Physicists have found a long-predicted twist in light from the big bang that represents the first image of ripples in the universe called gravitational waves, researchers announced today. The finding is direct proof of the theory of inflation, the idea that the universe expanded extremely quickly in the first fraction of a nanosecond after it was born. What’s more, the signal is coming through much more strongly than expected, ruling out a large class of inflation models and potentially pointing the way toward new theories of physics, experts say.
“This is huge,” says Marc Kamionkowski, professor of physics and astronomy at Johns Hopkins University, who was not involved in the discovery but who predicted back in 1997 how these gravitational wave imprints could be found. “It’s not every day that you wake up and find out something completely new about the early universe. To me this is as Nobel Prize–worthy as it gets.”
The Background Imaging of Cosmic Extragalactic Polarization 2 (BICEP2) experiment at the South Pole found a pattern called primordial B-mode polarization in the light left over from just after the big bang, known as the cosmic microwave background (CMB). This pattern, basically a curling in the polarization, or orientation, of the light, can be created only by gravitational waves produced by inflation. “It looks like a swirly pattern on the sky,” says Chao-Lin Kuo, a physicist at Stanford University, who designed the BICEP2 detector. “We’ve found the smoking gun evidence for inflation and we’ve also produced the first image of gravitational waves across the sky.”
Such a groundbreaking finding requires confirmation from other experiments to be truly believed, physicists say. Nevertheless, the result has won praise from many leaders in the field. “There’s a chance it could be wrong, but I think it’s highly probable that the results stand up,” says Alan Guth of the Massachusetts Institute of Technology, who first predicted inflation in 1980. “I think they’ve done an incredibly good job of analysis.” The BICEP2 detectors found a surprisingly strong signal of B-mode polarization, giving them enough data to surpass the “5-sigma” statistical significance threshold for a true discovery. In fact, the researchers were so startled to see such a blaring signal in the data that they held off on publishing it for more than a year, looking for all possible alternative explanations for the pattern they found. Finally, when BICEP2’s successor at the same location, the Keck Array, came online and began showing the same result, the scientists felt confident. “That played a major role in convincing us this is something real,” Kuo says.
The cosmic microwave background is a faint glow that pervades the entire sky, dating back to just 380,000 years after the big bang. Before then the baby universe was too hot and dense for light to travel far without bumping into matter. When it cooled to the point that neutral atoms could form, light was freed to fly through space unimpeded, and it became the CMB. This glow was discovered accidentally 1964 by Arno Penzias and Robert Wilson, who initially mistook it for interference caused by pigeon droppings on their antenna. Eventually, the scientists realized they had discovered an imprint from the primordial universe, a finding that won them the 1978 Nobel Prize in Physics. “It’s amazing what’s come out of the CMB,” Wilson says. “Initially I didn’t realize anywhere near how much information there might be coming from it. From my point of view, it’s been a wonderful ride.”
BICEP2 uses about 250 thumbnailsize polarization detectors to look for a difference in the CMB light from a small patch of sky coming through its telescope in two perpendicular orientations. The instrument collected data between January 2010 and December 2012 at the Amundsen–Scott South Pole Station, where the cold, dry air offers especially stable viewing conditions. Another experiment there, the South Pole Telescope, reported finding B-mode polarization last year, although the signal it saw was at a different angular scale across the sky and was clearly due to the known process of gravitational lensing (a warping of light caused by massive objects) of the CMB by large galaxies, rather than the primordial gravitational waves seen here.
BICEP2 has plenty of competition in searching for B-mode polarization in the CMB: other projects include the Atacama B-mode Search (ABS) led by Princeton University; the POLARBEAR experiment led by the University of California, Berkeley; the high-altitude balloon–borne E and B Experiment (EBEX) run by the University of Minnesota; the Cosmology Large Angular Scale Surveyor (CLASS) led by Johns Hopkins University; and numerous others. Given that the BICEP2 team saw such a clear signal, these searches should easily confirm the results if they are real. “Right now it’s just the tip of the iceberg,” Kamionkowski says. “In the coming years we’re going to be able to extract a huge amount from these measurements. It’s a great thing, not just for the guys who found it but also for the people they scooped” because the different experiments should collect complementary data.
The BICEP2 researchers have reported a surprisingly large number for r, the ratio of the gravitational wave fluctuations in the CMB to the fluctuations caused by perturbations in the density of matter. This value was previously estimated to be less than 0.11 based on all-sky CMB maps from the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck satellite. BICEP2’s value, however, is around 0.20. “Everything hinges on this little r,” Guth says, “and this measurement changes things quite a bit. In fact, the models that looked like they were ruled out last week are now the models that are favored this week.” Such a high value of r, for instance, indicates that inflation began even earlier than some models predicted, at one trillionth of a trillionth of a trillionth of a second after the big bang.
The timing of inflation, in turn, tells physicists about the energy scale of the universe when inflation was going on. BICEP2’s value of r suggests that this was the same energy scale at which all the forces of nature except gravity (the electromagnetic, strong and weak forces) might have been unified into a single force—an idea called grand unified theory. The finding bolsters the idea of grand unification and rules out a number of inflation models that do not feature such an energy scale. “This really collapses the space of plausible inflationary models by a huge amount,” Kamionkowski says. “Instead of looking for a needle in a haystack, we’ll be looking for a needle in a bucket of sand.” Grand unified theories suggest the existence of new fields that act similarly to the Higgs field associated with the Higgs boson particle discovered in 2012. These new fields, in turn, would indicate that other, heavier Higgs boson particles also exist, although with masses so high they would be impossible to create in any traditional particle accelerator. “This measurement is allowing us to use the early universe as a lab for new physics in energy ranges that are otherwise inaccessible to us,” Kamionkowski says.