Quasar

"We seem to live in a remarkably economical X-ray universe, in that the observed cosmic X-ray background (CXRB) is produced with almost the least cosmic effort possible. It is not dominated by luminous obscured quasars thundering out huge amounts of power at z ≈ 2–4 but rather by moderate-luminosity, obscured AGNs at z ≈ 0.5–2."

"LOFAR is a new European radio interferometer operating at frequencies 15–240 MHz (van Haarlem et al., 2013) and represents a milestone in terms of radio survey speed compared to existing telescopes. The LOFAR Surveys Key Science Project aims to carry out a tiered survey. ... These surveys will open the low-frequency electromagnetic spectrum for exploration, allowing unprecedented studies of the radio population across cosmic time and opening up new parameter space for searches for rare, unusual objects such as high-z radio quasars in a systematic way. Perhaps, one of the most tantalizing prospects are the 21 cm absorption line measurements using LOFAR along sight lines toward z > 6 radio quasars."

"The continuum spectrum of a quasar can often be described, over a broad frequency range, by a power law of the form S\nu \propto \nu–\alpha ... where \alpha is the spectral index. \alpha = 0 corresponds to a flat spectrum, whereas \alpha = 1 describes a spectrum in which the same energy is emitted in every logarithmic frequency interval."

"Quasars were several hundred times more numerous when the universe was much younger. They were most numerous when the universe was about twenty percent of its current age, a time in the history of the universe sometimes called "cosmic noon"."

"General relativity"