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See Inside Scientific American Volume 310, Issue 5

Proof that the Universe Inflated Rapidly After the Big Bang

Traces of primordial gravitational waves could tell us how and when the early universe went through its precipitous expansion


BLOWING ITSELF UP: A rapidly expanding universe spawns gravitational waves that stretch and compress spacetime.


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Score one for inflation. The idea that the universe ballooned rapidly after the big bang received a boost in March, when physicists confirmed a prime prediction of inflation theory. The Background Imaging of Cosmic Extragalactic Polarization 2 (BICEP2) experiment at the South Pole found evidence for primordial gravitational waves, ripples in the fabric of space and time, that were created when the early universe swelled. The discovery is not just a major validation of inflation, physicists say, but a good way to narrow down the many possible versions of inflation that might have taken place. “This really collapses the space of plausible inflationary models by a huge amount,” says Marc Kamionkowski of Johns Hopkins University, who was not involved in the discovery but who co-predicted back in 1997 how these gravitational-wave imprints could be found. “Instead of looking for a needle in a haystack, we'll be looking for a needle in a bucket of sand.”

BICEP2 found a pattern called primordial B-mode polarization in the light left over from just after the big bang known as the cosmic microwave background. This pattern, basically a curling in the polarization, or orientation of the electric field, of the light, can be created only by inflation-induced gravitational waves. “We've found the smoking-gun evidence for inflation, and we've also produced the first image of gravitational waves across the sky,” says Chao-Lin Kuo of Stanford University, who designed the BICEP2 detector and co-leads the collaboration.

Such a groundbreaking finding requires confirmation from other experiments to be truly believed, physicists say. Nevertheless, the result was heralded as a huge win for cosmology. “There's a chance it could be wrong, but I think it's highly probable that the results stand up,” says Alan Guth of the Massachusetts Institute of Technology, who first predicted inflation in 1980.

Physicists are now parsing the finding for clues about the timing and details of inflation. The BICEP2 measurement suggests that inflation began a trillionth of a trillionth of a trillionth of a second after the big bang, a time when the universe would have been so energetic that all the fundamental forces of nature—the electromagnetic, strong and weak forces, with the exception of gravity—might have been unified into a single force. The new results could also quell any remaining doubters of inflation. “If this discovery is confirmed,” says Andrei Linde of Stanford, one of the main authors of inflation, “inflationary theory does not have any real alternatives.”

FURTHER READINGS AND CITATIONS ScientificAmerican.com/may2014/advances

This article was originally published with the title "Our Inflated Universe."

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