In the Stars: Dark energy's tell-tale sign

WASHINGTON, March 17 (UPI) -- The phrase that seems to describe most precisely the nature of the universe, as unfolded by modern physics -- both theoretical and observational -- is "stranger and stranger." Case in point: the mysterious force known as dark energy.

In the first few moments after the Big Bang, some 14.7 billion years ago, space and time expanded at an unimaginable velocity -- hundreds of thousands of times faster than the speed of light. Within just a few seconds, the nascent cosmos had grown to a size much bigger than the solar system, billions of miles across, yet still super-hot and super-dense.

Very quickly thereafter, gravity took over and began to slow the expansion, pulling the particle soup of matter into the first stars and galaxies.

Except for the brief interval of initial stupendous growth, called the period of inflation -- in which space moved much faster than anything could within it -- astronomers have known about the universe's expansion since the 1920s, when Edwin Hubble first observed there were galaxies other than the Milky Way and they all seemed to be moving apart from one another.

Intuitively, gravity should have slowed the universe's expansion relatively uniformly to this day. The problem is that it has not. Instead, the expansion actually is accelerating, with galaxies at the farthest reaches of the universe moving away from one another more and more rapidly.

Sometime around 6.3 billion years ago, acceleration replaced deceleration. Something took hold of the entire universe and began acting like a gigantic reverse magnet, nullifying the effect of gravity and speeding up the outward push.

When this phenomenon was discovered -- about six years ago, in data gathered by the Hubble Space Telescope -- physicists struggled to come up with an explanation. The one that stuck was dark energy, a term credited to University of Chicago cosmologist Michael Turner.

Dark energy, it turns out, comprises most of the universe. Its constituency -- whatever it is -- dwarfs mere matter's agglomeration of stars, planets, gas and dust, which accounts for less than 5 percent of the total. The slightly less-mysterious material called dark matter contributes about another 25 percent.

That leaves 70 percent -- nearly three-quarters of the universe -- comprised of something no one has yet seen, observed or measured directly. For a while, dark energy lived only in the minds of theoretical physicists and in a growing body of computer simulations.

Still, indirect evidence exists -- including those first observations of distant galaxies in the process of acceleration, taken by measuring the properties of certain types of their light.

Now, a research team has taken those observations by the Hubble and combined them with newer and more powerful computer simulations to produce the best hint yet that dark energy is indeed real. Moreover, it seems to operate in every corner of the universe.

Fabio Governato, an associate professor of astronomy at the University of Washington in Seattle, and colleagues have

constructed a picture of the cosmos as a sea of dark energy, within which hundreds of billions of galaxies exist as tiny islands of visible matter.

Gravitational attraction exerts a counter influence against dark energy's outward push, but it is not enough to overcome the acceleration -- it just slows things down a little. For the past 6.3 billion years, gravity and dark energy have been battling for the destiny of the universe. If gravity wins, the outward expansion will slow down someday or even stop, suggesting a possible reversal ending perhaps trillions of years from now in a Big Crunch -- the opposite of the Big Bang.

When Governato designed his computer model some years ago, he found it could simulate the evolution of the universe from the Big Bang until the present, but it could not portray accurately the motion of galaxies located near the Milky Way. The model kept predicting speeds for the nearby galaxies three to seven times higher than astronomers had actually observed.

"The observed motion was small, and we could not duplicate it without the presence of dark energy," Governato explained, but "when we added the dark energy, we got a perfect match."

The model shows dark energy influences even neighboring galaxies. This strange, repellant force is active in every corner of the universe, pushing on visible matter, maybe sending galaxies on their way to a cold, lonely destiny, untold years from now.

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In the Stars is a weekly series examining new discoveries about the cosmos. E-mail: sciencemail@upi.com