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Gravitational waves unmask universe just after Big Bang

For first time, researchers see traces of superfast cosmic expansion

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6:23pm, March 17, 2014

COSMIC SWIRL  Gravitational waves generated during a period of cosmic inflation twirl light from the cosmic microwave background, as seen in this sky map from the BICEP2 telescope. The lines trace the alignment, or polarization, of photons released after the Big Bang; the line lengths show the light’s intensity. The colors indicate how strongly twisted the polarization is, both clockwise (red) and counterclockwise (blue).

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Astronomers have detected the earliest echoes of the Big Bang, confirming a decades-old hypothesis that describes the universe’s ultrafast expansion during its first moments. The findings provide researchers with the first direct measurement of conditions at nearly the instant that cosmic expansion began, and may have far-reaching implications for physicists’ understanding of general relativity, quantum mechanics and the origin of the universe.

“We now have a much stronger belief that we understand the early universe than we did yesterday,” says Sean Carroll, an astrophysicist at Caltech.

For decades, astronomers have tried to understand several quirks surrounding the Big Bang, the story that describes how the universe began and evolved over its 13.8 billion year lifetime. In the 1980s, MIT physicist Alan Guth suggested that many of the oddities could be explained if the universe underwent a period of hyperexpansion, known as inflation. During inflation, Guth proposed, the universe’s volume grew by a factor of at least 1075 in the first trillionth of a trillionth of a trillionth of a second after the Big Bang.

Astronomers have long predicted that inflation, if it occurred, would have left marks on the cosmic microwave background radiation, the flash of light released into space about 380,000 years after the Big Bang, when the universe had cooled down enough for light to travel freely.  One signature could be found in how the photons align with one another, what physicists refer to as polarization. Gravitational ripples induced by inflation would have set up swirling patterns in the polarization. Up until now, this “B-mode polarization” has been exceedingly difficult to detect. But a detection of B-mode polarization would strengthen the case for inflation: Primordial gravitational waves are the only known source.

That’s just what a group of researchers did, the team announced today. Led by John Kovac, an astronomer at the Harvard-Smithsonian Center for Astrophysics, the researchers measured subtle variations in the polarization of the cosmic microwave background using the BICEP2 telescope. The telescope, located in Antarctica, houses 512 detectors, each cooled to nearly one-quarter of a degree Celsius above absolute zero. The detectors alternate, half filtering out horizontally aligned light and half vertical light. By regularly scanning a region of the sky above the South Pole, the researchers were able to map a chunk of the cosmic microwave background with polarized light. They released their results in papers posted online.

The strength of the gravitational waves is significantly higher than researchers had expected. Data from the Planck mission — a space telescope that, from 2009 to 2013, mapped minute variations in infrared and microwave light from the cosmic microwave background (SN: 4/20/13, p. 5) — showed little indication that gravitational waves played any role during inflation. The surprisingly strong signal rules out several possible models for inflation, although it’s not yet clear why Planck and BICEP2 disagree. 

“This opens up a whole new window, a whole new research area,” says Scott Dodelson, an astrophysicist at Fermi National Accelerator Laboratory in Batavia, Ill., who compared the finding’s importance to that of the recent discovery of the Higgs boson (SN: 7/28/2012, p. 5). The high energies seen in the inflationary epoch, he says, make it possible to test some ideas from string theory, which many assumed to be untestable. “This is a playground for everyone to start testing their theories,” he says.

Both Carroll and Dodelson, from preliminary looks at the data, believe that the team has been careful to understand and account for possible statistical sources of error. “It’s not a fluke, that’s for sure,” Carroll says.  But he says that researchers don’t know for certain if the signal is really from the early universe or if it’s an artifact from the telescope or intervening galaxies. At least eight other telescopes continue to look for B mode polarization. Their findings will help nail down the true source of the signal.

“People are skeptical, that’s what we do for a living,” says Dodelson. But before he and others start to scrutinize the data more carefully, he’s enjoying the moment. “The main message is one of excitement,” he says. “This is a game changer.”

Editor's Note: This story was updated on March 19, 2014, to correct the dates of the Planck mission and to clarify the difference in results obtained by Planck and BICEP2.

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