In 2001, Riess found that the Hubble Space Telescope had made repeated images of an extremely distant Type 1a supernova, SN 1997ff, an object that was more than 10 billion years old. It turned out that the object appeared brighter than it would have been if the universe had been expanding at the same rate throughout its history. In other words, the universe had been slowing down way back then, 10 billion years ago. And then it had speeded up.
Later images made by Riess and his colleagues enabled them to determine that the transition occurred some five or six billion years ago. This was the Big Jerk. (Sorry—that’s what they call it.) That was when the universe had expanded to a point at which its dark matter had become dilute enough, and its attractive force had therefore become weak enough, for the anti-gravitational push of dark energy to rise up and overpower it. Says Riess: “As the universe moved through time, it slowly removed its foot from the brake pedal until the point when the accelerator became stronger than the brake and started jerking the car forward.”
It’s an open question where the universe is headed, but the three alternatives are biggies: the Big Lonely, the Big Crunch, and the Big Rip. If the repulsive force of dark energy remains constant, the universe will continue to expand at its present rate. This will make our immediate celestial neighborhood a solitary place, a consequence that the dark energizers refer to as the Big Lonely. If dark energy gets weaker, standard attractive gravity will take over and the universe will collapse in on itself—the Big Crunch.
But if dark energy gets stronger, the current acceleration of the cosmic expansion will speed up even more. The expansion will feed on itself so that the repulsive force of dark energy will get stronger still, with the physical universe finally rending itself apart in a fabulous bacchanal of disintegration—the Big Rip. Riess describes how it will occur: “Large gravitationally bound systems rip apart, and then progressively smaller bound systems rip apart. A cluster of galaxies rips apart first, then galaxies themselves rip apart, and then solar systems rip apart, then planets rip apart, then nuclei rip apart. It’s smaller and more tightly bound systems that will rip apart as there becomes more dark energy in them than binding energy from the ordinary gravity.”
Finally, the material universe will be gone. Where to? Don’t ask.
All of this was so insane, even to astrophysicists, that there just had to be alternatives to dark energy. One of them proved to be that staple of crackpottery, the claim that Einstein’s general relativity theory is wrong, that we don’t really understand gravity. “Perhaps the most radical idea is that there is no dark energy after all, but rather that Einstein’s theory of gravity must be modified,” wrote Michael Turner and Andy Riess in “From Slowdown to Speedup,” in the February 2004 Scientific American.
“Maybe the laws themselves need to be changed,” wrote Georgi Dvali, the NYU physicist, in the same issue. They’d changed before, when Newtonian laws were replaced by Einstein’s, so why not now? Dvali is a proponent of superstring theory, a complex mathematical effort to present a unified account of nature. One of the cardinal assumptions of string theory is that nature has more dimensions than the ones we’re familiar with. “The theory adds six or seven dimensions to the usual three,” wrote Dvali. “The extra dimensions are exactly like the three dimensions that we see around us.”
The existence of extra dimensions provided the perfect opportunity for Dvali to advance his “leakage scenario,” which is his explanation of why the universe’s expansion is accelerating. Dvali thinks that normal attractive gravity is leaking out of our universe’s three dimensions and into those other ones, causing the universe to accelerate its expansion. His theory has a strange sort of logic to it. Who needs dark energy if you have six or seven extra dimensions as escape routes for gravitons, the particles that, according to quantum field theory, are the carriers of gravitational force? “Real gravitons that leak away are simply lost forever,” wrote Dvali. For those of us stuck back at home, “it looks as though they have disappeared into thin air.”
Riess, for one, doesn’t find the idea so crazy. He compares Dvali’s leakage scenario to shining light down a fiber optic cable and getting less light out at the other end than you’d put in. If you lived inside the cable and the cable was your whole universe, then it would appear as if some of the light had simply vanished. “But what’s really happening is that light is leaking out a little bit from the cable,” Riess says, “and you’re missing that.”
Unconventional as it is, Dvali’s theory has the prime advantage of making a scientific prediction that could one day be tested. “I have calculated that graviton leakage would cause the moon’s orbit to precess slowly,” wrote Dvali. “Every time the moon completed one orbit, its closest approach to Earth would shift by about a trillionth of a degree, or about half a millimeter.”
Other experiments to detect, measure, and understand the nature of dark energy are in the works. Saul Perlmutter of the Lawrence Berkeley National Laboratory in California has proposed building a satellite observatory called the Supernova Acceleration Probe to trace the history of the universe through the past few billion years. In 2007, the European Space Agency plans to launch its Planck spacecraft, an observatory that will make yet finer measurements of the cosmic microwave background pattern that constituted one line of evidence for dark matter. And the U.S. Department of Energy and NASA have proposed the sinister-sounding Joint Dark Energy Mission, a space-based telescope dedicated to observing Type 1a supernovas.