Inflation is dead, long live inflation! The very results hailed this year as demonstrating a consequence of inflationary models of the universe – and therefore pointing to the existence of multiverses – now seems to do the exact opposite. If the results can be trusted at all, they now suggest inflation is wrong, raising the possibility of cyclic universes that existed before the big bang.
It was touted as the biggest cosmological discovery in decades: in March experimentalists announced that primordial gravitational waves had been discovered. The team behind the BICEP2 Telescope in Antarctica had observed telltale twists and turns in the polarisation of the cosmic microwave background radiation (CMB) – the remnants of the earliest light produced in the universe.
It was hailed as a preliminary confirmation of inflation: the idea that for a sliver of a moment after the big bang there was a blisteringly fast expansion of the universe. The theory, the most widely held of cosmological ideas about the growth of our universe after the big bang, explains a number of mysteries, including why the universe is surprisingly flat and so smoothly distributed, or homogeneous.
But very quickly, the BICEP2 finding was shrouded in doubt, as it was revealed that the polarisation pattern could have been caused by cosmic dust. Cosmologists are waiting for space-based Planck telescope to reveal whether the dust could really make that pattern, and preliminary results released last week suggest dust might be able to.
But this week a team of theorists decided to analyse the polarisation signature further and ask: if it isn't completely caused by dust, what exactly does it say about inflation?
Rather than just looking at the polarisation to see if it suggests the existence of gravitational waves, David Parkinson at the University of Queensland in Australia and his colleagues decided to look at the nature of those apparent gravitational waves to see if they were the type of waves predicted by inflation. And they weren't.
Counter to what the BICEP2 collaboration said initially, Parkinson's analysis suggests the BICEP2 results actually rule out any reasonable form of inflationary theory.
Mostly ruled out
Most inflationary models require that as you look at larger and larger scales of the universe, you should see stronger and stronger gravitational waves. Cosmologists call that a "gravitational wave spectrum".
"What inflation predicted was actually the reverse of what we found," says Parkinson. How many inflationary models does it rule out? "Most of them, to be honest."
It's not entirely impossible for inflation models to conform with what BICEP2 found if you do "really tricky" things to the mathematics, says Katherine Mack, a theoretical astrophysicist at the University of Melbourne in Australia. But such tricky things would break something called the inflation consistency relation, she says, which is "something that's considered pretty basic for inflation". The inflation consistency relation links the amplitude of the primordial gravitational waves with the distribution of matter in the universe.
Flipped on its head
There is still the very live possibility that no gravitational waves were observed at all, and that BICEP2's result was entirely caused by cosmic dust. Yesterday, that would have been a blow for the backers of inflationary theory. Today, seemingly, that is the best they could hope for.
That's what Nobel laureate Brian Schmidt at the Australian National University in Canberra, who has been critical of the theory of inflation, says he expects will happen.
"But on the other hand, if BICEP2 is shown to be correct, it's exciting," says Schmidt. "And it does potentially break standard inflation and therefore you are testing inflation and showing its wrong."
Paul Steinhardt of Princeton University, who helped develop inflationary theory but is now scathing of it, says this is potentially a blow for the theory, but that it pales in significance with inflation's other problems.
Meet the multiverse
Steinhardt says the idea that inflationary theory produces any observable predictions at all – even those potentially tested by BICEP2 – is based on a simplification of the theory that simply does not hold true.
"The deeper problem is that once inflation starts, it doesn't end the way these simplistic calculations suggest," he says. "Instead, due to quantum physics it leads to a multiverse where the universe breaks up into an infinite number of patches. The patches explore all conceivable properties as you go from patch to patch. So that means it doesn't make any sense to say what inflation predicts, except to say it predicts everything. If it's physically possible, then it happens in the multiverse someplace
Steinhardt says the point of inflation was to explain a remarkably simple universe. "So the last thing in the world you should be doing is introducing a multiverse of possibilities to explain such a simple thing," he says. "I think it's telling us in the clearest possible terms that we should be able to understand this and when we understand it it's going to come in a model that is extremely simple and compelling. And we thought inflation was it – but it isn't."
If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.
