Big Bang's "Smoking Gun" Confirms Early Universe's Exponential Growth

Observations confirm "inflation" of early universe to cosmological sizes in early instant.

Gravitational waves trigger ripples in space, seen in this illustration.


Surprisingly strong gravitational waves rippled through the fiery aftermath of the Big Bang, astronomers announced Monday, a finding that confirms the cosmos grew to a stunningly vast size in its very first moments.

The long-sought observations, taken from Antarctica, strongly support the cosmological theory of "inflation," which explains how the early universe smoothly expanded to unimaginable vastness in the first fractional second of its existence. (See: "Origins of the Universe.")

"This is a first detection of a gravitational wave signal," says physicist Clement Pryke of the University of Minnesota in Minneapolis, a member of the scientific team that reported the results Monday at the Harvard-Smithsonian Center for Astrophysics in Massachusetts. "We're firmly convinced these observations are real."

The finding means that in little more than a century, humanity has figured out not only the age of the universe—it was born about 13.82 billion years ago in the Big Bang—but also how its birth unfolded. (See also: "Proof of Big Bang Seen by Space Probe.")

"I never thought I would see these measurements made in my lifetime, and it is thrilling to learn of them," said Alan Guth of the Massachusetts Institute of Technology in Cambridge, who first described a theory of cosmic "inflation" in 1980. "These are very difficult measurements."

To make the gravitational wave discovery, the team studied the cosmic microwave background, leftover heat from the origin of the universe.

Imprinted across the entire sky, this relic heat reveals in minute temperature differences where matter condensed about 400,000 years after the Big Bang. Those patterns also serve as a frozen snapshot of the conditions inside the moment of creation that spawned it.

That matter later condensed further to form the clusters of galaxies that fill the universe today.

Scientists on the BICEP2 team, which reported the new observations on Monday, hailed from a wide number of scientific institutions. Their team is just one of many observing the cosmic microwave background in a long-running hunt for hints to how the Big Bang happened.

The sun sets behind BICEP2 and the South Pole Telescope.


The BICEP2 team did not observe gravitational waves directly. Instead, it observed their effect on the cosmic microwave background. The researchers looked for curling patterns in the distribution of temperature and matter seen there, which is a signal of gravitational waves.

Gravitational waves stretch and compress space as they travel, lumping together matter at intervals determined by the size of the stretching. Described by Einstein's theory of gravity, the waves are predicted as a signature effect of inflation, and are ruled out by competing theories.

"Thus, if this discovery is confirmed, inflationary theory does not have any real alternatives," Andrei Linde of Stanford University, who was not part of the study team, said by email.

Antarctic Observations

The discovery team relied on microwave-detecting telescopes in Antarctica, where clear skies and dry air allowed unparalleled views of the cosmic microwave background.

The South Pole measurements came over three seasons of observation from 2010 to 2012, in areas of the sky two to ten times as wide as the full moon. These wide views allowed telescopes to look for gravitational wave patterns in the cosmic microwave background.

Inflation theory held that these light particles, seen as microwaves today, should be preferentially twisted in their orientation, an effect similar to the light blocking of polarized sunglasses.

The BICEP2 Telescope's focal plane consists of 512 superconducting microwave detectors, developed and produced at NASA's Jet Propulsion Laboratory.


Powerful Polarization

Indeed, the team reports that a distinctive polarization pattern emerged. The effect was about twice as strong as expected from indirect measures of the effect, which in recent years had been estimated by satellite observing teams.

"That is a surprise, and it does mean that gravity was working more strongly than we expected during inflation," said MIT's Guth. "It will be very reassuring to see the result confirmed."

Pryke said the finding was so surprising that the discovery team spent three years checking the result. Statistics suggest that they have a 99.9997 percent certainty of being correct.

"This is not just a home run, it is a grand slam," says physicist Marc Kamionkowski of Johns Hopkins University in Baltimore, who spoke at the briefing but was not on the discovery team. "It is the smoking gun for inflation."

Graduate student Justus Brevik tests the BICEP2 readout electronics.


Jittering of Matter

The strong gravitational wave findings support some of the simplest models of inflation and explain how the mass of the universe first escaped from subatomic size without falling in on itself in its very first moments.

Inflation suggests that the dense energy of the Big Bang drove an exponential expansion of the boundaries of space (hence the name "inflation") within the first millionth of a trillionth of a trillionth of a second of the existence of the universe.

This rapid expansion produced the gravitational waves, Guth says, which are "just a jittering of matter in the universe."

That means that in its very first moments, the entire universe reached a size far, far larger than what is observable or will ever be observable to humanity (the "observable" universe is about 92 billion light-years across).

Correction: An earlier version of the story misstated the width of the observable universe, which is about 92 billion light-years across.

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