THERE were no conga lines or high fives, but this gathering of cosmologists and astronomers was about as close to a celebration as physics conferences get. Seven weeks ago, when the team behind NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) released its first set of results, the landscape of cosmology changed. One of the subject’s wilder ideas – that shortly after the big bang the Universe went through a brief period of extremely rapid expansion called inflation – became a proper science.
Last week’s meeting was the first chance for cosmologists to get together and discuss the results, and the researchers who have spent decades working towards this moment were jubilant. “Inflation is now part of experimental science,” announced David Spergel of Princeton University.
Inflation was the brainchild of Alan Guth at the Massachusetts Institute of Technology. He didn’t have a mechanism for the expansion, just an argument that showed it would solve a few cosmological puzzles. Other theorists then took up the idea and made predictions about what effects inflation ought to have on the Universe.
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Now those predictions have been borne out in style by the WMAP data (żěè¶ĚĘÓƵ, 15 February, p 12). The probe measured the way the temperature of the microwave background radiation, left over from the big bang, varies across the sky (see image, top right). Much of the data is summarised in just one innocuous-looking graph (see right) on which the temperature difference between pairs of points is plotted against their angular separation, producing what is known as a “power spectrum”. The temperature variations reflect the way structure is thought to have emerged in the early Universe, and theoretical predictions for the details of this spectrum closely match WMAP’s observations. Inflation, it turns out, was rather a good idea.
No one is happier about that than Guth. “I am very pleased and very surprised,” he told żěè¶ĚĘÓƵ. “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 is no small task. For a start, researchers are still in the dark about what actually caused inflation. Theorists ascribe it to a hypothetical particle called the “inflaton”, 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. So far, the data is not precise enough to distinguish between them.
The key to making that distinction may lie in the “spectral index”, a parameter that emerged as the star of the Davis show. 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.
It may even tell us whether there are hidden extra dimensions. One of the more surprising possibilities to emerge at the conference, presented by Lisa Randall of Harvard University, was that the inflaton might live in an extra dimension. This scenario gives a spectral index of 0.96 – within the range allowed by the WMAP data. It also predicts significant gravitational wave radiation during inflation, a key ingredient of many versions of the theory.
Indeed, proof of the existence of gravity waves is the next big test for inflation. This would be the smoking gun, Michael Turner of the University of Chicago told the conference: “Gravity waves are the real holy grail in all this.” They would also be a conclusive means of settling the arguments with inflation’s chief remaining rival, the cyclic Universe, which predicts there will be no gravity waves (see Box).
Another hot topic at Davis was the “anthropic principle”, which is used to explain why the Universe seems so finely tuned to support life. This controversial argument postulates the existence of many universes with different properties, and that we just happen to be in one that allows enough complexity to create life.
Physicists have started taking the anthropic principle seriously because inflation provides a mechanism by which multiple universes could have arisen. If a tiny part of space blew up to become the whole of the Universe that we see today, it is likely the same thing also happened in other regions that we can’t see, causing any number of other universes to emerge, all with different properties and too far away for us to ever reach. As astronomer Martin Rees of the University of Cambridge put it, the fundamental laws of physics may be nothing more than “parochial by-laws in our cosmic patch”.
Leonard Susskind of Stanford University told the assembly that this anthropic argument is also emerging as an overwhelming prediction of string theory, which allows a range of different scenarios to develop.
Guth says he now strongly believes that inflation is producing new and different universes all the time, an idea called “eternal inflation”. “Any inflationary scenario that cannot eternally reproduce would seem as implausible as discovering a species of rabbits incapable of reproduction,” he told the conference.
But there is no way to test the idea, and University of Cambridge cosmologist Stephen Hawking says he believes it “has serious flaws”. Indeed, Hawking burst the party balloons early in the Davis meeting. Clearly in a combative mood – despite the “Stop the war” badge on his lapel – he claimed 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 told the assembly. 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 suggested that only “top-down” cosmology – combing through our current observations without prejudice – will yield any final answers about the nature of the Universe.
But Hawking’s onslaught didn’t dampen spirits. The only moment of self-doubt came from Spergel, who admitted he spent most of last year worrying that one particular region of the power spectrum wouldn’t fit with inflation theory. He was right: it didn’t. When the temperature differences are compared across the largest distance scales, the WMAP data falls below the theoretical prediction (see Graph). “There’s no explanation for it,” he told żěè¶ĚĘÓƵ.
The mismatch is disturbing, he says, 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. Spergel sees no way to solve the problem in the near future. “Normally, faced with such an anomaly in physics, you do another experiment. But there’s not much we can do.”
The general feeling, though, was that the WMAP team has done enough: they even worked out the error on their errors – less than two per cent. Licia Verde of Princeton, another member of the WMAP team, put the success down to an obsessive concern over minimising sources of experimental and systematic error in the equipment. She requested similar care from the cosmologists. “The rest of the analysis should have the same level of obsession,” she warned.
There were repeated promises to comply, as cosmologists rejoiced in their new status as proper scientists with data on which to base their theories. Max Tegmark of Pennsylvania State University summed up the euphoria best. During his talk he repeatedly flicked his slides back and forth between data that cosmologists had at their disposal on 11 February and what was available to them the day before. “I just can’t see it enough times,” he said.
