When gravity rocked the universe
From a telescope at the South Pole, researchers find concrete evidence of the early universe’s incredible expansion
March 17, 2014
Right after its birth in the Big Bang, the infant universe threw the mother of all tantrums.
In an instant it ballooned from smaller than an atomic nucleus to something much larger than its currently observable size. (Yes, the universe is bigger than we can currently observe.)
This infinitesimally short period of expansion is called the inflation epoch, and it was rough. Inflation generated all kinds of particles and perturbations, including gravitational waves, which made gigantic ripples in the very fabric of space.
Or so the theory of inflation goes. But there has been scant hard evidence to bolster it. Until now.
A team of physicists, led by the University of Minnesota’s Clem Pryke along with colleagues from Harvard, Stanford, and Caltech, has just confirmed a key piece of the theory. Inflation predicts that gravitational waves will have left a certain “signature” on the cosmic microwave background (CMB) radiation—a form of light that fills the sky and has been called “the afterglow of the Big Bang.”
Working with the BICEP2 telescope at the National Science Foundation’s South Pole Station, the researchers detected the clear imprint of gravitational waves in the CMB.
“It’s the smoking gun for inflation,” says Pryke, an associate professor of physics. “There’s no other explanation for the gravitational waves [whose imprint] we’re seeing.”
A polarizing influence
The universe began with the Big Bang some 13.7 billion years ago. Inflation followed immediately, and went incredibly fast: It began somewhere around 10-32 seconds after the Big Bang and was over after another 10-32 seconds or so had passed. It produced high-amplitude gravitational waves, which propagated through the newly expanded universe.
About 380,000 years later, the universe had cooled enough for the first true atoms to condense, opening up a path for light to travel freely. That first light persists today as the CMB, which has been stretched to microwave wavelengths by the universe’s continued expansion.
As the CMB light was scattering for the last time, the presence of the gravitational waves imprinted a special curling pattern of polarization on the CMB. This observed curling pattern in the CMB, called B mode polarization, was not only a smoking gun for gravitational waves and the inflation theory, but unexpectedly strong.
“We were looking for a needle in a haystack, but instead we found a crowbar,” says Pryke.
Another triumph for Einstein
The existence of gravitational waves is predicted by Einstein’s theory of general relativity. The waves have never before been observed, but the BICEP2 finding means general relativity has passed another test, says Pryke.
It also brings physics closer to its holy grail: a theory of everything (TOE) that describes the four fundamental forces of nature within a single framework. Many physicists believe that in a period previous to inflation, known as the Planck epoch, the universe was unimaginably small, dense, and hot and only one force existed. But it split into four, which became electromagnetism, two forces that govern the behavior of the atomic nucleus, and gravity. Currently, quantum mechanics describes the first three, but theorists have been unable to bring gravity into the fold.
In observing the handiwork of gravitational waves through BICEP2, however, physicists are looking into gravity’s behavior during a window of time that is “as close as we will ever get” to the Planck epoch, Pryke says.
“This is a direct insight into physics at the Planck scale, where energies were very high and all the forces were unified,” he explains. “The discovery has implications for those trying to formulate [a unified theory that describes all the forces]. Observing B modes could be the only experimental evidence we’ll ever get, because we’ll never be able to build a machine that can recreate the energy of the Planck epoch.”
The inflation theory was first proposed by American physicist Alan Guth in 1980. Besides Pryke, principal investigators for the BICEP2 collaboration are John Kovac at Harvard/CfA, Jamie Bock at Caltech, and Chao-Lin Kuo at Stanford/SLAC.
Watch a podcast of Pryke's talk at the February 2014 Cafe Scientifique, sponsored by the University's Bell Museum of Natural History.
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