The era of cosmological reionization was the last great phase transition of the universe. Early stars, quasars and other high-energy sources could create huge ionised regions, or Strömgren spheres, around themselves, which in time grew, overlapped and completely filled the intergalactic space.

The NANOGrav Collaboration's Pulsar Timing Array experiment has confirmed the presence of a stochastic gravitational wave background, one of the possible sources of which is the merging of supermassive black hole binaries. Assuming that supermassive black hole binaries are formed in galaxy mergers, which also induce luminous quasar activity, the observed quasar luminosity function can be used to estimate the gravitational wave background.