Quasars are energetic beacons of light churned out by matter spiraling into black holes at the cores of active galaxies. The farthest ones have popped up during the Sloan Digital Sky Survey (SDSS), a multiyear effort to map most of the sky. To date, SDSS astronomers have found four quasars that shone brilliantly when the universe was less than a billion years old. Cosmologists presume that these early blazes were powered by titanic black holes that devoured gas from the first big galaxies. However, theorists struggle to explain how such massive galaxies arose so soon after the big bang. Gravitational lensing might be the reason: If some quasars are actually dimmer than they appear, then their host galaxies must be correspondingly smaller.

To determine how lensing affects our view of that early epoch, Harvard University astronomer Abraham Loeb and postdoctoral researcher J. Stuart B. Wyithe considered two key factors. First, it's more likely that any given galaxy will magnify a distant quasar than a closer object, because the quasar's light travels a much longer path. Second, gravitational lenses might let telescopes find far more faint quasars than they otherwise would, by amplifying their light. As the scientists report in the 27 June issue of Nature, their calculations suggest that the phenomenon boosts the apparent light output of 10% to 30% of the most distant quasars by a factor of 10 or more. "That's a surprisingly big fraction, and observers need to correct for it," Loeb says.

"If Wyithe and Loeb are right, the early universe will be all that much harder for us to understand because of the distorted view," comments Princeton University astrophysicist Edwin Turner. A test will come soon: Starting this fall, SDSS researchers will use the Hubble Space Telescope to examine dozens of quasars, including their four most distant ones, for multiple images--the signpost of a lensed quasar.

--ROBERT IRION