Exploring Dark Energy

On a rare cloudy October night in the Davis Mountains of West Texas, University of Texas at Austin astronomers wrangled with what they believe to be the largest mystery of the universe: dark energy.

These space explorers were not lazing around in beanbag chairs, eating snickerdoodles and discussing the sci-fi worlds of Star Wars, nor were they pondering the metaphysical karmic vibes of good and bad energy.

The dark energy they are studying is a bizarre phenomenon that is causing our cosmos to explode around us. And even though dark energy dominates the universe, we know nothing about it, yet.

“Dark energy represents our ignorance of what’s going on in the universe,” said Karl Gebhardt, professor of astronomy.

At the McDonald Observatory, seven hours west of Austin’s city lights, the astronomers were testing the prototype of a device that will enable them to measure the universe like none have done before and to describe the nature of its best-kept secret.

They believe their experiment, called the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), could be the first on Earth to begin answering questions about dark energy, propelling the university to the forefront of the worldwide scientific race to understand the universe.

Edwin Hubble discovered that the universe is expanding in 1929. But not until 70 years later was it realized that the rate of expansion is, in fact, increasing. This acceleration, seemingly contrary to the pull of gravity, is due to dark energy.

“It’s ‘dark’ because we can’t presently see it. It’s ‘energy’ because it has an effect,” said astronomer David Lambert, director of the McDonald Observatory, the most visited observatory in the world.

Matter (the stars, water, carbon, your dog) makes up only four percent of the universe. Dark matter, which is another mystery that interacts with matter only through gravity and is seen only on the scale of galaxies, makes up 23 percent.

Dark energy, seen only on the scale of the space between galaxies, composes the other 73 percent of the universe. While gravity attracts, dark energy repels. This is all we know for sure about dark energy. It could be a force, a particle, or a function of space itself.

“Whatever the answer is,” Gebhardt said, “it will take a revolution in our understanding of the laws of physics.”

HETDEX

Astronomers Gebhardt, Gary Hill and Phillip MacQueen aim to incite this revolution by conducting HETDEX.

The three forged an alliance over the past several years through hallway conversation. Their professional talents and interests dovetail perfectly, it seems, for tackling the problem of dark energy.

The HETDEX experiment calls for the most complete mapping of the universe to date. In all, they will map the positions of one million galaxies at a distance of 10 to 12 billion light-years from Earth, going back about 90 percent of the lifespan of the universe.

Then, they will compare the HETDEX map to an image of the universe when it was even younger, about 380,000 years old. The earlier image was obtained by a satellite Wilkinson Microwave Anistropy Probe (WMAP). The image is important because it is a picture of the universe at its smallest observable size, when early plasma cooled and condensed into the matter composing the galaxies.

The earlier WMAP image can be used as a standard ruler to which the later HETDEX map can be compared, said astronomy professor Eiichiro Komatsu, one of the scientists who helped analyze the data from WMAP. The differences between the two will be used to figure out exactly how the universe evolved.

Once they know how the universe evolved, they can figure out how the effects of dark energy have changed, if at all, said Komatsu, who has become the fourth member of the HETDEX team of Gebhardt, Hill and MacQueen.

The Big Rip

If dark energy has not changed over time, then it is something similar to “the cosmological constant,” an idea conceived by Albert Einstein.

“Einstein wanted the universe to be ‘static,’ not expanding or collapsing,” Komatsu said. “However, his Einstein equation predicted that the universe should expand. So, he added a cosmological constant to make the universe static.”

Einstein later rejected the idea of the cosmological constant after Hubble discovered the universe is expanding.

If the effects of dark energy increase with time, there is the possibility that dark energy eventually will overwhelm every other force and will tear apart galaxies, stars and planets. Ultimately, every single atom in the universe will be dispersed into free particles. This idea is called the “Big Rip.”

No one knows for sure what the universe after a “Big Rip” would look like, but Komatsu mused on the possibility that energy could be suspended in a strange state, perhaps like plasma, but with a temperature of absolute zero.

If the effects of dark energy decrease or stay the same over time, then the universe might meet a gentler fate, continuing to coast outward forever.

VIRUS and the future

Before dark energy can be described and the fate of the universe determined, HETDEX must be completed and a million galaxies mapped. The astronomers say they can do it over an astonishingly short period of 110 nights for an amazingly small amount of money: $34 million.

To make the galaxy map, they’ll use a specially constructed, very large instrument called a spectrograph to record properties of the light from stars and galaxies entering the Hobby-Eberly telescope.

The Visible Integral-field Replicable Unit Spectrograph, or VIRUS, will be made of 145 copies of a single spectrograph. The prototype, about the size of a scuba tank, is already built.

Each of the 145 spectrographs will contain 246 optic fibers. Each optic fiber can receive and transmit a single point of light. All together, VIRUS will enable the astronomers to survey a much larger area than if they were trying to use an ordinary spectrograph. It’s like using a shotgun rather than a sniper rifle. It will also be ten times faster than any existing spectrograph.

Hill said that VIRUS puts them way ahead of the competition. “This will be a unique instrument that nobody else can touch,” he said.

The smaller VIRUS prototype is already being used in the HETDEX pilot survey, which began in October 2006 and will continue through summer 2007.

Because the university owns the majority of the Hobby-Eberly telescope outright, a luxury in the world of astronomy, a significant amount of time can be set aside for HETDEX observations.

Once VIRUS is in place on the telescope, in 2009 or 2010, observations will occur over a period of three springs, MacQueen said.

A completion date of 2012 would mean HETDEX’s results will come out three years sooner than its quickest competitor, which is five additional years ahead of everyone else. HETDEX’s $34 million budget is $17 million less than its thriftiest competitor and a fraction of the most expensive, space-based experiments with projected tabs of $1 billion.

In addition to being the cheapest and the fastest, HETDEX will be the most accurate of all the experiments and will cover the greatest period of time, meaning a greater chance of detecting evolution of dark energy, Hill said.

However, Gebhardt said that although HETDEX is designed to be the most significant dark energy project, it will probably take more than one team and more than one experiment to nail down the mystery of dark energy.

“We have this gross ignorance,” he said. “It means we don’t understand something about physics. Once we figure it out, who knows where that takes us?”

Written by Patrick Brendel
Photos by Wyatt McSpadden