“I AM very pleased and very surprised. It was a pretty radical idea and I was pretty much a novice in cosmology. I was quite nervous that the whole thing could just blow up and fall apart. But now it’s just a matter of filling in the details.”
That was Alan Guth’s reaction when the Wilkinson Microwave Anisotropy Probe’s results were released in February 2003. More than 20 years earlier, Guth had come up with the idea that the universe went through a brief period of extremely rapid expansion shortly after the big bang. According to the WMAP data gleaned from the cosmic microwave background radiation left over from the big bang, Guth’s idea – known as inflation – was spot on.
WMAP measured the way the temperature of the microwave background radiation varies across the sky. It wasn’t the first such measurement: essentially, WMAP confirmed the results of projects such as MAXIMA and Boomerang – but with much more sensitive instruments, and thus more accurate results.
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Much of WMAP’s data is summarised in just one innocuous-looking graph (see below) on which the temperature difference between pairs of points is plotted against the angle between them on the sky, producing what is known as a “power spectrum”. The temperature variations reflect the way energy and matter were clumped in the early universe. Inflation predicts how the spectrum should look, and the prediction matches WMAP’s observations closely.
Blown apart
Nevertheless, researchers are still in the dark about what actually caused inflation. Some theorists ascribe it to a hypothetical particle called the “inflaton”. Others talk about an energy field that blew the universe apart. But no one is really any the wiser. And there are many different versions of inflation, each of which produces a universe with a slightly different power spectrum. Guth, for example, says he now strongly believes that inflation is producing new and different universes all the time, an idea called “eternal inflation”. So far, the data is not precise enough to distinguish between the various models.
The key to making that distinction may lie in the “spectral index”. It is roughly a measure of the overall slope of the power spectrum curve. Basic inflation theory suggests it is almost, but not quite, 1. Other variations of the theory give slightly different values. WMAP measured the spectral index at 0.99 ± 0.04, but as the data improves and pins down the figure further it should be possible to start ruling some of these variations in or out.
But it has to be said that the basic idea of inflation is still not home and dry. When the temperature differences are compared across the largest distances, the WMAP data falls slightly below the theoretical prediction (see Graph). As yet, there is no explanation for this. The mismatch is disturbing because it suggests there could be something fundamentally different about the universe on the largest size scales. It may behave differently over huge cosmological distances, or it may simply be finite.
This has been seen in other experiments too. In February this year, the CBI team working with the Atacama radio telescope in Chile reported that the temperature difference between nearby points on the sky tends to be less than between points that are further away, and falls again for points that are even more distant. This does not fit with inflationary theory, which predicts the same temperature differences no matter how far away the points are.
A second team taking data from the Very Small Array (VSA) in the Canary Islands saw the same effect. Although neither result is convincing by itself, says WMAP team member David Spergel of Princeton University, it is intriguing that more than one analysis is seeing the same thing. “It’s suggesting that something is going on,” he says.
One question mark over the independence of the two studies is that both reached their results by combining their own data with data from WMAP, which has its own hint that the universe’s temperature variations might be different on different scales. But the CBI and VSA teams point out that the effect increases when their data is added in. And in a fourth analysis, Uros Seljak’s team from Princeton University saw something similar when they combined data from WMAP with data from 3000 quasars in the Sloan Digital Sky Survey.
Unappealing choice
All four teams continue to collect data. If the results are confirmed, cosmologists face a choice between two unappealing alternatives. One is to come up with a new theory that explains why the universe’s expansion looks different at different scales. “We don’t have a theory like that,” says Dick Bond of the University of Toronto in Ontario, Canada, who worked on data from the Cosmic Background Imager (CBI) at Atacama in Chile. The other is to fine-tune inflation by arranging for the field that causes it to fluctuate in just the right way. But without any new ideas to explain why the field should do this, making such a change wouldn’t be convincing. “If you are willing to just put something in, you can fit whatever you want,” says Mathias Zaldarriaga of Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
So, though inflation looks good, it doesn’t look perfect. Indeed, University of Cambridge cosmologist Stephen Hawking says he believes inflation “has serious flaws”. He claims inflationary models are hopelessly inconsistent and possibly irrelevant. Even if inflation works, it won’t tell us why the universe is as it is, Hawking says. Because inflation has no defined link to what kicked off the universe, he believes it has nothing to say about the fundamentals of physics. “It simply shifts the problem from 13.7 billion years ago to the infinite past,” he says.
Proof of the existence of gravity waves is the next big test for inflation. According to Einstein’s theory of general relativity, these travelling warps in space-time are produced when large masses are strongly accelerated. Although they have not been detected directly, most scientists are convinced they exist, and experiments have been set up to look for them (see “Grappling with gravity”).
During inflation, powerful gravitational waves should have been wrenched into existence. Gravitational waves would have passed unaffected through the primordial fireball, so they could be carrying a signal from the dawn of time. They would also be a conclusive means of settling the arguments with inflation’s chief remaining rival, the cyclic universe (see “Cosmic trigger”), which predicts there will be no gravity waves.