In the 1920s, Edwin Hubble showed that the universe, thought since the Greeks to be changeless, instead is expanding. But at what rate? Some theoreticians predicted the expansion, driven by the momentum of the Big Bang, should be slowing. Eventually the galaxies would either stop their outward travel and the cosmos become static, or gravity would pull everything back together for a Big Crunch. Other theoreticians felt cosmic expansion would continue at a steady pace essentially forever. Since the galaxies must long ago have overcome the gravity of the Big Bang in order to be rushing outward, this reasoning went, they'll simply keep going.
Numerous attempts were made to measure the movements of distant galaxies, in order to estimate whether the expansion is steady or slowing. In the mid-1990s, a team led by Adam Riess of Johns Hopkins employed images of very distant supernovae to derive what was considered the most accurate measurement to date of cosmic speed. The researchers reviewed their data many times, always coming to the same vexing conclusion: the expansion of the universe is speeding up.
Riess and his associates found that for the last 7 billion years or so -- roughly the halfway point of the apparent lifetime of the cosmos - galactic expansion has been gaining speed. This would be possible only if something is pushing the galaxies, adding energy to them. So Riess posited "dark energy," a force that is real but eludes detection.
A quick trip to the blackboard for calculations showed that in order for the galaxies to be accelerating in their observed manner, three-quarters of the universe must be dark energy, while a mere fraction of the content of the universe is the "ordinary" matter and energy that's found in our solar system. But though Riess is a founder of the dark energy concept, he says, "I have absolutely no clue what dark energy is." Dark energy appears strong enough to push the entire universe - yet its source is unknown, its location is unknown and its physics are highly speculative.
The whole dark-energy concept is so strange, so utterly lacking antecedent either in theory or in common-sense experience, it was as if someone walked up and said, "Hey, did you hear Britney Spears just found the cure for cancer?"
For a century, physicists have elaborately fine-tuned a concept of the natural world in which there are four fundamental forces: gravity, the "strong force" that holds atoms together, the "weak force" that keeps electrons in place around atoms, and electromagnetism (light, radio waves, X-rays, etc.). Entire academic careers have been made on work refining four-forces thinking. Suddenly there's a fifth force, dark energy. And although it appears to comprise three-quarters of the universe, nobody's noticed it till now.
Gravity is inverse to distance, meaning as objects draw close together the attraction becomes more powerful; as they move apart, fades significantly. Dark energy appears to operate the other way around, proportional to distance: feeble between objects close by, steadily stronger as objects get farther apart.
"Dark energy is incredibly strange, but actually it makes sense to me that it went unnoticed, because dark energy has no effect on daily life, or even inside our solar system," Riess says. "We know there is gravity because apples fall from trees. We can observe gravity in daily life. If we could throw an apple to the edge of the universe, we would observe it accelerating. Until the 1990s, there were few reliable observations about movement at the scale of the entire universe, which is the only scale dark energy effects. So dark energy could not be seen until we could measure things very, very far away."
Unable to explain why the universe had not folded back onto itself as his General Theory of Relativity seemed to suggest should have occurred, Albert Einstein inserted an anti-gravity term into his equations -- the "cosmological constant," a hypothetical force that pushed outward on creation. Later Einstein retracted the idea, admitting there was no evidence for a cosmological constant nor any physical explanation of how one might function. Now there does seem to be a repulsive force, dark energy, whose role is similar to Einstein's conjecture.
And dark energy is - what, exactly?
Physicists use the paradoxical term "vacuum density." Occasionally in vacuum-chamber experiments, subatomic particles swirl in and out of existence, seeming to come from nowhere and return to nowhere. It does not appear to be possible to make a vacuum in which there is nothing at all, so it is now assumed that intergalactic space is not a true void. Perhaps the interstellar void contains both ghost particles and energy potential, the latter of which is expressing itself by causing cosmic acceleration.
That vacuum could be the source of energy appears to make no sense: but also appears to be what's happening. Because space keeps expanding, every day there is more vacuum, which would mean every day there is more vacuum-produced dark energy. At the Big Bang, dark energy was a trivial constituent of the cosmos; now it's 74 percent of creation and growing. An eon from now, the universe might be 99 percent dark energy, with the galaxies expanding away from each other at unfathomable speeds .
Here is a spooky thought. Physicist Lawrence Krauss of Arizona State University calculates that three trillions years from now -- about the time the federal budget is expected to be balanced -- dark energy will have accelerated the universe to such speeds that no galaxy will be able to see any other galaxy. Intercourse between galaxies will become unimaginable, even if Captain Kirk's warpdrive is invented. Each galaxy will perceive itself as the whole of creation, a lonely cluster of stars surrounded by infinite emptiness. At this far-future point, all traces of the Big Bang will have dissipated from the cosmos. Any intelligent beings evolving under those circumstances would not be able to figure out how the universe formed. We're lucky, Krauss thinks, to have come into existence while the cosmos is "young," and dark energy has not yet hurled away all the clues about why we are here.
Of course there may also be some force or factor at work that no one has yet guessed. Perhaps our descendents will say, "Can you believe that in the 21st century, Nobel Prize winners thought the entire universe could be pushed by an invisible unknown force no one can locate? And Atlantic writers fell for it!"
For today, Adam Riess of Johns Hopkins, Saul Perlmutter of Cal Berkeley and Brian Schmidt of Australian National University are the state of the cosmic art.