Quasars are the most powerful sources of energy known in the universe, and so
bright that they can be seen at greater distances than any other kind of
astronomical object. The most distant quasars known have redshifts of about
4.9. It is now generally accepted that quasars are the most extreme versions of
active galactic nuclei, where energy is generated by matter cascading into a
supermassive black hole. This is the only known physical mechanism that could
account for the phenomenal energy output.
Large redshifts correspond to great distances in the universe (the Hubble Law
relationship). Due to light travel time, the most remote quasars are being seen
when the universe was only a few percent its present age. Since quasars are not
seen nearby, they must have been much more common in the early universe.
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