DAILY NEWS 31 August 2007 ‘Swiss cheese’ universe challenges dark energy By Anil Ananthaswamy

Dark energy may not be needed to explain why the expansion of space appears to be speeding up. If our universe is like Swiss cheese on large scales – with dense regions of matter and holes with little or no matter – it could at least partly mimic the effects of dark energy, suggests a controversial new model of the universe.

In 1998, astronomers found that distant supernovae were dimmer, and thus farther away, than expected. This suggested the expansion of the universe was accelerating as a result of a mysterious entity dubbed dark energy, which appears to make up 73% of the universe.

But trying to pin down the nature of dark energy has proven extremely difficult. Theories of particle physics suggest that space does have an inherent energy, but this energy is about 10120 times greater than what is actually observed.

This has caused some cosmologists to look for alternative explanations. “I don’t have anything against dark energy, but we ought to make all possible efforts to see whether we can avoid this exotic component in the universe,” says Sabino Matarrese of the University of Padova in Italy.

So he and colleagues, including Edward Kolb of the Fermi National Accelerator Laboratory in Batavia, Illinois, US, decided to model the universe as having large-scale variations in density.

That contradicts the standard model of cosmology, which assumes that the universe is homogeneous on large scales. In the homogeneous model, known as the Friedmann-Robertson-Walker (FRW) universe, the effect of dark energy is to stretch space, thus increasing the wavelength of photons from the supernovae.

Testing assumptions

A similar effect was seen when the researchers added large-scale spherical holes to the FRW universe. They allowed the density of matter within each hole to vary with radius and found that in certain cases, photons travelling through under-dense voids had their wavelengths stretched, mimicking dark energy.

The extent of the effect depends on the exact location of the supernovae and how many under-dense regions the photons have to cross before reaching Earth. And Matarrese cautions that the deviations are not enough to explain away all of the observed dark energy. He says their model is still very preliminary: “We are very far away from getting the full solution.”

Cosmologist Sean Carroll at Caltech in Pasadena, US, says the Swiss-cheese model is interesting and useful as a test of more mainstream theories. “The overwhelming majority of cosmologists think that the completely smooth approximation is a very good one,” Carroll told New Scientist. “But if you want to have confidence that you are on the right track, you better not just make assumptions and cross your fingers, you better test it.”

Up for debate

Astrophysicist Niayesh Afshordi of Harvard University in Cambridge, Massachusetts, US, is less impressed. Astronomical observations suggest that the density of matter in the universe is relatively smooth – and not like Swiss cheese – at scales of about 100 million light years or larger, he says. The new research, however, suggests that space is holey on scales of 500 million light years.

“The model is very inhomogeneous on scales that we observe as homogeneous,” Afshordi told New Scientist. “What we can learn from the toy model is not really applicable to this universe, because the properties of this model are very different what we see in our universe.”

However, team member Antonio Riotto of the University of Geneva in Switzerland argues that their Swiss-cheese model is realistic in the sense “that the universe is characterised by under-dense regions”.

“We know that the universe has voids, you can debate about their size,” he says. In fact, recent observations suggest that voids can extend across nearly a billion light years. Riotto says their model was worked out independently of that discovery, but “this observation is welcome by us”.

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