快猫短视频

SOMETIMES cosmology talks can be exciting 鈥� riveting even. Take, for instance, the occasion when a young graduate student called Alan Guth heard all about the serious problems with the big bang theory. It was so provocative and stimulating it led Guth, now a professor at the Massachusetts Institute of Technology, to make one of the most audacious suggestions in science.

Guth鈥檚 idea is called inflation and it suggested that the major problems in cosmology could be solved if the universe had blown up like a balloon, inflating faster than the speed of light in the moments after its birth.

The lecture that inspired Guth was more than 25 years ago and the idea of inflation is still king in cosmology. How ironic, then, that the talk now threatening to dethrone inflation was such a snoozefest. 鈥淚t was the most boring talk you鈥檝e ever heard,鈥� says , a cosmologist at the University of Illinois, Urbana-Champaign.

And Wandelt should know 鈥� he was doing the talking. Speaking at a conference on , held last December in Cambridge, UK, he described how satellite measurements of the cosmic microwave background radiation (CMB), the echo of the big bang, seem to contradict the predictions of inflation. Wandelt claims his analysis puts inflation to its most precise test yet 鈥� and that the theory seems to have failed.

Most physicists find this hard to swallow, so to get anyone to even think about accepting his analysis Wandelt had to spend his allotted hour refuting a seemingly endless litany of possible flaws. That鈥檚 why his talk was so dull. 鈥淚 had to examine effect after effect after effect,鈥� he says. It was worth the effort, though. Wandelt鈥檚 analysis was published last month in the prestigious journal Physical Review Letters (), an outcome that will surprise anyone who thought inflation was ironclad. It is, in fact, one of a number of recent setbacks for inflation, which is starting to look a little vulnerable.

Inflation is arguably the most important theoretical idea in cosmology since the big bang 鈥� and it has been successful too, within its limits (快猫短视频, 3 March 2007, p 33). It explains why the universe is essentially flat, despite Einstein showing us that space-time curves, and why the temperature at opposite horizons of the universe is almost identical, even though heat should not have had time to spread that far. Inflation even explains how stars and galaxies formed from the quantum jitters present after the big bang.

But it has its flaky side, too. Michael Turner at the University of Chicago compares it to a patchwork of duct tape repairs. 鈥淚t might last for 10 years, but it won鈥檛 last for ever,鈥� he says. There are also big questions inflation doesn鈥檛 answer. And it is still no more than a paradigm; an idea in search of a solid foundation. So is it time to call off the search and move on?

The first reason to doubt inflation鈥檚 majesty is not a positive result, but the lack of one. The rush of inflation would have likely created gravitational waves, patterns of distortion in the space-time fabric of the primordial universe. Detecting them would mean inflation is very hard to dismiss. So far, though, we haven鈥檛 鈥� and this may soon become a difficult issue for inflation鈥檚 supporters, according to Paul Steinhardt of Princeton University in New Jersey, one of the original architects of the theory.

The classic map of the CMB is a chart of the way its temperature fluctuates as you look at different points around the sky. But the CMB has another subtle property: it is polarised, meaning that its electric and magnetic fields point in particular directions as you look across the sky. This polarisation arose from the way in which radiation bounced off free electrons that filled the early universe. Primordial gravitational waves should have left their mark on the CMB, too. As they rushed through the universe, they would have stretched and squeezed space-time and, in turn, the wavelength of the radiation in the inflating cosmos. These wavelength variations would affect the way the radiation scattered from the electrons, leaving it with a distinct pattern of polarisation.

Trouble is, this predicted polarisation signal is much weaker than the temperature fluctuations 鈥� anything from 40 per cent down to just 5 per cent as strong 鈥� and this makes it difficult to measure. Over the last few years, the sensitivity of the experiments looking for this signature has been steadily improving, allowing us to move down that percentage scale. The signature wasn鈥檛 there when we could have seen a 40 per cent signal, and it wasn鈥檛 there when measurements improved enough to detect polarisation at 20 per cent. 鈥淲e are already in the middle of the interesting region,鈥� Steinhardt says. Yet so far we have found no sign of gravitational waves.

This does not in itself disprove inflation. For starters, a significant minority of physicists believe gravitational waves are not an inescapable product of inflation. And it may be that our equipment is not yet sensitive enough, or that the signal is, in fact, unexpectedly small.

Ongoing searches for the CMB gravitational wave signal, in particular by the at the South Pole, will ultimately decide. 鈥淚f the gravitational wave imprint on polarisation is the 鈥榮moking gun鈥� of inflation, then BICEP鈥檚 evidence will be crucial to our understanding of the inflationary universe,鈥� says Brian Keating, the project leader, who is based at the University of California, San Diego. BICEP has completed two of its three years of observation, and is performing in line with expectations, Keating says. So we may not have too long to wait for significant news on the gravitational wave signature.

In the meantime, Wandelt鈥檚 very boring talk, which focuses on a rather esoteric characteristic of the radiation, has given inflation鈥檚 supporters a whole new reason to bite their nails. It relates to the temperature fluctuations across the sky, whose magnitude should, according to inflation, be distributed according a bell-shaped, gaussian curve. Crucially, inflation says there should be as many hot spots as cold spots. Wandelt鈥檚 analysis suggests this is not the case. 鈥淗e claims that the distribution is skewed to have more cold spots,鈥� says cosmologist David Wands at the University of Portsmouth, UK.

This skewing, known as 鈥渘on-gaussianity鈥�, is a truly tiny effect. 鈥淭he temperature variations are of the order 1 in 100,000,鈥� Wands says, and Wandelt was looking at 鈥渧ariations within those variations鈥�.

As small as the effect might be, non-gaussianity is a big deal. The important measure of non-gaussianity is known to cosmologists as f_NL and it quantifies how distorted the temperature distribution is, compared to the bell-shaped curve. For the simplest inflation models, f_NL should be between 0 and 1. Wandelt鈥檚 analysis of the radiation puts f_NL somewhere between 50 and 80. For inflation, that鈥檚 a problem.

There is a get-out, though. Wandelt鈥檚 analysis is not yet statistically significant enough for him to be able to claim that he has detected non-gaussianity. His error bars are too big: in physics parlance he has a 2.8 sigma result, or better than 99 cent chance that the result is trustworthy and replicable. He鈥檇 need something like 5 sigma, or 99.99995 per cent, before anyone is sure the non-gaussianity cannot be dismissed.

His paper in Physical Review Letters claims 鈥渆vidence for鈥� non-gaussianity, not a 鈥渄etection of鈥�, and Steinhardt thinks this is pitching it about right. Wandelt鈥檚 result still has to be taken with a huge pinch of salt, he says. Turner calls it 鈥渧ery tentative鈥�.

It may not stay like that for long, though. 鈥淒etection may not be that far away,鈥� Steinhardt says. 鈥淭here are many analyses occurring around the world that may yield weaker or stronger results.鈥�

If Wandelt鈥檚 evidence is strengthened and turns into a detection, what would that mean? 鈥淚t would mean that the models of inflation that you鈥檙e beginning to find in the cosmology textbooks are wrong 鈥� or at least that they are too simple,鈥� says Wands. 鈥淵ou鈥檇 need a different kind of model.鈥� That might be a little upsetting for the authors, as the ink in the textbooks is barely dry, but inflation鈥檚 inclusion in them might have been a little premature anyway. 鈥淚nflation has much of the truth, but it may need significant modification,鈥� Turner says.

鈥淚nflation鈥檚 inclusion in the latest cosmology textbooks may have been a little premature鈥�

One of the problems is the idea鈥檚 flexibility. Inflation is now widely considered to be an ongoing process, with universes blowing up out of other universes. This 鈥渆ternal inflation鈥� gives us an infinity of possible universes to explore in our theories, each with different properties that would give different observations. This troubles Turner. 鈥淭here are all kinds of conceptual problems with trying to do the measurements,鈥� he says (快猫短视频, 18 August 2007, p 26).

Then there is the lack of any solid scientific idea for why or how inflation might have happened. 鈥淭he number one challenge is to find a specific model for inflation that is rooted in fundamental physics,鈥� Turner says. If no such model can be found, we may have to go deeper and look for something that will replace inflation altogether. 鈥淭he distance between looking for the right model of inflation and finding something better might not be that far,鈥� he says.

It is a view that Wandelt thinks his result can only strengthen: as it stands, he reckons inflation is effectively falsified. Although the theory solves some problems with the big bang, it has yet to pass any stringent test. The best results only fit inflation鈥檚 prediction to within a couple of percentage points, putting them some way behind Wandelt鈥檚 analysis.

That鈥檚 the situation for the simple versions of inflation, at any rate. In these versions, a single quantum field is used to describe matter in the extreme conditions just after the big bang, and it is random fluctuations in this quantum field that should lead to an uncomplicated gaussian signature being imprinted into the CMB. It鈥檚 rather like throwing a single pebble into a pond: it makes a nice, simple circular pattern of ripples.

Recycled universe

There are, however, more complex scenarios. Throw two or more pebbles in, and the ripples are more complex. Similarly, a non-gaussian signature in the CMB requires that there was more than one field in the universe during those crucial early moments. Though possible, this is a major departure from the simple scenario that most cosmologists feel provides the most pleasing explanation for inflation.

Wands doesn鈥檛 see this complication as a problem, though. 鈥淚t鈥檚 not that inflation has to be thrown out,鈥� he says. 鈥淵ou can have different fields doing different jobs.鈥� In 2001, working with David Lyth at the University of Lancaster, UK, Wands produced just such a variant of inflation. It produces a non-gaussianity very similar to that which Wandelt claims to have seen, while still solving the universe鈥檚 horizon and flatness problems.

Wands is untroubled by the idea that simple has to be better. 鈥淚n the absence of knowing what the model is, we should have an open mind about how it happened,鈥� he says. 鈥淲e should investigate different possibilities 鈥� as long as they are possibilities you can test.鈥� Turner is less keen on endless fiddling with inflation. 鈥淧erhaps, instead of trying to tweak it, maybe we should be looking for a grander paradigm,鈥� he says.

Steinhardt certainly thinks we should look much harder at alternatives. He is now the prime mover behind inflation鈥檚 biggest competitor, the . Here the cosmos goes through phases of expansion, contraction and rebirth. And because in cyclic models space-time doesn鈥檛 undergo the same rapid expansion seen in inflation, they predict an absence of primordial gravitational waves. Cyclic models also predict f_NL will be at least 100 times the inflationary value, putting it in the ballpark of the value Wandelt is claiming.

It is too early to say that simple inflation is definitely on the skids. It could yet be redeemed if satellite experiments find the gravitational wave signature in the CMB. Measurements sensitive enough to tie this down will arrive within five years, Steinhardt reckons. Any sight of gravitational waves would destroy the cyclic universe idea at a stroke.

Within the same timescale, a spacecraft to be launched later this year will tie down the non-gaussianity that Wandelt claims to have found. The European Space Agency鈥檚 will reduce Wandelt鈥檚 error bars to the point where we know whether the cosmology textbooks really do need a rewrite. 鈥淚f f_NL is in the region of 50 to 80, Planck will see it. It will be completely unambiguous,鈥� Wandelt says. It is also entirely possible that Planck will make Wandelt鈥檚 results disappear. 鈥淭here have been plenty of cases where a 3 sigma result turned out not to be correct,鈥� Turner points out.

Whichever way it goes, there鈥檚 a lot we are about to learn. 鈥淭he next five years are going to be very exciting,鈥� Wandelt says. Perhaps the most stimulating of the possibilities is that we may see gravitational waves plus a significant amount of non-gaussianity. That will leave the cyclic models dead and inflation struggling to stay alive. Once again we鈥檇 be saddled with the original problems of the big bang.

Why would this be good news? Because we might then be forced go back to the drawing board and conjure up a deeper, more satisfying theory. This could be based on existing alternatives to the inflation and cyclic universe ideas 鈥� theories that invoke a varying speed of light or modified gravity, for instance 鈥� but being forced to consider an entirely new approach could bring out some useful surprises. We might, for instance, find a theory for the primordial universe that ties in with the efforts of quantum gravity researchers to create one consistent description of how the universe works. 鈥淚t might mean there鈥檚 something about quantum gravity, about the way we emerged from the big bang, that we have yet to discover,鈥� Steinhardt says.

If we do make such a discovery, history will recast Wandelt鈥檚 talk as a seminal moment in physics, the trigger for a brilliant cosmological insight. Future generations need never know just how dull it really was.

鈥淗istory may recast Wandelt鈥檚 talk as one of the seminal moments in science, a brilliant new cosmological insight鈥�

Cosmology 鈥� Keep up with the latest ideas in our .