Astrophysicists interrogate one of their most successful theories

By Alexandra Witze

Web edition: July 13, 2012
Print edition: July 28, 2012; Vol.182 #2 (p. 20)

Ask any astronomer what inflation is, and you’ll hear about the moment when the universe’s primordial fireball expanded like a balloon on steroids, smoothing and flattening its initial wrinkles before it grew into the cosmos seen today.

Now, some physicists are trying to let a little air out of that scenario.

Generally regarded as one of the most successful theories about the early universe, inflationary cosmology is not exactly under attack. But a few scientists are questioning whether it deserves its reputation as completely untouchable. Inflation may be the best-developed explanation for many features seen in the modern universe, these researchers say, but it still has problems.

“The picture doesn’t really hold together,” says Paul Steinhardt, a theoretical physicist at Princeton University. “Either inflation needs a major overhaul or we have to think about some other approach to cosmology.”

In a paper posted online at arXiv.org in April, physicist Robert Brandenberger of McGill University in Montreal argues that scientists should continue exploring alternatives to inflation rather than just taking for granted that it’s right.

One such alternative, developed over the last decade, holds that the universe may not have begun with a single Big Bang, but rather experiences cycle after cycle of contraction and expansion. Another approach posits a world with a collection of tiny vibrating strings whose movements generate cosmic features currently explained by inflation.

Within the next few years, telescopes may collect enough data to distinguish among the options. Only then, say the inflation agnostics, will the picture hold together or fall apart.

“We really don’t know what happened in the early universe,” says Jean-Luc Lehners, a cosmologist at the Max Planck Institute for Gravitational Physics in Potsdam, Germany. “We know what the result was, but we don’t know how the universe got there.”

Those on both sides of the issue are quick to point out that inflation could turn out to be right. Inflation, says Andrei Linde of Stanford University, is “the only presently existing internally consistent theory of the early universe.” Linde developed some of the first versions of inflation, and thinks those who question it are either intellectually off-course or led astray by journalists looking for a story. “It is quite possible that eventually this theory will be generalized and extended,” he says. “But so far all attempts to replace it by something better failed.”

The big balloon

In 1981 Alan Guth, now of MIT, proposed inflation and showed that it could explain two mysteries about the universe: why it is so smooth and why it is so flat.

Distant reaches of the cosmos look very much alike, even though they are too far apart to have had any contact after the universe was born in the Big Bang, 13.7 billion years ago. But if the infant universe had ballooned outward ridiculously quickly before slowing to a more leisurely expansion rate, this inflation would have smoothed out primordial disorder differentiating one region of space from another. Just a minuscule fraction of a second of inflation would have spread matter out uniformly except for tiny clumps — fluctuations in the background density — to serve as seeds around which the first protogalaxies could grow.

Such an “inflationary” period would also put the universe into the finely balanced state seen today, in which space on large scales seems very close to flat.

In 1992, the Cosmic Background Explorer satellite, or COBE, confirmed a key part of Guth’s idea by measuring slight fluctuations in the leftover heat from the Big Bang. Later data from the Wilkinson Microwave Anisotropy Probe (WMAP for short) brought those cosmic fluctuations into even greater focus. Astronomers began talking about the arrival of an era of precision cosmology, in which detailed observations produced hard numbers that supported inflation.

But others aren’t so sure. “We can’t count the fact that our calculations agree with current observations as a success,” says Brandenberger.

He has several problems with inflation. For starters, its math doesn’t mesh nicely with emerging notions of particle physics; inflation doesn’t play well with ideas like string theory that attempt to unify quantum mechanics with general relativity.

Another problem, he says, is that inflation requires density fluctuations in the infant universe to have wavelengths smaller than the Planck length, below which regular notions about space break down. “This is not to say that the calculations are wrong, but the calculations are extrapolations into regions where we cannot trust them,” Brandenberger says.

Other recent work attempts to deal with new problems that inflation created. One phenomenon of concern is eternal inflation — inflation that never stops.

Guth’s original concept called for inflation to end after a fraction of a second. But Steinhardt and others soon discovered that inflation would continue forever in a few rare spots, spawning rogue areas that went on ballooning. “That now turns the story inside out,” Steinhardt says. “Instead of most of the universe being like us, most of the universe is inflating.”

With eternal inflation, an infinite number of “pocket” universes can pop into existence. And in a universe where anything that can happen will happen an infinite number of times, it becomes impossible to determine what events are more or less likely.

Guth himself has wrestled with that last point, known as the “measure problem.” In a paper posted at arXiv.org last year, he and Stanford’s Vitaly Vanchurin describe efforts to define probabilities of events in an eternally inflating universe.

Not understanding eternal inflation doesn’t mean inflation is wrong, though. “Many cosmologists, including me, believe that eternal inflation is the almost unavoidable consequence of our best understanding of the fundamental laws of physics,” says Guth — meaning even alternative theories would have to cope with eternal inflation somehow.

Other pasts

Of alternative ideas, the one with the most traction comes from scientists including Steinhardt and Neil Turok, now of Canada’s Perimeter Institute. It involves cycles of contraction and expansion (SN: 9/22/01, p. 184).

In this “cyclic scenario,” the Big Bang isn’t the beginning of space and time, but simply a transition from an earlier period in which the universe was contracting. It gets around the eternal inflation problem by smoothing out matter clumps during contraction. Any rogue areas are thus shrinking and don’t become a problem.

Only after a period of contraction does the universe reverse itself and expand outward, so that astronomers today see distant galaxies rushing away at an accelerating rate. Yet this universe has its own problems, most notably that researchers can’t properly describe the change from contraction to expansion.

Other alternatives to inflation include the “matter bounce” — which also relies on a switch from a contracting to an expanding universe, but using different mathematics. Brandenberger’s favorite, “string gas cosmology,” calls for a gas of tiny vibrating strings in the early universe, rather than a gas of particles, and thus meshes with string theory, he says.

Still, most scientists say inflation remains a much stronger candidate than any of the other proposals. “All these alternative models are not justified either by observations or theoretically,” says Viatcheslav Mukhanov of Ludwig Maximilians University Munich.

Ongoing experiments should reveal whether inflation will triumph in the end. Several efforts are now looking for a sign of inflation called gravitational waves. These disturbances ripple through spacetime from violent cosmic events like colliding black holes — or the Big Bang. Other clues may come from the European Space Agency’s Planck satellite, launched in 2009 to build on the success of COBE and WMAP (SN: 4/11/09, p. 16). Planck is hunting for another subtle imprint on the cosmic microwave background, with initial results expected next spring.

One final approach may be to bundle the alternatives to inflation together. Lehners, for instance, has been working to combine eternal inflation with the cyclic universe. Each pocket universe created by eternal inflation, he says, could replay the cyclic scenario over and over again, in a sort of best of both worlds.

In the end, physicists will undoubtedly keep exploring both inflation and its alternatives, and the final solution may lie somewhere in between. “It’s really harmful,” says Lehners, “to assume that we know what the answer is going to be.”