also allows us to recover the linear (post-recombination) power spectrum for small scales, which have turned nonlinear by now.
Lines of sight to quasar pairs, be they optical pairs or pairs that are a consequence of gravitational lensing, provide additional clues to the clustering transverse to the line of sight (Wu et al., 1999).
The statistical analysis of the distribution of quasars and Ly- clouds has provided additional evidence for the large scale homogeneity in the universe (Andreani et al., 1991, Carbone and Savaglio, 1996).