In Depth

Searching the universe for dark energy

July 11, 2012 | Author: Freelance Writer Brad Hooker
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.