Researchers with an experiment based at the South Pole have discovered the long-sought "smoking gun" for inflation.
Researchers with the BICEP2 experiment have set the world’s cosmologists buzzing with the announcement that they’ve detected the fingerprints of inflation — the exponential expansion that put the “bang” in the Big Bang.
About 10 teams of researchers around the world have been actively looking for this signal, called primordial B-modes. But I have to admit that, of all the announcements that might have hit the air waves St. Patrick’s Day morning, the discovery of this polarization signal was not at the top of my list. Two teams did reach an important stepping point in this hunt several months ago, by finding another signal that could muck up the primordial data. But from work presented at the American Astronomical Society meeting this past January, I figured astronomers were at least a year away from the announcement made today.
I’m glad to be wrong.
B-modes are a particular pattern of polarization. As a wavelength of polarized light travels through space, it wiggles at a preferred angle to its direction of motion. If inflation happened, it should have sent gravitational waves rippling through spacetime. These waves would have imprinted the B-mode polarization pattern on the cosmic microwave background radiation (CMB).
There’s one other way to create a B-mode pattern in the CMB: when the gravity of large-scale cosmic structures works as a lens on the CMB, distorting its polarization pattern. But these lensed B-modes exist at an angular scale only one-tenth of the primordial ones. With a lot of careful analysis, researchers can weed these out.
The discovery of the primordial B-modes comes from the second round of the Background Imaging of Cosmic Extragalactic Polarization (BICEP) experiment. It’s among one of several projects observing the CMB in what’s called the Southern Hole, a patch of sky visible from Antarctica that’s a direct sightline out of our galaxy and into the cosmic depths. (See the sky map.)
The BICEP2 scope has an aperture of less than 30 centimeters (12 inches), but it doesn’t need to be big. Cooled to 4 kelvin, it gazes at a 20° patch of sky 24/7, detecting the CMB’s faint microwaves — and, crucially, how they’re polarized.
Using 3 years of data, the BICEP2 team meticulously analyzed their polarization measurements. They also compared their data with observations from BICEP1 and from the team’s new Keck Array, which is basically like five BICEP2s in one. It was this ability to combine three data sets that ultimately allowed the team to make the discovery.
After a year of intense work — including ruling out more than a dozen alternate explanations — the team is confident that they’re seeing the signal of inflation, on a scale of about 2° on the sky. In statistical terms, their signal is better than 5 sigma, which is the gold standard a detection has to meet before physicists accept it as a discovery.
“We are convinced that the signal is really coming from the sky, and that it’s coming from the cosmic microwave background,” says Clem Pryke (University of Minnesota), who headed up the analysis.
The other researchers present at the technical briefing were also swayed. “This looks as solid as any result that I’ve seen,” says Alan Guth (MIT), co-developer of the inflation paradigm. He and everyone (including the team) want other groups to confirm it, but the signal sure looks like it’s from inflation.
“I am extremely excited,” said gravitational waves physicist Scott Hughes (MIT), beaming. “Of course there are these implications for cosmology and inflation — just as a scientist it’s bloody awesome.”
The So What
Until now, astronomers have really only had one line of evidence to investigate whether inflation happened: the CMB’s speckled pattern of temperature variations. Studies of these patterns — particularly as seen by ESA's Planck satellite — support the simplest version of inflation.
But having B-modes in hand is another ballgame. “This is not something that’s just a home run, but a grand slam,” says Marc Kamionkowski (Johns Hopkins University), one of the theorists who first suggested inflation-triggered B-modes might be detectable in the CMB. “It’s the smoking gun for inflation.”
The B-modes carry with them specific information about the size of the gravitational waves, when inflation happened, and how much energy inflation involved. So having an actual detection in hand shrinks the theoretical playing field — and not just a little, both Kamionkowski and Guth stress.
From the BICEP2 results, it looks like inflation happened roughly 0.5 × 10-37 second after the Big Bang, says Kamionkowski — but he cautions that’s from a quick calculation he did on a scrap of paper.
The measurement also suggests that inflation might have had something to do with the unification of three of the four fundamental forces of nature — the strong, weak, and electromagnetic. The energy level implied by the BICEP2 data — we’re talking 2 × 1016 GeV, according to Guth, or roughly a trillion times the energy of the Large Hadron Collider — matches the energy of grand unified theories, or GUTs. That’s an idea theorists have toyed with since the 1970s, but the BICEP2 result is the missing link they’ve sought for decades.
The data also tell us something about the size of the gravitational waves. This information comes in the form of the ratio of gravitational waves (which are a type of density perturbation) to the run-of-the-mill density fluctuations in the CMB. The technical term for this number is the tensor-to-scalar ratio, where the gravitational waves are tensors and the “normal” density fluctuations are scalars.
The BICEP2 team came up with a ratio of about 0.2, which means the gravitational waves were “pretty big,” Kamionkowski says. (Sorry, I don’t have an ironclad number for you.) The Planck team had come up with an upper limit of 0.11 from their data, but Pryke says that, while there’s a bit of tension here with his team’s result, the discrepancy is not much to worry about. It could be solved by simple extensions to the standard cosmological model, for example. They don’t know yet.
The results do not tell us what set inflation in motion, only that it happened. Nor do they answer the question of whether inflation is eternal, setting off an endless series of big bangs and creating pocket universes. This cosmological landscape is usually referred to as the multiverse. (You can read my in-depth discussion of the search for evidence of multiple universes in the December 2012 Sky & Telescope.) However, it’s hard to tune inflation such that pocket universes don’t happen, Guth points out.
A few “smaller” results that have been lost in the inflation hubbub:
1. This is the first hard evidence that gravity is quantized, or comes in discrete packets as light does. The gravitational waves that produced the B-modes were born as quantum fluctuations in gravity itself, then stretched during inflation’s faster-than-light-speed expansion. “I think this is the only observational evidence that we have that actually shows that gravity is quantized,” says cosmologist Ken Olum (Tufts University). “It’s probably the only evidence of this that we will ever have.”
2. This is the first detection of gravitational waves’ action on matter other than their source. Astronomers have observed neutron stars spiraling inward toward each other just as they should if the system was radiating gravitational waves, but they’ve never seen these waves affecting other matter in the cosmos.
3. This is the first detection of Hawking radiation. Hawking radiation is usually associated with the slow evaporation of black holes, as photons emitted from the event horizon. But the observable universe also has a horizon. Hawking radiation should be coming from this horizon, and also from every horizon in the universe — in other words, from every point in the universe, says cosmologist Max Tegmark (MIT). Today the cosmic horizons are huge and their Hawking radiation is utterly insignificant. But in the universe’s first fraction of a second, the horizons were tiny and sharply curved. The gravitational waves announced today are these horizons’ Hawking radiation.
Other teams will be working arduously to confirm the BICEP2 result. Planck’s polarization measurements aren’t expected until later this year, and the last word from the team was that those results wouldn’t include primordial B-mode analysis. But Planck's all-sky coverage might reveal B-modes on larger angular scales than BICEP2 can, and also show something called the "reionization bump," a result of primordial B-modes being rearranged by intervening ionized material, says Planck scientist Charles Lawrence (JPL). Whether Planck can do it at all, though, is for now uncertain.
In the meantime, the excitement is palpable. As Tegmark put it, “I think this is one of the most important discoveries of all time.” (See his blog post from the event for why.)
Here is the BICEP team's website for their papers, detailed information about the data, explanations, images, and videos.
Senior editor Alan MacRobert contributed to the reporting for this news article.