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“Dark energy” an age-old phenomenon, study finds

Nov. 16, 2006
Courtesy Johns Hopkins University 
and World Science staff

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.

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


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.

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.


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Homepage image: The Hubble Space Telescope in orbit. (Courtesy NASA)

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A mysterious force that makes the universe expand faster and faster isn’t new: it has existed for most of cosmic history, a research team has concluded. This “stuff we call ‘dark energy’ was present as long as 9 billion years ago, when it was starting to make its presence felt,” said Adam Riess of The Johns Hopkins University in Baltimore, Md., leader of the group. This is in line with Albert Einstein’s early prediction that a repulsive form of gravity is an intrinsic property of empty space, the astrophysicists said. Conversely, they added, the finding tends to conflict with some competing explanations holding that the dark energy’s strength changes over time. The findings are to appear in the Feb. 10 issue of Astrophysical Journal. “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. Riess and colleagues used NASA’s Hubble Space Telescope for the study. To study dark energy long ago, Hubble had 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. The researchers studied a particular type of exploding star, called supernovae. These stars are 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. 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 universe would be crushed by the combined gravity of everything in it. But he eventually dropped the idea. It remained a historical curiosity until 1998, when Riess and others detected the acceleration of the expansion of space from observations of supernovae. Astrophysicists concluded that Einstein may have been right after all: there was a repulsive form of gravity. They dubbed it dark energy. Since then, they’ve been trying to uncover two of dark energy’s key properties: its strength and permanence. 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 dark energy’s 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.