Published online 15 April 2008 | Nature | doi:10.1038/news.2008.755


Deflating inflation?

A controversial analysis questions the standard model of the early universe.

Could the Big Bang have come not at the beginning of the universe, but after a long, slow period of shrinkage?

That's one theory bolstered by a new analysis of the Big Bang's afterglow, which shows that the early universe did not inflate with the smoothness that many theorists expected.

The earliest days of the universe may not have been a smooth ride.Hubble Deep Field/NASA

“The standard, canonical models will be ruled out if this holds,” says Amit Yadav, an astronomer at the University of Illinois at Urbana-Champaign. “The simplicity is gone.”

Yadav presented his result on 13 April at the American Physical Society conference in St. Louis, Missouri. His paper1, written with his adviser Benjamin Wandelt, has been accepted for publication in Physical Review Letters.

In the beginning

The provocative result suggests that models of inflation – a furious hyperexpansion in the instant after the Big Bang, 13.7 billion years ago – have to be much more complicated than previously thought, or else that inflation never occurred at all and that the Big Bang came after a period of contraction. “The impact will depend on whether the result gets verified,” says Paul Steinhardt, a theorist at Princeton University in New Jersey.

If the result sticks, he says, it would be the first time that one of the predictions of simple inflation failed. And it could also lead to a radical reinterpretation of what the Big Bang was and whether it marked the universe's beginning. “There's a lot at stake here.”

Standard, simple inflation – needed to achieve a flat, smooth universe – holds that, just after the Big Bang, a uniform negative gravitational field drove a brief period of accelerated expansion. Then the field died out, creating the matter and energy known in the universe today and leaving an afterglow of microwave radiation just a few degrees above absolute zero. If simple inflation theory is right, this imprint should be almost, but not quite, perfectly gaussian – a pattern with smooth-looking noise.

The Illinois researchers measured the degree of gaussianity in a map built with data from the Wilkinson Microwave Anisotropy Probe (WMAP), a satellite launched in 2001 to measure the afterglow, which is known as the cosmic microwave background (CMB). WMAP has already made many measurements that mesh well with simple inflation. But the analysis of Yadav and Wandelt, which took months of supercomputer time, showed that the map was not at all gaussian.

The key is how confident the researchers are in making that statement. They claim that they are 99.5% certain that the CMB is not gaussian. But Steinhardt says he has seen many purported discoveries, supposedly this certain, turn out to be nothing more than noise. Most cosmologists and particle physicists demand higher levels of confidence.

And Eiichiro Komatsu, a cosmologist at the University of Texas at Austin and a member of the WMAP science team, recently published a similar analysis with fresher data that did not draw the same conclusion. 2 “We don't see any compelling evidence yet that non-Gaussianity exists. So simple inflationary models still fit the data pretty well,” he says.

The WMAP team used Yadav's computational method, but on a CMB map constructed from five years of data rather than three. But they also discarded more data associated with noise from the Milky Way galaxy, which meant more uncertainty in their final number. And so the WMAP team's results can be explained as supporting either a gaussian or non-gaussian scenario.

Komatsu, Yadav and others interested in this crucial question will refine their computational models and await the completion of the next stage of WMAP data collection. The next version of the map, based on seven years of data, is due to be finished in August.


And in October, the European Space Agency plans to launch Planck, a mission that will measure the gaussianity an order of magnitude more precisely. Until then, theories for the early universe might remain unsettled, with simple inflationary theorists defending their turf against the radical insurgents.

Steinhardt says he'll remain agnostic during this exciting time. “But to be honest, I want to know which one's right. It's science, not a sports event.” 

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