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Why do distant quasars show astronomers the early universe?

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six quasar host galaxies — 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).

Distant quasars serve as windows into the early universe primarily due to the finite speed of light. Because light takes a significant amount of time to travel across the vast distances of space, observing objects that are billions of light-years away allows astronomers to see them as they existed in the distant past—a concept known as "look-back time".12

Quasars are among the most luminous objects in the universe, which makes them visible even at these extreme cosmological distances. By studying these distant, ancient objects, astronomers can observe the universe as it was when it was much younger, including eras less than a billion years after the big bang. This provides critical insight into the evolution of galaxies and the growth of supermassive black holes during the early stages of cosmic history.341