Is dark energy an illusion?

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Over time, matter pulls together into a web-like structure of 'superclusters' of galaxies, leaving ever larger voids behind. The greater rate of expansion in the voids – which make up a larger fraction of the universe's volume – may account for the observations usually attributed to dark energy (Illustration: Center for Cosmological Physics/U Chicago)

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• Syksy Rasanen, CERN
• Niayesh Afshordi, CfA
• 2-degree Field Galaxy Redshift Survey

The quickening pace of our universe's expansion may not be driven by a mysterious force called dark energy after all, but paradoxically, by the collapse of matter in small regions of space.

Astronomers were astonished to discover in 1998 that the expansion of the universe is happening at an ever-increasing rate. The mysterious repulsive force responsible for this was dubbed dark energy, though scientists still do not know what it is (see Dark energy: seeking the heart of darkness).

Now, physicist Syksy Rasanen of CERN in Geneva, Switzerland, says we might not need dark energy after all. As counter-intuitive as it sounds, the increasing rate of expansion might be due to the collapse of small regions of the universe under gravity, he says.

Gravitational brake

Cosmologists have long assumed that the overall expansion of the universe is not affected by the properties of small regions within it, since these properties should average out on the largest scales.

But in any given region of space, the force of gravity between bits of matter acts as a brake on expansion. This means that expansion should slow down quickly in regions with lots of matter, while continuing without much change in mostly empty regions.

It is this difference in the expansion rate between different regions that could produce the illusion of dark energy, Rasanen says. Strangely enough, even though the expansion rate decreases or stays about the same in every region, the average rate of expansion for the universe as a whole can increase.

This is because over time, denser regions suck even more matter into them by the force of gravity, which further empties the less dense regions. This process has led to the condensation of matter into galaxies and clusters of galaxies, with ever larger voids in between.

Unchecked expansion

Although the expansion slows down more and more in the dense regions, these regions become less and less important to the overall expansion of the universe, because they make up a smaller and smaller percentage of its volume.

In the voids, by contrast, the expansion continues relatively unchecked, so that they become a larger and larger part of the universe's total volume.

As the influence of the dense regions declines, the average expansion rate of the universe actually increases, even though there is no repulsive dark energy at work. "I find this to be a very plausible explanation for accelerated expansion," Rasanen says.

Stark differences

But Niayesh Afshordi of the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Massachusetts, US, says although the idea is an attractive possibility, it is "very unlikely".

In order for the effect of density differences to be important enough to notice, they would need to be very pronounced on large scales of tens to hundreds of millions of light years.

But some measurements show the density differs by just 0.001% on such large scales, Afshordi says. He cites data from surveys that measure the density of different regions by the way their gravity bends light.

Rasanen counters that 3D maps of galaxies, such as the 2-degree Field Galaxy Redshift Survey, show differences much closer to the minimum 20% level needed to account for the cosmic acceleration.

Open question

Rasanen admits that he has worked out only a simplified theoretical description of the effect of the differing expansion rates in voids and dense regions.

But until this has been determined, he argues that astronomers cannot say for sure that the accelerating expansion is due to dark energy. "There's no doubt that this question has not been solved," he told New Scientist.

If the density differences really do have an important influence on the universe's expansion, then future measurements of the expansion rate based on supernova surveys might show different rates of expansion in different directions, he says.

Rasanen presented his work on Wednesday at a cosmology conference called "Outstanding Questions for the Standard Cosmological Model" in London, UK.