Cosmic roller coaster Exploding into life, sending out ripples that would last for aeons (Image: Henning Dalhoff/SPL)

Slivers of a second after the universe was born, it ballooned exponentially in the blink of an eye. This vision of cosmic expansion, called inflation, got a big boost last week with the announcement of the first sighting of primordial gravitational waves, ripples in space-time linked to the universe’s rapid growth spurt.

The findings – reported by the team that ran the BICEP2 experiment at the South Pole – are already helping physicists sort through the mountain of theories for how inflation happened (see our video of gravitational waves explained with a towel and apple).


But there are a few wrinkles, including the fact that hints of the waves seem much more pronounced than they should be, according to previous observations of the early universe. Resolving the discrepancies – perhaps using results due out later this year from the Planck space telescope – might give a glimpse of physics from before the big bang. Or it might mean inflation is out, and that we actually have the first whiff of evidence for string theory.

“Everyone was celebrating and so forth, but you have to be careful with this result,” says Avi Loeb at the Harvard-Smithsonian Center for Astrophysics. “That’s why the Planck data is so interesting to watch for.”

Inflation’s particle

The BICEP2 telescope used its vantage point at the South Pole to hunt for hints of gravitational waves in a patch of the cosmic microwave background (CMB) – the relic light emitted about 380,000 years after the big bang. Existing CMB maps from Planck depicted an early universe that was almost uniformly smooth, with only tiny variations in density. This seemed to support inflation, because the variations we see are too small for matter to have expanded slowly after the big bang.

The Planck team reported no signs of gravitational waves at the time, and said the density variations picked up were so small that any such waves must be weak. That made BICEP2’s sighting all the more startling, as its gravitational wave signal is twice what Planck suggested.

“How do we reconcile them?” says Laura Mersini-Houghton at the University of North Carolina at Chapel Hill. “That’s the million-dollar question.”

If the BICEP2 results hold up, they support the simplest models of inflation, which say that a hypothetical particle, the inflaton, drove the process. The issue would then be how the inflaton’s energy changed over time – a detail that could tell us why inflation ended, and whether we live in a multiverse.

One leading model is known as chaotic inflation (see diagram, above right), put forward in the 1980s by Andrei Linde at Stanford University in California. Building on work by Alan Guth, it features an inflaton that decays quickly, but also allows quantum fluctuations to trigger new bursts of inflation, giving rise to other universes. (See “Who is the father of inflation?“.)

Quantum seeds

Another contender is natural inflation, proposed by Katherine Freese at the University of Michigan in Ann Arbor in 1993. In this model, the inflaton retains its peak energy for a relatively long time before decaying. That would keep inflation going long enough to explain the smoothness seen by Planck (arxiv.org/abs/1403.5277).

A third possible theory is Higgs-like inflation, in which the inflaton behaved like the recently discovered Higgs boson. The Higgs is the only known particle with an associated scalar field – one that does not act in a specific direction – which is a property the inflaton should share.

If the Planck team does see gravitational waves, but at a lower strength than BICEP2, the models get more complex. One solution is to have inflation that starts out fast and slows down abruptly, says Marco Peloso at the University of Minnesota in Minneapolis. Another is to assume that inflation was faster in one direction (arxiv.org/abs/1403.4596v1). This could explain an anomaly in the Planck data that suggests the universe has a “preferred” direction, nicknamed the axis of evil.

Pre-big bang

An even wilder way to square the discrepancy between Planck’s smooth cosmos and BICEP2’s strong ripples is to include physics from before the big bang.

In 2006, Mersini-Houghton and her colleagues devised a model in which the universe was seeded by a quantum particle, one in a sea of particles that existed before the big bang. Only those with high enough energies inflated and turned into universes, and all of those particles remained connected via the quantum property of entanglement.

“You have a second source of energy that comes from quantum entanglement with other universes,” says Mersini-Houghton. Gravitational waves would still exist and could be as strong as BICEP2 reports, but the entanglement energy would suppress the density variations, making them even smaller than inflation should allow.

Connecting strings

The upcoming Planck data may even help rule out inflation. Instead, the universe could be the result of string gas cosmology, based on the multiple dimensions of string theory.

Picture the cosmos as a rolled-up piece of paper held in place with rubber bands, says Robert Brandenberger at McGill University in Montreal, Canada, who was part of a team that came up with the model in 1989.

The paper is a nine-dimensional universe, and the rubber bands are vibrating strings. If two strings meet, their edges can form a single, twisted loop. That would release three dimensions of space and one of time, which can then swell to the scales we see in the universe today. This process can account for the tiny density variations seen in the CMB and strong gravitational waves – no inflation required.

The BICEP2 results slightly favour this model. If Planck sees the same signal, it could be the first observational evidence for string theory. “For string theorists this is very important,” says Brandenberger. “Opponents can no longer say string theory does not connect with data.”

Russia’s relativistic road to inflation Read more: Click here to read a longer version of this story Hints of gravitational waves, widely hailed as validation for inflation, could net someone a Nobel prize if they can be confirmed. But who came up with inflation in the first place? In 1980 Alan Guth, now at the Massachusetts Institute of Technology, gave his first public lecture on inflation, and he is often cited as a founder. But Guth wasn’t the only one pondering the theme. Aleksei Starobinksy of the Landau Institute for Theoretical Physics in Moscow, Russia, had published his own theory of inflation in December 1979. The paper appeared in a prominent Russian journal, and was later translated into English. Still, it was largely overlooked. Viatcheslav Mukhanov of the Arnold Sommerfeld Center for Theoretical Physics in Munich, Germany, thinks this is because Starobinsky based his work on Einstein’s general theory of relativity. People in the field wanted testable predictions, and at the time any experiment to detect inflation seemed almost impossible to carry out, Mukhanov says. In contrast, he says, Guth’s ideas drew on particle physics – a playground for theorists – and so sparked widespread interest as a mathematical exercise. Starobinsky’s model was a better fit for our observations of the early universe until last week’s sighting of gravitational waves. Now it is unclear which of the competing theories of inflation will reign (see main story). “If the BICEP2 data are confirmed, apostles of inflation may celebrate the dramatic confirmation of the general idea of inflation, but not of any concrete simple model of it,” says Starobinsky. Stuart Clark

This article appeared in print under the headline “Cosmic ripples put inflation to the test”