HPCwire: Finding a Way to Test Dark Energy

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September 09, 2005

Finding a Way to Test Dark Energy

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What is the mysterious dark energy that's causing the expansion of the universe to accelerate? Is it some form of Einstein's famous cosmological constant or an exotic repulsive force, dubbed "quintessence," that could make up as much as three-quarters of the cosmos? Scientists from Lawrence Berkeley National Laboratory and Dartmouth College believe there is a way to find out.

In a paper to be published in Physical Review Letters, physicists Eric Linder of Berkeley Lab and Robert Caldwell of Dartmouth show that physics models of dark energy can be separated into distinct scenarios, which could be used to rule out Einstein's cosmological constant and explain the nature of dark energy. What's more, scientists should be able to determine which of these scenarios is correct with the experiments being planned for the Joint Dark Energy Mission (JDEM), which has been proposed by NASA and the U.S. Department of Energy.

"Scientists have been arguing the question 'how precisely do we need to measure dark energy in order to know what it is?'" said Linder. "What we have done in our paper is suggest precision limits for the measurements. Fortunately, these limits should be within the range of the JDEM experiments."

Linder and Caldwell are members of the DOE-NASA science definition team for JDEM, which has the responsibility for drawing up the mission's scientific requirements. Linder is the leader of the theory group for SNAP (SuperNova/Acceleration Probe), one of the proposed vehicles for carrying out the JDEM mission. Caldwell, a professor of physics and astronomy at Dartmouth, is one of the originators of the quintessence concept.

In their paper, Linder and Caldwell describe two scenarios, one they call "thawing" and one they call "freezing," which point toward distinctly different fates for our permanently expanding universe. Under the thawing scenario, the acceleration of the expansion will gradually decrease and eventually come to a stop, like a car when the driver eases on the gas pedal. Expansion may continue more slowly, or the universe may even recollapse. Under the freezing scenario, acceleration continues indefinitely, like a car with the gas pedal pushed to the floor. The universe would become increasingly diffuse, until eventually our galaxy would find itself alone in space.

Either of these two scenarios rules out Einstein's cosmological constant. In their paper, Linder and Caldwell show, for the first time, how to cleanly separate Einstein's idea from other possibilities. Under any scenario, however, dark energy is a force that must be reckoned with.

According to Linder, "Because dark energy makes up about 70 percent of the content of the universe, it dominates over the matter content. That means dark energy will govern expansion and, ultimately, determine the fate of the universe."

In 1998, two research groups rocked the field of cosmology with their independent announcements that the expansion of the universe is accelerating. By measuring the redshift of light from Type Ia supernovae, deep-space stars that explode with a characteristic energy, teams from the Supernova Cosmology Project, headquartered at Berkeley Lab, and the High-Z Supernova Search Team, based in Australia, determined the expansion of the universe is actually accelerating, not decelerating. The unknown force behind this accelerated expansion was given the name "dark energy."

Prior to the discovery of dark energy, conventional scientific wisdom held that the Big Bang had resulted in an expansion of the universe that would gradually be slowed by gravity. If the matter content in the universe provided enough gravity, one day the expansion would stop altogether and the universe would fall back on itself in a Big Crunch. If the gravity from matter was insufficient to completely stop the expansion, the universe would continue floating apart forever.

"From the announcements in 1998 and subsequent measurements, we now know that the accelerated expansion of the universe did not start until sometime in the last 10 billion years," Caldwell said.

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