Throw a baseball up toward the sky, and gravity will slow its travel from the moment it leaves your hand. For decades, astronomers assumed that the post-Big Bang universe worked the same way. Even though galaxies are flying apart as space expands, their motion should be decelerating as the eons go by, due to the pull of their gravity on each other.

But a decade ago, cosmologists discovered something totally unexpected. The expansion of the universe has not been slowing down but speeding up in the past few billion years. It's as if the baseball you threw upward, instead of slowing down, suddenly sprouted a rocket engine and took off toward the clouds.

Something akin to anti-gravity — dubbed "dark energy" for lack of a better term — has apparently been inflating space. The evidence? Extremely far-off galaxies are traveling away from us at the wrong speeds (as measured by their redshifts) for their distances (as measured by the brightnesses of supernovae within them).

In recent years cosmologists seeking confirmation of dark energy's existence have looked for its effects another way: by analyzing distant clusters of thousands of galaxies with masses totaling up to a million billion (10¹⁵) Suns. Such galaxy clusters are the largest structures in the universe that are held together by their own gravity. They offer tantalizing hints that something like anti-gravity has indeed been retarding their evolution.

A team led by Alexey Vihklinin (Smithsonian Astrophysical Observatory) has discovered that very ancient galaxy clusters are much more massive than those that formed more recently. "What we find is that the growth of these structures has slowed down during the past 5½ billion years," Vihklinin explains, "and this is the unmistakable signature of dark energy."

The researchers used NASA's Chandra X-ray Observatory to map the hot, X-ray-bright gas filling dozens of clusters — some relatively young, others much older — to determine their masses. Their results won't be formally published until next February 10th's issue of the Astrophysical Journal, but a press briefing on Tuesday offered a preview (in easier-to-digest form!) about the years-long effort.

"A cluster's growth is really a competition between gravity's pull and accelerating expansion" of space, explains Smithsonian coauthor William Forman. And the Chandra observations indicate that younger clusters grew to be less massive than they should have, compared to the ones that came together early on. Roughly five billion years ago, with galaxies getting farther apart and their gravitational pull on each other weakening, the repulsive force of dark energy started to win out over gravity.

The work of Vihklinin and his team builds on earlier cluster work based on Chandra observations. In fact, Tuesday's announcement seems to echo a similar pronouncement linking galaxy clusters and dark energy made in mid-2004.

There's more to the story. The new result has buoyed cosmologists' confidence that they know dark energy's equation of state — that is, how its behavior changes, or doesn't, as the universe expands. It increasingly seems that the amount of dark energy in a given volume of space (a cubic centimeter, let's say) remains the same no matter how much space expands and how many cubic centimeters exist. This implies that dark energy is somehow associated with empty space itself, rather than being some kind of particles or field in space — which would thin out as space expands, as atoms and galaxies have done.

In other words, dark energy seems to match a notion conceived — and then rejected — by Albert Einstein nearly a century ago. Einstein invoked a gravity-defying "cosmological constant" to explain how a seemingly static universe (this was before cosmic expansion was even discovered) could resist collapse due to its own gravity.

The new results help to confirm that what we think of as "normal" matter — everything from stars and galaxies down to subatomic particles — represents only 4% of all the matter and energy that exists. The rest consists of 24% "non-baryonic dark matter" (made of something yet unknown, though particle physicists have their ideas), and 72% dark energy. The same ratio is indicated by the supernova-based galaxy distances measured a decade ago — and, with greater precision, by analyses of the cosmic microwave background radiation. It's yet another feather in the cap for the new era of "precision cosmology."

Besides Einstein's cosmological constant, theorists have proposed that dark energy might be explained in different ways: by an imperfection in general relativity requiring a modified law of gravity; or by an invisible energy field called "quintessence" that pervades the universe (though the equation-of-state value argues against this); or by an effect of unseen extra dimensions of space on a microscopic scale as implied by string theory.

"The simplest explanation," says David Spergel (Princeton University), "is that there's energy associated with empty space," as Einstein proposed and particle physicists have long speculated. In other words, Spergel quips, "even nothing weighs something" — and the weight of nothingness has a negative value. The universe contains so much nothing, that nothing's slight negative weight has begun pushing apart everything that's not already gravitationally bound. Spergel calls the new result a "triumph of general relativity."

Veteran cosmologist Michael Turner (University of Chicago) agrees, to a point. "Chandra's standalone evidence opens the door to a new technique," he tells Sky & Telescope. "All the observations are still consistent with the simple explanation of a cosmological constant, but 10 years later we’re still scratching our heads." There's still wiggle room for other ideas, Turner cautions, such as quintessence. "I think there’s a deepening appreciation of this being a very profound problem."

Now, more confident that dark energy really exists, cosmologists can imagine a universe destined literally to fly apart faster than the speed of light*. We'll always have the Milky Way's stars to gaze upon, as well as those of the Andromeda Galaxy (which is actually heading toward us) and, probably, other galaxies of our Local Group. But things farther away are not gravitationally bound to us, so some tens of billions of years from now, the accelerating expansion of space will carry them beyond all possibility of view.

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* Yes, space can expand faster than light. Einstein's rule that no matter or energy can move faster than light, a rule that has been confirmed ever more firmly for a century, only refers to motion through space.

Bear with me. Imagine that galaxies are like ships sitting dead in the water, with the water being space. Imagine that the ocean itself is expanding, due to a huge upwelling current, so that the ships move apart from each other. Even though each ship is sitting dead in the water, in this way it can end up moving away from the other ships much faster than its maximum hull speed — which only limits its velocity through the water.

We didn't say cosmology was easy. . . .