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Essay: Will Dark Energy Please Come to Light?

Alex_Filippenko

Alex Filippenko at University of California’s Lick Observatory

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In 1998, after months of disbelief and checking the math, scientists announced that they had discovered a new force of nature. "Our jaws just dropped," said Alex Filippenko, a professor of astronomy at the University of California at Berkeley. "Initially we thought that we had done something wrong."

But two teams working independently came to the same conclusion: An invisible force is separating the matter in space at an increasing pace. It seems to act opposite gravity. Gravity pulls, but this force pushes. Gravity makes galaxies cluster together. This force drives those clusters apart.

Although scientists didn't know what this powerful force was, they came up with a name: dark energy. More than 10 years later, dark energy remains barely more than a concept. Astrophysicists are still struggling to identify its basic physical properties. In fact, they have yet to confirm that it even exists.

Many are now looking to a new device, the Dark Energy Camera, to probe the farthest reaches of space for some answers. The camera may find abundant evidence of dark energy's influence on the cosmos. Or it may show that the effects attributed to dark energy result from something even more provocative: a flaw in the theory of gravity itself.

Our Expanding Universe

Although the idea of dark energy is new, the fact that the Universe is expanding has been understood since the late 1920's. Telescope observations show that galaxies, no matter where they are in space, are becoming more distant all the time. The farther away a galaxy is, the faster it is receding. This relationship is known as Hubble's Law after the astronomer Edwin Hubble, who formulated it in 1929.

In other words, the space between the galaxies is growing. Think of the galaxies as raisins in a cake. As the cake bakes, it expands, separating the raisins farther and farther from one another over time. From the perspective of any one raisin, its neighbors appear to be moving away.

Current theory says that the Universe's expansion was launched by the Big Bangits first explosive moment. The Big Bang occurred 13.7 billion years ago, when all matter in the Universe was condensed into an infinitesimally tiny space. Then space began to expandand it hasn't stopped since.

Scientists long reasoned that gravitational attraction among galaxies would act like a brake on the expansion rate. Gravity pulls galaxies toward each other and thus would slow the expansion. So in the late 1990's, the two groupsFilippenko's University of California team and a second team of international astronomersset out to confirm this reasoning. They planned to measure how the Universe's expansion rate has changed over time.

Measuring Ancient History

To measure the Universe's rate of expansion, the teams observed not galaxies but exploding stars called type Ia supernovas. These are extremely energetic events so they can be detected at much greater distances than typical galaxies. Stars that become type Ia supernovas all start with about the same mass. Therefore, each explodes with the same amount of energy. Thus, the brightness of type Ia supernovas depends on only one factor: their distance from Earth. The dimmer the supernova, the more distant it is from Earth. It's the same principle that governs how much you squint at the headlights of an oncoming car. The headlights of a distant car appear dimmer than those of a car closer to you. Thus a supernova's brightness provides an estimate of its distance.

The light of a supernova serves as a time machine. Say a supernova is calculated to be five billion light-years away. That means it took five billion years for the light to reach Earth, so the supernova actually occurred five billion years ago. "We are seeing [the supernovas] as they were billions of years ago," says Filippenko. "That allows us essentially to view a movie of the past history of the Universe."

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The farther away an object is, the more stretched its wavelengths of light become. This shifts the light toward the red end of the electromagnetic spectrum.

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A supernova's light also offers a clue to the expansion rate at the time it exploded. Because the Universe is expanding, the supernovas appear to be moving away from us. The expansion actually stretches the wavelengths of light that reach us from the supernovas. Astronomers call the amount of stretch "redshift," and they measure it with a device called a spectrograph. Redshift corresponds to how quickly an object is receding from Earth--the rate of expansion of the intervening space.

The two teams plotted the redshift of a number of supernovas on a graph. Some were very bright, thus close by and occurring relatively recently. Others were very dim, thus very distant and occurring much earlier in the Universe's history. Consequently, the graph showed the change in the rate of the Universe's expansion over time.

As the two teams analyzed their data, the expansion rate proceeded as expected for the first eight billion years of the Universe's history. It was slowing down over time because of the counteractive effect of gravity. But curiously, five billion years ago, the expansion rate reversed. It started to accelerate and is still doing so. "A repulsive effect of unknown origin began to take over," says Filippenko. Dark energy, they reasoned, was propelling cosmic acceleration.

Einstein's Biggest Blunder?

The existence of a repulsive force of nature has been proposed before. In 1917, a dozen years before the Universe was found to be expanding, Albert Einstein was working on his theory of gravity. Einstein thought the Universe was neither expanding nor contracting. So, he theorized that a force must exist to balance gravity's constant pull and keep the Universe at a constant size. He called that force the cosmological constant. He even described the cosmological constant's properties: a "negative pressure" that could, if gravity weren't pushing back, drive space to expand. But Einstein admitted defeat when Hubble's Law proved his notion of a static Universe wrong. He even called the cosmological constant "his biggest blunder."

But the data on dark energy hint that Einstein's idea wasn't so far off the mark. "Einstein's greatest blunder may turn out to be better than the ideas that most of us have had on our very best days," says Robert Kirshner, an astronomer at the Harvard-Smithsonian Center for Astrophysics.

Indeed, many astrophysicists think that dark energy may turn out to have similar characteristics to the cosmological constant that Einstein theorized. "Determining exactly what dark energy is is really hard, because we can already tell that its properties are very close to what would have been expected for Einstein's cosmological constant," says Filippenko. "If it is like the cosmological constant, then it's just an unchanging property of space. It's the simplest possibility." Only very precise measurements of dark energy's effects on the Universe will reveal if Einstein was onto something.

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Installing CCD’s (digital “film”) behind the lens of the Dark Energy Camera

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The Dark Energy Camera

In a cleanroom at Fermilab, a research center of the U.S. Department of Energy in Batavia, Illinois, the guts of a massive camera are exposed. A scientist in a light blue lab coat and a white hairnet carries a glassy tile toward the device and slides it gently into a socket in the back of the camera's lens.

The tiles are called CCD's, or charge-coupled devices. They're what digital cameras use to capture light to make a picture. The CCD of a household digital camera is the size of a fingernail and can capture a few megapixels of information. This camerathe Dark Energy Camerahas 74 CCD's that are each 3 by 6 centimeters across and can detect five hundred megapixels in total.

When it is mounted on a telescope in Chile in 2011, the camera will seek evidence of dark energy. It will measure the redshift of many more supernovas to add to the graph, allowing more precise measurements of the Universe's expansion over a wider span of history. It will also spot hundreds of millions of the most distant galaxy clusters ever detected to examine how they're attracted to one anotherand how they're being pulled apart.

Some of the methods that the Dark Energy Camera will use will double-check the theory of gravity as it applies to the large-scale structure of the Universe. This is where a breakthrough might occur.

"Either there's new stuff out in the Universe that we call dark energy that is indeed gravitationally repulsive," says Josh Frieman, a scientist working on the Dark Energy Camera at Fermilab, "or we have to admit our understanding of gravity, which comes from Einstein's theory, has some holes in it."

If the Dark Energy Camera discovers that dark energy is a property of space that opposes gravity in our expanding Universe, it will transform Einstein's greatest "blunder" into his greatest success. But it's also possible that gravity might become weaker at very large distances in the Universe, and thus be responsible for the ever-increasing acceleration that scientists observe. In fact, there's a universe of possibilities out there. Astrophysicists would very much like to narrow them down to one.

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