Searching the universe for dark energy

AAAS fellow Joshua Frieman standing in front of the Dark Energy Camera, known as DECam, which will survey about 4,000 supernovae looking for evidence of dark energy. (Photo: Brandon Schulman)

When theoretical cosmologist Michael Turner gathered particle physicists and cosmologists into the same room for the first time, many scoffed at the idea that elementary particles could have anything to do with the cosmos. Yet the years following this 1984 meeting developed into a period of renaissance for cosmology. New evidence suggested that an exotic hidden force, unlike anything observed in physics before, was dominating the universe. Turner coined the phenomenon "dark energy."

One of Turner's first students at the University of Chicago, AAAS fellow Joshua Frieman, is today pointing a powerful digital camera to the night sky over the high Chilean Andes in an ambitious survey to study the nature of dark energy and what Turner now believes is the most profound problem in all of science: Why is the expansion of the universe actually speeding up?

"Scientists live for really big puzzles, really rich puzzles," says Turner, also a fellow at AAAS. "Dark energy controls the destiny of the universe." It may give clues, he says, to complex theories that, if accurate, would introduce a new era of particle physics.

His particle astrophysics collaboration between the University of Chicago and nearby Fermi National Accelerator Laboratory became what he calls the "mother church for quarks and the cosmos." Discoveries in particle physics began seeding new theories on the Big Bang and the early universe, while astronomy surveys were raising new mysteries about the nature of the cosmos.

The "hottest thing that could possibly be happening anywhere" was the measurement of cosmic microwave background (CMB), according to AAAS fellow Martin White, who completed a doctoral degree in particle physics as he was being drawn to the exciting discoveries in cosmology. "The CMB fluctuations are the seeds for all the stuff we see today," says White, now a theorist at the University of California, Berkeley. "So then it was natural for us to ask: Well what do these seeds grow into?"

In the 1990s it was becoming clear that something was pushing the universe outward and that gravity would not pull the universe back together again. Turner and White co-authored a paper on the shape of the universe based on this inflation theory. "So there was one out if you wanted to maintain a flat universe," says Turner. "That was to put something in like a cosmological constant."

Albert Einstein's famous fudge factor, the cosmological constant, claims that empty space has an energy pushing against gravity. This modification filled in blanks for Einstein's theory of general relativity. But after a heated debate with astronomer Edwin Hubble, he redacted this version of the theory, calling it the "biggest blunder" of his life.

By factoring in the cosmological constant, Turner's calculations showed that against his expectations, the speed of the universe was not slowing down but speeding up.

Then everything changed one day in 1998 when astronomers monitoring supernovae found evidence that the universe is indeed accelerating in its expansion. For this, three of those scientists won the Nobel Prize in physics.

"It was absolutely a sea change," says Turner. "What's sort of odd about it is that, who would have thought that something as crazy as the universe speeding up would cause everything else to make sense? But that was the missing part of the puzzle."

With the discovery of cosmic acceleration, the race was on to find out why.

Something from nothing

With news of the discovery, theorists eventually narrowed the search to three suspected causes.

A curious aspect of the dark energy phenomenon is that it's extremely elastic, says Joshua Frieman, now an astrophysicist at Fermilab. When ordinary matter spreads out, particles disperse and the matter thins out. "As the universe expands, in the simplest models of dark energy, it doesn't get more dilute," he says. "It's very strange. The density of the stuff stays the same." This allows dark energy to blanket 72 percent of the universe, while the weakly interacting particles that form dark matter account for about 25 percent and ordinary matter is less than five percent.

When things are highly elastic in physics, they lead to a negative pressure and a repulsive gravity, as when a rubber band stretched too far snaps in half and fires off in opposite directions.

One way to explain this dark energy phenomenon was for Turner and White to look to particle physics, specifically the murky subatomic world of quantum mechanics, where the natural laws of physics don't seem to apply. Dark energy, retaining its density as it does, would not break down into smaller and smaller particles like matter. Instead, it may exist as a sort of vacuum energy.

In an absolute vacuum, where the current understanding of the world would say nothing should exist, experiments have found actual energy fluctuations in this nothingness. If this is a constant pattern throughout the universe, then the simplest explanation for dark energy would be that it's the vacuum energy defined in Einstein's controversial cosmological constant.

Another possibility is that the empty space could be filled with temporary ultra-light particles. But when theorists try to factor in these particles and compare that calculation to the known weight of the fluctuations within the vacuum, the two numbers are wildly different.

The alternative is that Einstein was wrong on two fronts.

Aside from the cosmological constant, general relativity and its explanation of gravity is a well-understood and well-tested theory. Yet Turner suggests that on the cosmic scale it may come up short.

"So, basically, the idea is that in Einstein's theory of general relativity space-time is no longer sort of a static stage in which action plays out," says White. "It's one of the actors in the drama."

This means that gravity is an active force that fluctuates with the temperature of space-time. As it's been understood since Isaac Newton, gravity is attractive. But if it's possible for something to become repulsive when it gains negative pressure, why shouldn't gravity, instead of dark energy, swing the opposite way on the cosmic scale and push the universe outward?

To find out whether dark energy is actually vacuum energy or if general relativity must be replaced with a new theory of gravity, far more information is needed. In this rare case, observation is leading theory.

Following the light

Back in the 1990s, Frieman's interest in the question of dark energy led him and a few colleagues to develop models for cosmic acceleration. "But I became convinced that to make progress and to point us down the right path we needed to get more and better data," he recalls. "So I started thinking with some colleagues about a new kind of survey that could do that."

Frieman's team then began building an international collaboration, accruing more than 120 scientists from 23 institutions spanning three continents. They forged the parts, assembled a camera and developed the complex software needed to analyze 400 gigabyte-sized images each night. Now eight years on, Frieman, as director of the Dark Energy Survey (DES), will finally point their camera to the sky, flip the switch, and try to see what the universe is made of.

The Dark Energy Camera, known as DECam, recently traveled from its home at Fermilab to Chile, where it is now being mounted to the end of a telescope at a remote observatory in the high Andean mountains. In capturing images with the extremely sensitive 570-megapixel camera, the survey team will look as far as eight or nine billion years back in time with a degree of precision one million times more powerful than the naked eye. 

The survey will look at about 4COMMANUMBER000 supernovae — an astonishing number when compared to the tens of supernovae the 1998 survey studied. DECam will track these imploding stars by returning to images already taken and searching for changes in light. The astronomers will then calculate how far away each supernova is and, since light travels at a finite speed, how far back in time the supernova occurred and how quickly it's pushing away from Earth — all clues to the speed of the universe.

The survey hopes to also gather this expansion rate by analyzing regions where hundreds of thousands of galaxies are clustered. This is measured by gravitational lensing — a method of analyzing light that bends around the pull of a gravitational field. The four DECam survey probes will provide "robust" crosschecks along with several points of comparison.

"So it's basically more powerful and it's going to use this multiplicity of techniques," says Frieman. "That's what I think makes it stand out from previous generations of these kinds of studies."

The collaboration planned to begin taking data in November 2012, using 524 nights on the telescope to conduct the survey, with definitive results within two years.

"We're just now at the point of actually seeing it come to fruition," says Frieman. "Very exciting. Lots of nail biting to make sure that everything works."

With so little known about dark energy, it's difficult for scientists to predict whether this puzzle will be solved in 10, 30 or 300 years, which is what makes it such a thrill for Turner. "We've just barely gotten our heads around this question," he says. "And it's connected to all kinds of other fundamental problems in physics and cosmology. It just isn't clear how long it will take to solve or even precisely what you need to do to solve it." Many more twists and turns can be expected.

"Science is about living in interesting times," he continues. "And we're so privileged to be here at this time, where we kind of put together a first draft of the universe."

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