How big is the universe? It is one of the oldest questions in science, and the answer could be anything from "slightly bigger than the area of the universe that we can see" to "infinite".

Until now. Cosmologists scrutinizing patterns in the microwave radiation 'afterglow' of the big bang have taken a big chunk out of that uncertainty. They calculate that the universe cannot possibly be smaller than a hefty 78 billion light years across.

That rules out earlier suggestions that the universe could be a relatively small shape wrapped around itself. A recent suggestion that the cosmos could be shaped like a soccer ball¹, for example, would have meant that the universe was just 60 billion light years across.

"There is not much room left for the small universe hypothesis," says Neil Cornish, a physicist from Montana State University, Bozeman, who led the study, due to be published in Physical Review Letters^2,3.

If the universe were relatively small, it would not necessarily be that obvious because it would not have to have an edge. Space could be wrapped in on itself, like a video game where characters disappearing off one side of the screen instantly reappear on the other.

If that were the case, light from a distant object would be able to reach us along more than one path, just as one could travel from the North Pole to the South Pole along any number of different straight paths around our planet's curved surface. So we should be able to see light from the same object arriving from apparently different directions.

"In principle, it would not be ridiculous to see light from the Earth that has wrapped around the Universe, so we could see the Earth as it was when, say, life formed 4 billion years ago," says Cornish.

To test whether light was being wrapped around in this way, Cornish and his team analysed data from NASA's Wilkinson Microwave Anisotropy Probe (WMAP), which detects microwave radiation from just 379,000 years after the universe began.

If light from the same object was arriving from different directions, the researchers calculated that this should produce circular patterns of hot and cold spots in the radiation.

"But we did not find any statistically significant circle matches," says Cornish. He concludes that the universe must be larger than 78 billion light years across, much larger than the 28 billion light years or so that we can see with our telescopes.

Cornish believes that further observations by WMAP may push that minimum size limit up to about 90 billion light years. The probe lies 1.5 million kilometres from Earth, where it can detect temperature differences of just 20 millionths of a degree in the microwave background radiation.

References

1. Luminet, J.-P. et al. Nature, 425, 593 - 595, doi:10.1038/nature01944 (2004). |Article|
2. Cornish, N. J., Spergel, D. N., Starkman, D. N. & Komatsu, E. Preprint, http://arxiv.org/abs/astro-ph/0310233 (2004). |Article|
3. Cornish, N. J., Spergel, D. N., Starkman, D. N. & Komatsu, E. Phys. Rev. Lett., in press, (2004).

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