If imagining the big bang makes your head ache, what happened an instant later might make it explode. Cosmologists think the just-born universe—a hot, dense soup of matter and energy—went through a burst of expansion faster than the speed of light. Like a magical balloon, the cosmos doubled its size 60 times in a span of 10-32 seconds. This phase, known as inflation, ended well before the universe was even a second old.
Now, 13.7 billion years later, cosmologists have detected what they say is the first direct evidence of this inflation—one of the biggest discoveries in the field in 20 years. From studying the cosmic microwave background (CMB)—the leftover radiation from the big bang—they have spotted traces of gravitational waves—undulations in the fabric of space and time—that rippled through the universe in that infinitesimally short epoch following its birth. The imprint of these gravitational waves upon the CMB matches what theorists had predicted for decades. The findings, announced this morning at a scientific presentation at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, also show that gravity—at the smallest scale—follows the rules of quantum mechanics, similar to other forces such as electromagnetism.
“This is an astounding result,” says Alan Guth, a cosmologist at Massachusetts Institute of Technology in Cambridge and one of the original proponents of inflation. Guth—who was not involved in the work being highlighted today—says the researchers showed him a draft paper a week ago, after swearing him to secrecy. “The observations are at a very high level of significance,” he says. Andrei Linde, a cosmologist at Stanford University in California who developed one of the most theoretically successful models of inflation, agrees: “If these results are right, inflationary theory has passed its most difficult test ever.”
The discovery comes from observations by a small but sophisticated telescope at the South Pole dedicated to a project known as the Background Imaging of Cosmic Extragalactic Polarization (BICEP). Just like visible light and other kinds of electromagnetic radiation, a cosmic microwave’s electric and magnetic fields could be oscillating along any of an infinite number of orientations. The telescope used by the BICEP researchers is designed to map the orientation—or polarization—of the CMB as it varies in different parts of the sky. In data taken from a small patch of the sky between January 2010 and December 2012, the researchers found a random pattern of faint pinwheel-like swirls in the CMB. Such swirls, called B modes, are the hallmark of gravitational waves in the primordial universe and—many cosmologists say—are the smoking gun for inflation.
“We believe that gravitational waves could be the only way to introduce this B-mode pattern,” says John Kovac, a cosmologist at Harvard and one of the four principal investigators of BICEP.
According to the standard model of cosmology, when the universe sprang into existence, it contained a quantum field similar to an electric field that was roiled with quantum fluctuations. Inflation magnified those infinitesimal fluctuations to enormous size, seeding differences in density, energy, and matter that ultimately led to the cosmic structure we see today, with galaxies and other features. The fluctuations also created variations in temperature of the CMB across the sky, from which cosmologists have determined the content of the universe in terms of ordinary matter, mysterious dark matter whose gravity binds the galaxies, and weird space-stretching dark energy.
But, thanks to quantum mechanics, not only did the quantum field created in the big bang fluctuate—so did spacetime itself. Inflation stretched that spacetime jittering into gravitational waves billions of light-years in wavelength. The waves left their own imprint on the CMB, scattering CMB photons in a way that created the polarization pattern that Kovac and his colleagues observed.
BICEP scooped a gaggle of other experiments, including the European Space Agency's Planck spacecraft. Suzanne Staggs, a cosmologist at Princeton University who works on the Atacama B-mode Search in Chile, says she was shocked when she heard the news. "The more I think about this, the more excited I am because the signal is so big," she says.
Kovac says the experiment’s success was due in large part to advances in the technology for measuring CMB polarization. The detectors for the telescope used to make the observations were made by the same group—led by Jamie Bock at the California Institute of Technology—that developed detectors for Planck. Whereas Planck was launched in 2007, “we have been able to go down to our telescope every year and upgrade the detectors,” Kovac says. Also, Planck was surveying the entire sky while BICEP’s telescope was focused on a small region. “We are now eagerly awaiting Planck’s polarization results,” Kovac says.
The discovery of B modes is just the beginning, researchers say. The strength of the current signal tells theorists the energy density during inflation. By studying the statistical distribution of larger and smaller swirls on the sky, they may be able to piece together a detailed picture of the energy and density distribution in the primordial universe. That would help in nailing down a precise model of inflation.
“For us, it’s huge, as big as it gets,” says Marc Kamionkowski, a cosmologist at Johns Hopkins University in Baltimore, Maryland, of the results. “It’s not every day that you wake up and find out what happened one trillionth of a trillionth of a trillionth of a second after the big bang.”