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 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. [Did the Universe Begin as a Simple 1-D Line?]
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, a sister site to SPACE.com. 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.
In short, Tsagas' explains our observations of the expansion of space-time nearby and far away without invoking dark energy, or any other mysterious mechanism. According to Tsagas' work, the acceleration of the universe in our immediate vicinity is caused by its motion alone. The universe beyond our region isn't accelerating outward; rather, it is safely rolling to a stop.
Tsagas' theory is supported, in part, by other recent observations that have puzzled cosmologists. Some data collected from space, such as the cosmic microwave background [CMB] radiation and light from supernovas, seems to show that the universe has a "preferred axis": In its outward expansion, it appears to be stretching more one way than another.
As detailed in a new paper recently posted to the physics arXiv, Zhong-Liang Tuo and colleagues at Key Laboratory of Frontiers in Theoretical Physics in China have identified such a "preferred axis" in the expansion of space-time by looking at light from more than 500 supernovas.
By measuring how much the light from each of the stellar explosions is red-shifted — stretched — they detected the rate of expansion of different parts of space, and found that the universe looks to be stretching more toward the constellation Vulpecula in the northern sky than it is in any other direction.
Previously, a "preferred axis" in the expansion of space-time was also detected in the cosmic microwave background radiation, and pointing in the same direction. Tsagas said this alignment is no mere coincidence: the axis is another illusory effect of the "dark flow" of our space-time bubble.
"Peculiar motions have a very characteristic signature," Tsagas wrote in an email. "Observers will 'measure' slightly faster acceleration in one direction and slower in the opposite, as a result of their own drift motion alone."
To see why, imagine swimming in a river: If you're swimming with the current, you move faster than when you're trying to swim upstream or across the river. Similarly, our galactic bubble is also "swimming." Tsagas argues that this is why we perceive the expansion of space-time as faster in one direction — the direction of our motion — than any other. [Does the Universe Have an Edge?]
Kashlinsky, the cosmologist who discovered dark flow, said Tsagas' theory is interesting, but that it might not yet explain everything we observe. "In general, I find this to be an interesting idea. But I am skeptical that it can account for many other observations such as the spatial distribution of the cosmic microwave background anisotropies or the observed pattern of galaxy clustering among others," Kashlinsky said. "Still, it'd be interesting to see how — or whether — these observations can be accounted for by models such as proposed in [Tsagas'] paper."
In response to these points, Tsagas replied: "There should be no extra effects on the CMB, since the very large-scale kinematics [motions] remain essentially unaffected [by my theory]. There might be some small effects on galaxy clustering, but one needs to look into it to make sure."
Dominik Schwarz, a cosmologist at the University of Bielefeld in Germany who also studies cosmic expansion, finds Tsagas' theory plausible, and believes local or "peculiar" accelerations really could obscure our measurements of the global behavior of the universe. "The task for the community will be to find out how to distinguish these peculiar accelerations on large scales from an acceleration of the global expansion," Schwarz said. If we can do that, he said, we can determine if there really is a global acceleration at all.
Cosmologist Dejan Stojkovic of the University of Buffalo, who has found evidence that calls dark flow into question — or at least dark flow as fast as that measured by Kashlinsky — said: "If the dark flow of that magnitude is real, then Tsagas is pointing out that it could trick us into thinking the universe is accelerating. This is plausible."
In short, Tsagas may have shown that the universe either has dark flow or dark energy, but not both. Dark flow is by far the less mysterious of the two: While no one knows what dark energy is, or how we might find it, dark flow is merely movement.