New Light on Dark Energy

Steady flames. Two type Ia supernovae captured by the Hubble Space Telescope (top panels) and closeups (bottom panels).

PHILADELPHIA--Beacons from halfway across the universe are revealing the mysterious "dark energy" that suffuses the cosmos. At the American Physical Society meeting here this week, astrophysicists presented tantalizing glimpses of new data that are beginning to help physicists understand the antigravity force that is causing the universe to expand faster and faster.

In 1998 and 1999, astrophysicists such as Saul Perlmutter of the Lawrence Berkeley National Laboratory and Adam Riess of the Space Telescope Science Institute in Baltimore, used a certain type of supernova, known as a type-Ia, to gauge how fast the universe was expanding at different times in the past. Because all the type-Ias explode in roughly the same way and with the same brightness, physicists can use their apparent luminosity in the sky to figure out their distance, age, and speed. This, in turn, allowed the teams to figure out how fast the cosmos was expanding at various times in the universe's history. The answer they came up with was baffling: Rather than slowing down, as everybody expected, the universe was growing at an ever-quickening pace, presumably thanks to an antigravity effect known as dark energy.

Since the first supernova data came in, other cosmological observations supported the same weird picture of the universe: only 30% of the "stuff" in the universe is matter, while the rest is dark energy. Now the supernova researchers have gathered even more support from a wide array of ground-based and space-based telescopes. Though the scientists presented few details, Perlmutter, Riess, and Alexei Filippenko told the meeting that they have now tripled the size of their data set to about 230 supernovae. "It's certainly a confirmation of dark energy," says Filippenko. In fact, he adds, the data are so good that there is even a hint of a slowing down of the expansion that must have happened in the early universe before the dark energy began to dominate the cosmos.

Better yet, the data are beginning to illuminate the nature of the dark energy. Berkeley Lab's Eric Linder says the new supernova data point to an unwavering dark energy rather than an ever-changing "quintessence" that some theorists have proposed. The new data set "allows you to probe what the dark energy is," he says, adding that more supernova data, along with other types of observations, will really begin to pin down the nature of dark energy. "I'm really looking forward to the next 5 to 10 years."

Related site
The Supernova Cosmology Project at the Lawrence Berkeley National Laboratory

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