Does the Universe go on forever?

October 08, 2003

PERPLEXING observations beamed back by a NASA spacecraft are fuelling debates about a mystery of biblical proportions- is our universe infinite? This week, a team of scientists announced tantalising hints that the universe is actually relatively small, with a hall-of-mirrors illusion tricking us into thinking that space stretches on forever.

However, work by a second team seems to contradict this, and scientists are now busy trying to resolve the conundrum. "Whether space is finite is something people have been asking since ancient times, and probably before that," says mathematician Jeffrey Weeks from Canton, New York. "If we resolved this and confirmed that space is finite, this would be an enormous step forward in our understanding of nature."

At the centre of the debate are observations by NASA's Wilkinson Microwave Anisotropy Probe (WMAP), which was launched in 2001. The probe measures temperature ripples in the "cosmic microwave background", the afterglow radiation from the big bang fireball.

Astronomers are interested in how strong different sizes of ripples are, as this reveals vital information about the early universe, and might tell us how big the universe is today. Many astronomers suspect that the universe is infinite.

In that case, the microwave background ripples should have an unlimited range of sizes. But while WMAP's observations of small-scale ripples have matched predictions for an infinite universe almost perfectly, the large-scale measurements have not.

On the largest scales, WMAP has shown that the ripples almost disappear (see Graphic, top). It could just be down to chance. Computer simulations that model the birth of the microwave background create this pattern once every few hundred runs. But it could mean that space itself is not big enough to support the broadest ripples, according to Weeks.

"Just as the vibrations of a bell cannot be larger than the bell itself, any fluctuations in space cannot be larger than space itself," he says.

Weeks and his colleagues, a team of astrophysicists in France, say the WMAP results suggest that the universe is not only small, but that space wraps back on itself in a bizarre way (Nature, vol 425,p 593).

Despite being finite, the universe wouldn't have any kind of edge. If a spacecraft blasted off in what we'd perceive to be a straight line- the line a beam of light would follow- it would eventually end up back where it started. Because of this odd wraparound effect, the light from one galaxy could follow two different routes to the Earth, so the same galaxy would appear in two different parts of the sky.

Effectively, the universe would be like a hall of mirrors, with the wraparound effect producing multiple images of everything inside. According to Weeks, the WMAP results point to a very specific illusion- that our universe seems like an endlessly repeating set of dodecahedrons, football-like shapes with a surface of 12 identical pentagons.

If you exit the football through one pentagon, you re-enter the same region through the opposite face and you keep meeting the same galaxies over and over again (see Graphic, bottom). Weeks says the match between the predictions of his repeating-football model and the WMAP observations is striking: "I was just blown away, the results are far better than I could have imagined." If confirmed, they would indeed be stunning. They would mean that the universe is relatively small, something like 70 billion light years across.

What's more, we could in theory see the entire cosmos and check that there are no hidden corners where the laws of physics are different. For instance, it would rule out exotic ideas such as chaotic inflation, which suggests our local universe is just one of myriad expanding bubbles beyond eyeshot, each with slightly different physical laws. It would also banish the philosophical paradoxes of an infinite universe, such as the idea that every person on Earth has an infinite number of alien doubles leading parallel lives.

"If we could prove that the universe was finite and small, that would be earth-shattering," says David Spergel of Princeton University in New Jersey. "It would really change our view of the universe." However, in response to Weeks's report, Spergel and his colleagues have announced evidence that contradicts the findings.

They showed previously that if the universe does produce a hall-of-mirrors effect, it should be possible to find a pattern of matching circles in the microwave background around which the fluctuations are identical (New Scientist, 19 September 1998, p 28).

Weeks's theory predicts six specific pairs of matching circles in the sky, but Spergel's team has had no luck finding them in WMAP data. "Weeks's team has a very powerful model that's nice because it makes a very specific prediction about the pattern we should see on the sky," says Spergel. "However, we've looked for it, and we don't see it."

Spergel and his team are now working with Weeks to see if they might somehow have missed the circles. And there is a further test of the dodecahedron model. It predicts that a key measure of the density of matter in the universe, which governs its curvature, is equal to 1.013.

Completely flat space corresponds to 1, while values greater than or less than 1 would create a curved universe. Observations of the microwave background radiation so far suggest the value lies somewhere between 1.00 and 1.04. Further observations by WMAP and other instruments should give a more accurate answer within the next few months.

In the meantime, Spergel's team is continuing to use the matching-circles technique to see whether the universe might be small and finite but with some other possible shape. Computers at two universities and at NASA's Goddard Space Flight Center are scanning the WMAP results for all the possible patterns of circles that might exist on the microwave background.

"We're burning up a lot of supercomputer cycles on this," says Spergel's colleague Neil Cornish of Montana State University in Bozeman. Cornish says his team believes it has already ruled out almost half of the possible small-universe shapes- including football and doughnut shapes- and he suspects the work will probably turn up nothing, meaning that the universe is either very large or infinite.

"We're disappointed because we favoured the small-universe idea," says Cornish." But I guess you've just got to take the universe you're given."
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New Scientist issue: 11th October 2003

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Author: Hazel Muir

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New Scientist

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