A major area of research in X-ray astronomy is the accretion of
matter on to black holes and neutron stars in the context of AGN
and galactic X-ray binaries. Accretion is an energetic process
and can give high luminosities. During the epoch of reionization
the first stars and black holes in the Universe (re-) ionized
the Intergalactic Medium. Studying the epoch of reionization
will tell us about the first galaxies. Observations of nearby
galaxies and high-redshift quasars suggest that black holes are
present in the majority of galaxies. The first quasars harbor
already black holes as massive as several billion solar masses.
The intergalactic medium (IGM) and not galaxies contain most of
the baryons (e.g. H, He, C, N, O) in the Universe. We use
spectroscopic observations of high redshift backgound quasars to
study this material.