Published online 2 November 2007 | Nature | doi:10.1038/news.2007.215

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Bursting dark energy's bubble

Theorist suggests mysterious force could be an 'artefact' of a void in space.

A chart of the density of our Universe. Different colours have been used to show different types of galaxies; the black spaces show low-density gaps.Sloan Digital Sky Survey Team, NASA, NSF, DOE

It has been called one of the greatest discoveries in modern cosmology, and most astronomers believe that it makes up more than two-thirds of the stuff in the Universe. Now one theorist is suggesting that dark energy might be little more than a mirage.

Subir Sarkar, a theoretical physicist at Oxford University, UK, has written a treatise that suggests that dark energy, a mysterious force that seems to be pushing the Universe apart, might actually be the result of an enormous bubble of empty space around our galaxy. The paper is available on the physics preprint server ArXiv1.

Sarkar says he has floated the idea to encourage astronomers to make more careful measurements of the rate of expansion of our Universe using tools already available, before being so certain of the existence of dark energy. "I'm simply concerned that people are not doing mundane checks they ought to be doing before making these claims," he says. "People seem to be more concerned about planning mega-projects for the future," he says.

Dark force

Dark energy was first measured in 1998, when two teams of astronomers discovered that certain kinds of supernovae appeared dimmer than they should. If these supernovae were as far away as they should be based on calculations of the acceleration of the Universe, then they ought to have looked brighter. Instead they seemed to be farther away than theory would allow.

The findings could be explained if dark energy was pushing the Universe apart.

In combination with evidence from the cosmic microwave background — the faint afterglow of the Big Bang — theorists have estimated that this energy must make up roughly 70% of the stuff in the Universe.

But Sarkar says that the astronomical community has been too quick to accept dark energy, which has continued to defy astronomer's attempts to characterize it (see A constant problem). As an alternative hypothesis, he suggests that our Galaxy might lie in a huge bubble of comparatively empty space. The denser area of space outside this bubble would then pull material towards it. To an observer inside the bubble, it would seem as though a dark-energy-like force was pulling the Universe apart.

Hubble bubble

Small bubbles of low density are known to exist in the Universe, and researchers have previously considered that a local 'Hubble bubble' around the Milky Way might explain the supernovae results, says Adam Riess, an astronomer at Johns Hopkins University in Baltimore, Maryland. But measurements have shown that a small, local bubble would not have much of an effect on dark energy's strength, he says. "We put in the void, took the void out, and it doesn't change the answer," he says.

Sarkar is proposing a much, much bigger void — about 800 million light years in radius. A void of that size would be extremely unusual. But Sarkar notes that astronomers earlier this year found what could be a 450-million-light-year radius bubble about 6 billion light years away2. That's still smaller than what Sarkar proposes, but it's the right order of magnitude.

"Maybe the Universe is not really as smooth as people believe," he says.

Double check

Sarkar says that his theory could be easily checked by making more extensive measurements of the Hubble constant, which describes the rate of expansion of the Universe. If we are in a low-density bubble, this 'constant' should be unusually large in our vacinity, and have a smaller value farther away from us.

Current measurements of the Hubble constant are variable from one place in the sky to another. But researchers cannot tell if this variation is due to error in the measurements, or is something real.

Riess takes issue with a number of Sarkar's interpretations of data, and says such a huge bubble is very unlikely. Both agree that better measurements are called for. "We should do the easiest things first that inform the cosmology the most," Riess agrees. He says his team is now making measurements to help reduce the Hubble constant's uncertainty. 

  • References

    1. Sarkar, S. et al. arXiv , arXiv:0710.5307
    2. Rudnick, L. , Brown, S. & Williams, L. R. arXiv, arXiv:0704.0908v2
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