Astronomers who are using the Hubble Space Telescope (HST) to observe the gravitational lensing of light from distant quasars have discovered new evidence about the rate at which the universe is expanding.
The lensing study shows that the universe is expanding at rates slightly slower than, but similar to, rates calculated from the HST Key Project to measure the size and age of the universe. The distance scale was one of the primary science problems the HST was built to address. That findings from the gravitational lensing study and the HST Key Project agree is good news.
Also good news: The new research shows a slow enough expansion to explain the ages of the oldest globular star clusters, as long as the universe has a low density. Low density is indicated by a variety of pieces of evidence. Until recently, astronomers have been confounded by discoveries of some globular star clusters older than the calculated age of the universe.
The bad news is that a slowly expanding, low density universe spells trouble for the favorite version of the big bang theory. The big bang ultimately traces the birth of the universe back to an explosion of unimaginable violence and power.
The big bang theory can only be patched up in three ways, each of which is unpalatable, say astronomers who report the new results. Theorists can explain the old, low density universe by appealing to special conditions in the first fractions of a second of the big bang. They can postulate new and unseen forms of matter. Or they can appeal to the cosmological constant, which was originally proposed by Einstein to explain why the universe does not collapse under the force of its own gravity. Unfortunately, there is no physical basis for the cosmological constant, so adding it is just an arbitrary way to "fix up" the big bang model, they add.
Astronomers from the Harvard-Smithsonian Center for Astrophysics and The University of Arizona in Tucson have been using the infrared and optical cameras on the Hubble Space Telescope to image two dozen gravitational lens systems. Their project, the CfA-Arizona Space Telescope Lens Survey (CASTLES), aims to carry out a variety of cosmological measurements. The first results - a direct measurement of the distance scale of the universe - will appear in the December 1998 Astrophysical Journal. The results say that the universe is expanding no faster than 48,000 mph for every million light years increase in distance from Earth.
The team studied the quasar PG 1115+080, one of the rare cases where a quasar is almost perfectly lined up with an intervening galaxy.
"Only one in every 500 quasars has light that is gravitationally lensed," said Christopher D. Impey of the UA Steward Observatory. "The gravity of a closer galaxy bends light from a single distant quasar so that astronomers on Earth see two, three, four or even five identical images of that quasar. Because light from the source quasar splits into slightly different paths around the lensing galaxy, light from the multiple images arrive at slightly different times at the telescope. Knowing the path difference and the speed of light, we can calculate the distance scale directly," Impey said.
The time delays for the PG 1115+080 system had previously been measured using ground-based data, but the CASTLES team used their sharp infrared imaging with the Hubble Space Telescope to make an accurate model of the mass distribution of the lens. The project team members are Joseph Lehar, Emilio Falco, Chris Kochanek, Brian McLeod, and Jose Munoz at the Harvard-Smithsonian Center for Astrophysics, and Impey, Hans-Walter Rix, Chuck Keeton, and Chien Peng at the UA Steward Observatory.
The team is releasing an infrared image that shows the four quasar images from the single quasar PG 1115+080, a ring of lensed light that connects them, and the elliptical lensing galaxy at the center of the ring. The ring is the stretched and amplified starlight of the galaxy that contains the quasar, some ten billion light years away. In the case of perfect alignment, the starlight would be distorted into a perfect ring, as Einstein predicted.
"The combination of the NICMOS instrument and the Hubble Space Telescope has excellent resolution," Impey said. "We're able to see details 20 times smaller than would be visible with a typical telescope on the ground. This resolution would allow you to read the date on a quarter a mile away."
Christopher D. Impey
University of Arizona, Steward Observatory
University of Arizona