Wrenching findings: homing in on dark energy

Science News, Feb 28, 2004 by R. Cowen

Something is pulling the universe apart, causing galaxies to flee from each other at an ever-faster rate. Since 1998, when astronomers discovered this bewildering state of affairs, theorists have been struggling to comprehend the mysterious source driving the runaway expansion. Now, researchers have taken one of the first steps toward identifying this bizarre influence, often known as dark energy.

In an analysis of a group of bright but distant exploding stars called type 1a supernovas, researchers have found hints that dark energy is distributed uniformly throughout space and that its strength will remain constant throughout time. That would make dark energy resemble the cosmological constant, a term that Albert Einstein introduced into his general relativity theory in 1917 and quickly abandoned, but which physicists have resurrected several times since. The cosmological constant refers to an unspecified property of space that could add to or oppose gravitational attraction.

Adam G. Riess of the Space Telescope Science Institute in Baltimore announced the new findings during a teleconference last week. He and his colleagaes will also describe their analysis in the June Astrophysical Journal.

In the study, Riess and his collaborators analyzed the brightness and colors of 16 type 1a supernovas, all of which the Hubble Space Telescope had discovered. The group includes six of the seven most distant supernovas known.

"These results are going to be an important foil for [testing] ideas about dark energy," says Robert R. Caldwell of Dartmouth College in Hanover, N.H.

Because all type 1a supernovas have about the same intrinsic brightness, they serve as cosmic markers, enabling researchers to measure the size and expansion rate of the universe at different times in the past. From the expansion rate calculated from the new data, Riess' team suggests that the universe experiences a constant push. This finding is consistent with dark energy being the cosmological constant.

Because the cosmological constant would exist even in the absence of matter or radiation, dark energy might be an intrinsic property of space itself. Space on the sub-atomic scale isn't empty but seething with elementary particles that pop in and out of existence on extremely short timescales. Dark energy might result from the activity of some of these particles.

The fate of the universe hinges on whether the strength of dark energy varies over time, notes Paul J. Steinhardt of Princeton University. In the cosmological-constant scenario, the steady push provided by dark energy causes space-time to expand and the galaxies that lie within it to become ever more distant from one another, but they don't fall apart. In such a rarefied universe, a resident of the Milky Way billions of years from now would not see a single other galaxy in the sky.

In a competing theory known as quintessence, which Steinhardt and other theorists have proposed, dark energy is not a fundamental property of space. Instead, it's associated with some unidentified energy field that has variable strength. If this field grows stronger, it will not only expand space-time but also shred every galaxy, star, and atom, ending the universe in what's called the Big Rip (SN: 3/8/03, p. 48). If the energy field weakens sufficiently, the gravitational tug of matter will eventually overwhelm it, and the universe will ultimately collapse, ending in a Big Crunch.

The new study of exploding stars doubles the precision of previous supernova-derived data on the character of dark energy. At the same time, both Steinhardt and Riess agree, the new data don't rule out most versions of quintessence.