A new test that takes data from several realms of physics could explain what really happened in the first sliver of a second after the Big Bang.

Most cosmologists believe the universe burst from an extremely dense, hot state around 13.7 billion years ago, and has been expanding and cooling ever since. The universe ballooned ridiculously fast in its first moments, doubling in size thousands of times in less than a trillionth of a trillionth of a second.

"That would take a region the size of an atomic nucleus or a proton, and stretch it to a size exponentially greater than our observable universe at present," said cosmologist Paul Steinhardt of Princeton University. "Superlatives are not enough here. Incredible, remarkable, unbelievable amounts of stretching."

This idea, known as inflation, is the most popular theory for explaining why the universe looks the way it does. But so far, no one has proved it.

"At the moment it's our best theory, but it could be literally on the wrong track," said Latham Boyle, a cosmologist at the Perimeter Institute in Canada. "It's important to remember that it's not a fact."

In a paper published Dec. 6 in Physical Review Letters, Boyle and Steinhardt show how a cluster of unrelated observations could clinch the case for inflation.

"You take two completely different sets of measurements," Steinhardt said. "If those two numbers match, either that's a remarkable coincidence, or inflation was the cause. This is the new test that we're introducing."

Cosmologists dreamed up inflation in the 1980s to account for some weird coincidences that the original Big Bang theory, which assumed the universe expanded at a relatively slow, constant rate, couldn't explain. The universe looks nearly the same in every direction, even in regions so distant from each other that they shouldn't know about each other. The time for light to travel from one point to the other is longer than the age of the universe.

"Why would you expect two regions of the universe to have identical properties if they never had a chance to communicate with one another?" said Steinhardt, who was one of the original authors of inflation theory. "Before inflation, the only thing you had to say was, 'I don't know the answer, but we have to suppose it is so.'"

Inflation offered an explanation: Those two distant points in the universe started out next door to each other, but blew apart almost immediately. Later observations of the cosmic microwave background, the subtle glow of the first atoms to release light, fit closely with what cosmologists expected if inflation were true.

Those observations pushed inflation ahead of all the competing theories, but didn't rule the other theories out. Steinhardt himself is working on an alternative theory "where the Big Bang is not the beginning, but it's kind of a bounce." Other theories call on funny features of particle physics or extra dimensions.

In the new study, Boyle and Steinhardt suggest a way to show that inflation was really responsible for the universe's unlikely uniformity.

The key, they say, is to compare two different times when the universe stretched out: the extremely rapid stretching that happened during inflation, and the slower stretching that has been going on ever since.

During inflation, space expanded so quickly even light couldn't keep up. That means two particles sitting right next to each other before inflation would vanish from view once inflation began.

After inflation ended, though, light started to catch up. The universe kept expanding, but slowly enough that light could start making its way across the universe. Eventually, the light from those two particles traveled far enough for the particles to see each other again.

The amount of stretching the universe suffered from the instant those particles lost sight of each other until inflation ended should be exactly equal to the amount of stretching from the end of inflation until the particles were reunited, Boyle and Steinhardt point out. Otherwise, the particles would remain hidden from each other.

Conveniently, cosmologists can calculate the different amounts of stretching "using different bits of information that seem to have nothing to do with each other," Steinhardt said.

Knowing the amount of matter, radiation and dark energy in the universe gives a good estimate of the amount of expansion the universe has gone through since inflation ended.

"Once you know what it is in the present, you can extrapolate back to any time you like, and ask how much expansion has there been from then to now," Steinhardt said.

Observations of the cosmic microwave background plus data on gravitational waves (ripples in space-time predicted by general relativity) give a close sense of how much the universe expanded during inflation. Astronomers haven't detected gravity waves yet, but several observatories are already searching for them.

If the amounts of stretching turn out to be equal, "you declare victory," Steinhardt said. "Then you've really proven that inflation was the cause in a way that would be virtually impossible to explain by any other idea."

If they're not equal, though, that doesn't mean inflation is wrong. But all the competing theories would still be just as likely.

"If the test succeeds, it would eliminate the alternatives I've been thinking of, and seal the case," Steinhardt said. "If it fails, it's wide open."

"I think in retrospect these predictions will become very famous," said cosmologist Arthur Kosowsky of the University of Pittsburgh, who was not involved in the new work. "It's not going to shake up theoretical physics in the next year. But I think that in 20 years, if the real model of inflation comes out in this class of models that they write about, it will be looked back on as a very important contribution."

Image: A timeline of the universe, based on data from the WMAP probe. Credit: WMAP/NASA

See Also:

Follow us on Twitter @astrolisa and @wiredscience, and on Facebook.