The Wide Angle: Exciting Times Ahead for Dark Energy Search

The scoop: Dark energy is one of the greatest scientific hurdles humans have ever faced. An astronomer gives her take on the state of our knowledge about this mysterious force.

Only a decade ago, light from distant exploding stars showed us that the universe's size is increasing at a faster and faster rate. Leftovers created 400,000 years after the Big Bang, called the Cosmic Microwave Background (CMB), also show evidence of universe-wide acceleration.

This speeding up was quite unexpected -- Einstein's theory of general relativity predicts that the expansion should be slowing down. To describe the missing physics needed to explain this mystery, the term "dark energy" was coined.

Recent results from NASA's Chandra satellite now provide an important and alternative look: measuring the effect of dark energy on the growth of clusters of galaxies, some of the largest structures in the universe that each span millions of light-years across.

Dark energy is distinct from dark matter, which is responsible for the formation of cosmic structures such as galaxies. The nature of dark energy remains a mystery to us, and so understanding dark energy is one of the major challenges facing cosmologists and particle physicists today.

As such, it's vital to measure dark energy's effect on the universe in as many different ways as possible. Both the supernovae and CMB observations use "standard rulers" to measure the effects of accelerated expansion by measuring distances. Enter Chandra: an X-ray observatory positioned one-third of the way to the moon.

Chandra measured the X-rays emitted by hot gas present in galaxy clusters. Such X-ray signatures not only allow clusters to be spotted, but also give a way to measure mass: the more X-ray, the more massive a cluster. More specifically, the latest results measure how mass is spread among two groups of galaxy clusters: One with 49 bright clusters which are relatively close by on cosmic scales and another group of 37 far more distant clusters. X-rays from the more distant clusters have taken longer to reach us, and therefore give us a snapshot of how the clusters appeared earlier in the universe's history -- a time when the acceleration was only beginning.

When the rate of cosmic expansion accelerates, it slows down the formation of galaxies and clusters of galaxies. So the masses of clusters in an accelerating universe should be smaller than they would be in one that is decelerating. Comparison of the two groups of clusters allowed the Chandra team to measure the effect of dark energy.

What did the team of scientists find? First and foremost, that the X-ray measurements agree with evidence of dark energy from the two other sets of evidence -- supernovae and CMB data. These show that dark energy makes up at least 70 percent of the universe.

Two other benefits come out of the Chandra data. First, in combination with previous findings, the new information better reels in the properties of dark energy. For example, with all the datasets combined, the "equation of state" of dark energy -- which describes its effect on the expansion of the universe -- is now measured to within a 10 percent rate of precision. Second, the observations are a new way to learn about dark energy: through the growth of structure rather than effect on cosmic distances.

Even better is that predictions about the universe using both approaches (growth and distance) might be different depending on what, exactly, dark energy is. That means comparing rather than combining these two methods of hunting for dark energy's influence could eventually provide vital discrimination of the nature of dark energy.

The quest to understand dark energy is a major focus for NASA and the Department of Energy. Together they're funding a satellite -- the Joint Dark Energy Mission -- that is expected to increase by 10-fold the precision of the dark energy equation of state within 10 years. It will do this by measuring the properties of dark energy using both cosmic distance and the growth of cosmic structures.

Although extrapolating such results to the future lies in the realm of conjecture, precision measurements of dark energy's properties could give us some inkling of what might be in store for the universe's future. What we know now suggests that the universe's expansion isn't likely to slow down, trigger a contraction and an eventual "Big Crunch." What's more likely is that expansion will continue at an ever accelerating rate, our cosmic neighborhood fated to become increasingly desolate.

Conjecture or not, the next few years promise to be some of the most exciting ever when it comes to dark energy -- one of the greatest challenges in science we have ever encountered.

Rachel Bean is an Assistant Professor in the Department of Astronomy at Cornell University. The views expressed here are the author's alone and do not represent the official position of the Discovery Channel.

Article posted December 16, 2008.

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