Mirages in the Sky Probe Dark Matter in the Universe

Scientists from the Sloan Digital Sky Survey project (SDSS) have announced two intriguing discoveries relating to the phenomenon of gravitational lenses.

“We have discovered that a system appearing to consist of four separate quasars—the most luminous class of objects in the universe—actually contains four mirages of just one quasar,” says Donald Schneider, professor of astronomy and astrophysics at Penn State and a member of both SDSS discovery teams. Schneider also is the chair of the SDSS Quasar Science Group and its coordinator of scientific publications.

Albert Einstein’s Theory of General Relativity predicts that the gravitational pull of a massive body can act as a lens, bending and distorting the light of a distant object. A massive structure located between a distant quasar and Earth can “lens” the light of the quasar, making the image substantially brighter and producing several mirage images from the one object. “The image of this quasar is being split into four copies and projected onto the sky at the largest image separation ever recorded,” Schneider explains. “This lensing must be caused by an unexpectedly large amount of dark matter that is invisible to us on Earth.” This discovery is detailed in a paper published in the journal Nature.

In a second paper, published in the Astronomical Journal, an SDSS team used the high resolution of the Hubble Space Telescope to examine four of the most distant known quasars—located as far back in time as astronomers have been able to see objects—for signs of gravitational lensing. “Theories predict that a large fraction of the most distant quasars should be mirages because a massive lensing object is more likely to exist between them and the Earth,” Schneider explains. These theories help to explain how such luminous objects could have formed so early in the history of the universe. High luminosity at a great distance generally requires a violent clash of large amounts of matter with a large black hole in order to create fireworks colossal enough to be seen on Earth. “If the observed brightness of distant quasars were significantly boosted by the magnification of a gravitational lens, the quasars’ black holes could be of modest size, which would remove the requirement that very massive black holes formed when the universe was so young,” Schneider explains. “But when we examined the Hubble Space Telescope images of four of the most distant known quasars, we were quite surprised to find that not even one of them shows any evidence of gravitational lensing; therefore, the supermassive-black-hole issue remains a puzzle.”

Gordon Richards, a former Penn State postdoctoral scholar, also is a member of both SDSS teams that made these discoveries.

Barbara K. Kennedy