“Dark energy” an Age-old Phenomenon, Study Finds
 

A mysterious force making the universe expand faster and faster—discovered eight years ago—isn’t new, a study has concluded: it has existed for most of cosmic history.

As long as nine billion years ago, when the cosmos was about one third its present age, “the stuff we call ‘dark energy’... was starting to make its presence felt,” said Adam Riess of The Johns Hopkins University in Baltimore, Md. He led a research team conducting the work.

The finding, the scientists said, is in line with Albert Einstein’s early suggestion that a repulsive form of gravity is an intrinsic feature of empty space.

On the other hand, they added, the result tends to conflict with some competing views holding that the dark energy’s strength varies over time.

“Although dark energy accounts for more than 70 percent of the energy of the universe, we know very little about it, so each clue is precious,” said Riess, who is credited with co-discovering the dark energy in the late 1990s.

Einstein first conceived of a repulsive force in space in an attempt to explain how the universe held its size against the inward pull of its own gravity. If this opposing force didn’t exist, he reasoned, the cosmos would collapse under the combined gravity of everything in it.

But he eventually dropped the idea. It remained an historical curiosity until 1998, when Riess and others detected the acceleration expansion of space. Astrophysicists concluded that Einstein may have been right after all: there is a repulsive form of gravity. They dubbed it dark energy.

Since then, they’ve been trying to uncover two of its key properties: its strength and permanence. These could ultimately help explain what it is.

For the new study, Riess and colleagues used NASA’s Hubble Space Telescope to peer far across the universe. This equates to looking back in time, because it takes time for the distant objects’ light to reach us. Thus we see them as they were that much time ago.


Cosmic expansion as summarized in a diagram by the Space Telescope Institute in Baltimore, Md.

The researchers studied a class of exploding stars, called supernovae, used to trace the expansion and expansion rate of the universe at various times. The method, Riess said, is akin to watching fireflies at night. Because they all glow with about the same strength, you can judge their distance from their apparent brightness.

A similar technique was used to uncover the cosmic acceleration to begin with. Later work filled in some details. Hubble sightings of far-off supernovae, reported in 2004 by Riess and colleagues, indicated that matter dominated the early cosmos, slowing down its expansion with its gravity. But the swelling began to speed up some five billion to six billion years ago, as the eerie repulsion began to overpower gravity.

The new findings, Riess said, show that dark energy was obstructing the gravitational pull of the matter in the universe even before it began winning that gravitational “tug of war.”

The results stem from an analysis of the 24 most distant supernovae known, most found within the last two years. By measuring the universe’s relative size over time, astrophysicists tracked its growth spurts, much as a parent gauges a child’s growth spurts using marks on a doorframe. Distant supernovae provide the “marks” for Hubble.

“After we subtract the gravity from the known matter in the universe, we can see the dark energy pushing to get out,” said the University of Western Kentucky’s Lou Strolger, a supernova hunter on Riess’ team. The findings are to appear in the Feb. 10 issue of The Astrophysical Journal.


- Courtesy Johns Hopkins University
and World Science staff