Quasar absorption lines at high redshift: through a glass darkly

Thanks to the sloan digital sky survey, we now know that the universe becomes opaque to Lyα photons at z ∼ 6. This presents an obvious challenge for attempts to study the universe at high redshifts with quasar absorption lines. I discuss two ways to extract more information about reionisation than the simple presence of complete gunn-peterson absorption. First, we still expect to see metal line absorption red-ward of Lyα. This will allow us to probe the early phases of metal enrichment in the intergalactic medium; the lines can be quite strong because the enriched regions are relatively compact. This is particularly important because metals are a crucial feedback mechanism in the early universe. Second, we expect to see isolated Lyα transmission features even before the final stages of reionisation. These features allow us to learn about the topology of reionisation and the condensation of the cosmic web at z ∼ 6.