Repulsive gravity

The simplest models of inflation, which the BICEP2 results seem to support, require a particle called an inflaton to push space-time apart at high speed.

Inflation depends on a kind of material that turns gravity on its head and causes it to be repulsive,” says Alan Guth at  the Massachusetts Institute of Technology, another author of inflationary theory. Theory says the inflaton particle decays over time like a radioactive element, so for inflation to work, these hypothetical particles would  need to last longer than the period of inflation itself. Afterwards, inflatons would continue to drive inflation in whatever pockets of the universe they inhabit, repeatedly blowing new universes into existence that then rapidly inflate before settling down. This “eternal inflation” produces infinite pocket universes to create a multiverse. For now, physicists don’t know how they might observe the multiverse and confirm that it exists. “But when the idea of inflation was proposed 30 years ago, it was a figment of theoretical imagination,” says Marc Kamionkowski at Johns Hopkins University in Baltimore, Maryland. “What I’m hoping is that with these results, other theorists out there will start to think deeply about the multiverse, so that 20 years from now we can have  a press conference saying we’ve found evidence of it.” In the meantime, studying the properties of the swirls in the CMB might reveal details of what the cosmos was like just after its birth. The power and frequency  of the waves seen by BICEP2 show “Finding these primordial waves does tell us that gravity obeys quantum laws” that they were rippling through  a particle soup with an energy of about 1016 gigaelectronvolts, or 10 trillion times the peak energy expected at the Large Hadron Collider. At such high energies, physicists expect that three of the four fundamental forces in physics – the strong, weak and electromagnetic forces – would  be merged into one. The detection is also the first whiff of quantum gravity, one of the thorniest puzzles in modern physics. Right now, theories of quantum mechanics can explain the behaviour of elementary particles and those three fundamental forces, but the equations fall apart when the fourth force, gravity, is added  to the mix. Seeing gravitational waves in the CMB means that gravity is probably linked to  a particle called the graviton, which in turn is governed by quantum mechanics. Finding these primordial  waves won’t tell us how quantum mechanics and gravity are unified, says Kamionkowski.  “But it does tell us that gravity obeys quantum laws.” “For the first time, we’re directly testing an aspect of quantum gravity,” says Frank Wilczek at MIT. “We’re seeing gravitons imprinted on the sky".

Given the huge potential of these results, scientists will be eagerly anticipating polarisation maps from projects such as the POLARBEAR experiment in Chile or the South Pole Telescope. The next full-sky CMB maps from the Planck space telescope are also expected to include polarisation data. Seeing a similar signal from one or more of these experiments would shore up the BICEP2 findings and make a firm case for inflation and boost hints of the multiverse and quantum gravity. One possible wrinkle is that previous temperature maps of  the CMB suggested that the signal from primordial gravitational waves should be much weaker that what BICEP2 is seeing. Those results set theorists bickering about whether inflation really happened and whether it could create a multiverse. Several physicists suggested that we  scrap the idea entirely for a new model of cosmic birth. Taken alone, the BICEP2 results give a strong-enough signal to clinch inflation and put the multiverse back in the game. But the tension with previous maps  is worrying, says Paul Steinhardt at Princeton University, who helped to develop the original theory of inflation but has since grown sceptical of it. “If you look at the best-fit models with the new data added, they’re bizarre,” Steinhardt says. “If it remains like that, it requires adding extra fields, extra parameters, and you get really rather ugly-looking models.” Forthcoming data from  Planck should help resolve  the issue, and we may not have long to wait. Olivier Doré at the California Institute of Technology is a member of the Planck collaboration. He says that the BICEP2 results are strong and that his group should soon be adding their data to the inflation debate: “Planck in particular will have something to say about it as soon as we publish our polarisation result in October 2014.”  ■