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When were quasars most common in the history of the universe?
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Quasar
Six quasar host galaxies, as observed by the Hubble Space Telescope.
Shown are apparently normal, solitary galaxies (left), colliding galaxies (centre), and merging galaxies (right).
Quasars were most common in the early history of the universe, specifically around three billion years after the big bang.1
Because of the finite speed of light, observing objects at great distances allows astronomers to see them as they existed in the distant past. Observations show that the number density of quasars increases with distance (or "look-back time") up to that point, approximately 10.5 billion years ago. At ages earlier than three billion years post-big bang, the number density of quasars decreases sharply, reflecting an era when the population was still in its initial stages of development.1