Dark Energy: Astronomers Hot on Trail of Mysterious Force

Dark Energy: Astronomers Hot on Trail of Mysterious Force
By Robert Roy Britt
Senior Science Writer
posted: 09:00 am ET
10 April 2003



PHILADELPHIA -- At first there was disbelief. Then widespread befuddlement. Then a period of quantification. Now, five years after discovering that the universe is expanding at an ever-faster pace, scientists know exactly how much mysterious "dark energy" is behind the acceleration and have turned to figuring out what it is.

The task may eat up a lifetime, researchers admit. Or perhaps some new Einstein will figure it out next year in a light-bulb moment. The reward will be a far more complete understanding of the history and fate of the cosmos.

At a meeting of the American Physical Society (APS) here this week, front-line observational astronomers and big-thinking theorists said important clues to one of the most vexing mysteries of modern science are already rolling in. Cautious optimism infused the gathering.

Much important but unpublished data are already collected, SPACE.com has learned, and by the end of this year the history of the universe's expansion could become a bit clearer, further illuminating the path toward understanding dark energy.

Change of pace

Our universe has always been expanding, with practically all galaxies receding from each other, except for those bound in clusters.

The expansion was decelerating until about 6.3 billion years ago, however. Then an important switch to acceleration occurred. Something caused the universe to step on the gas, driving a growth that now speeds up each day. It is like a rocket whose speed increases 100 mph in the first mile, then by the same amount the next half-mile, then in a quarter-mile, and so on.

Scientists admit they've made almost zero progress in understanding dark energy. They have no idea what it is or how it works.

Various researchers describe the phenomenon as a repulsive force, as vacuum energy, as anti-gravity, and as possibly no more than a different manifestation of gravity over large distances. Some say the repulsion could be a response to dark matter, unseen stuff that is known to make up nearly a quarter of the universe, but such a link has never been established.

All that's clear is that dark energy comprises 73 percent of the mass-energy budget of the universe, and that it is no longer an arguable point for theorists but instead is a viable quarry for astronomers.

Surprise finding

While not even trying, the Hubble Space Telescope just spotted two very distant exploding stars that represent baby steps ahead of a footrace of expected observations.

The so-called supernovae, announced this morning, are 5 billion and 8 billion light-years away and were found serendipitously by Hubble's new Advance Camera for Surveys, installed a year ago, while it was making a calibration run. Hubble officials, who have been saying the new camera would turn Hubble into a supernova hunting machine, offered the discoveries as proof of that claim.

The supernovae bracket the presumed time of the switch from deceleration to acceleration, and examination of them has helped build the observational case that the shift occurred. Two other groups have used Hubble to purposely collect data on several more supernovae and results will likely be reported by the end of the year, researchers told SPACE.com.

Hubble is but one tool being used to probe dark energy.

Scientists are also exploring so-called cosmic microwave background radiation that carries an imprint of the baby universe's structure. Just two months ago, that effort led to the first firm determination of the age of the universe, a solid estimate for when the first stars were born, plus undisputed confirmation of dark energy's expansive role.


Other teams are examining the structure of the space through time by noting how interstellar hydrogen absorbs light. Still other researchers are pushing back the frontier of time, finding galaxies that formed when the universe was less than 10 percent its present age.

For the first time in history, experts suggest, cosmological data is accumulating faster than the wild theories that try to describe it all.

"By far the best is yet to come," said Max Tegmark, a cosmologist at the University of Pennsylvania. "We're not even halfway through this avalanche of data in cosmology."

Going back in time

Theorists have known since the 1920s that the universe was expanding. They wondered if that expansion would go on forever, or if common gravity might eventually win out and pull everything back together in a sort of Big Crunch.

Then in 1998 two separate groups hunting faraway supernovae found several that were dimmer than they should have been, indicating that the universe is not just expanding, but accelerating.

The supernovae are of a particular variety, known as Type IA, that all shine with the same intrinsic brightness. Astronomers use them as "standard candles," their observed brightness revealing their distance. Light from the objects is analyzed to determine how much the waves have stretched, which bears an exact relationship to how much the universe has expanded since the light left its source -- the exploded star.

The 1998 finding of an accelerating universe was initially met with disbelief by its discoverers. Once digested -- in some cases only in the last couple of years by skeptics -- its profound implications for the composition and fate of the cosmos brought the term dark energy into common use.

Meanwhile, theorists had already figured out that an accelerating universe would necessarily be preceded by a period of deceleration, which would have followed an initial phase of rapid inflation associated with the Big Bang.

Here is why things must have slowed down:

"Early on the universe had lots of mass in a small volume," explains John Blakeslee of Johns Hopkins University. "The pull from gravity must have been enormous." As the universe expanded, gravity would have become less effective over the larger distances, and dark energy would have taken over.

One previously detected supernova, at about 10 billion light years away, supported this idea when reported in 2001, but the object proved difficult to study. The Hubble observations presented today also support the switch, said Blakeslee, lead author of a paper on the findings that will be published in the June Astrophysical Journal.

Had the universe always been accelerating, the supernova that's 8 billion light-years distant would have been dimmer, Blakeslee said in a telephone interview.

"It's not conclusive at all," Blakeslee said of his work. Another 20 or so very distant supernova are needed to make a strong case, he added.

Those 20, and then some, will not take long.

Hubble's new eyesight "should allow astronomers to discover roughly 10 times as many of these cosmic beacons as was possible with Hubble's previous main imaging camera," Blakeslee said.

Already in the bag

A separate Hubble project, led by Adam Riess of the Space Telescope Science Institute, has already bagged several distant supernovae. Riess, who worked on one team that made the 1998 acceleration breakthrough, reported preliminary results of his latest work at the APS meeting and is expected to publish a paper soon, possibly later this year.

Eleven other distant supernovae have been examined by Hubble in another study headed up by Saul Perlmutter of the Lawrence Berkeley National Laboratory. Perlmutter led the other team involved in the 1998 discovery of acceleration.

Perlmutter was not involved in the two new Hubble discoveries, but he told SPACE.com they are among many important steps that could lead to a firm determination of when acceleration began. Many of the discoveries are coming from ground-based telescopes, he noted, but Hubble "is really becoming a key for everybody in terms of follow-up" to glean the necessary detail.

By building a strong historical timeline of the universe, astronomers and cosmologists hope to answer a pressing question: Do the properties of dark energy change over time?

The answer would help them determine what dark energy is and would allow refined predictions about the origin and fate of the universe. No one expects a quick resolution, however.

Michael Turner, one of the world's foremost cosmologists from the University of Chicago, said solving the dark energy problem "is going to require a crazy idea." While most leading theories project the universe will accelerate forever, perhaps even to the wild point that it rips all matter apart, the notion that it might eventually collapse has not been ruled out, Turner said at the APS meeting.

"The destiny question is wide open," Turner said.