Wednesday, September 28, 2011

'Accelerating universe' could be just an illusion

In 1929, cosmologists discovered that the universe is expanding — that space-time, the fabric of the cosmos, is stretching. Then in 1998, light coming from exploding stars called supernovas suggested that the universe is not only expanding, but that it has recently begun expanding faster and faster; its expansion has entered an "accelerating phase." This was bad news for the fate of the cosmos: An accelerating universe is ultimately racing toward a "Big Rip," the moment at which its size will become infinite and, in a flash, everything in it will be torn apart.

The discovery was bad news for the state of cosmology, too. Because gravity pulls stuff inward rather than pushing it out, cosmologists believed that the expansion of the universe ought to be slowing down, as everything in it felt the gravitational tug of everything else. They didn't understand the mechanism that seemed to be opposing the force of gravity, so to explain their observations, they invoked the existence of "dark energy," a mysterious, invisible substance that permeates space and drives its outward expansion.

Now, a new theory suggests that the accelerating expansion of the universe is merely an illusion, akin to a mirage in the desert. The false impression results from the way our particular region of the cosmos is drifting through the rest of space, said Christos Tsagas, a cosmologist at Aristotle University of Thessaloniki in Greece. Our relative motion makes it look like the universe as a whole is expanding faster and faster, while in actuality, its expansion is slowing down — just as would be expected from what we know about gravity.

If Tsagas' theory is correct, it would rid cosmology of its biggest headache, dark energy, and it might also save the universe from its harrowing fate: the Big Rip. Instead of ripping it to bits, the universe as Tsagas space-time envisions it would just roll to a standstill, then slowly start shrinking.

Cruising through space-time
Tsagas' alternative version of events, detailed in a recent issue of the peer-reviewed journal Physical Review D, builds on a recent discovery by Alexander Kashlinsky, a cosmologist at NASA's Observational Cosmology Laboratory. In a series of papers over the past three years, Kashlinsky and his colleagues have shown that the huge region of space-time in which we live — a region at least 2.5 billion light-years across — is moving relative to the rest of the universe, and fast.

Some cosmologists remain skeptical about the newfound "dark flow," as it's called, and say that more evidence is needed to persuade them that the strange phenomenon is real. But the evidence that does exist is compelling. Based on light collected from galaxy clusters, our enormous bubble of space-time appears to be drifting at a rapid clip of up to 2 million miles per hour. No one knows why, exactly — there may be something beyond the part of the universe we can see, tugging on us — but Tsagas argues that the dark flow is skewing our perspective on the behavior of the universe as a whole.

"My article discusses how observers living inside such a large-scale 'dark flow' could arrive at the (false) conclusion that the universe is accelerating, while it is actually decelerating," Tsagas told Life's Little Mysteries. In his paper, he illustrates that dark flow would cause the space-time within our moving bubble to expand faster than the space-time outside of it (which is not accelerating). Without considering the dark flow, but just knowing that light we observe from nearby galaxies left its source more recently than light from galaxies farther away, we get the false impression that the whole of space-time recently entered an accelerating phase.

Read more