Big bang waves: direct evidence of universe's extraordinary expansion
Scientists have posited that inflation, a fleeting period of exponential growth just after the big bang, was central to the universe's structure today. Now they have a hint that they are right.
A vanishingly subtle twist in the orientation of light from the edge of the visible universe has provided the first direct evidence for cosmic inflation – a fleeting period of exponential growth when the universe was only a tiny fraction of a second old.
Inflation is of fundamental importance to scientists' theories about how the universe formed. Without it, scientists are unable to fully explain how the big bang could have created the universe we see today.
While scientists have found some evidence for inflation before, some cosmologists have argued that recent research raised some serious difficulties for the idea. But these researchers, which included Harvard University's Avi Loeb, note that measurements such as those reported Monday could help resolve the difficulties.
If the results released Monday are confirmed, as many expect, they will allow researchers to probe "a new regime of physics – the physics of what happened in the first, unbelievably tiny fraction of a second in the universe," says John Kovac, an astrophysicist at the Harvard Smithsonian Center for Astrophysics in Cambridge, Mass.
The team's detection "is not something that is just a home run; it's a grand slam," adds theorist Marc Kamionkowski, a researcher at The Johns Hopkins University in Baltimore who works at the crossroads of cosmology and particle physics and was not part of the team making the discovery. "It's the smoking gun for inflation."
"If confirmed, this result would constitute the most important breakthrough in cosmology over the past 15 years," says Dr. Loeb, who says he now counts himself more firmly in the inflation camp than he did last year.
Several researchers say they expect to see these results join the ranks of Nobel-winning work, such as the discovery of dark energy, a repulsive force that is driving the cosmos apart at an ever faster clip, and detailed measurements of the cosmic microwave background, the afterglow of the big bang.
Inflation first was proposed in 1980 – at the time "a figment of theoretical imagination," Dr. Kamionkowski says. "But it was a really interesting idea."
The universe's apparent uniform structure in all directions and its flat geometry ran afoul of what scientists thought should be happening based on their understanding of the physics behind the big bang. The big bang should have led to a curved universe with a far more random structure.
Inflation posited that this could be solved if the universe grew enormously – at a rate faster than the speed of light – in the trillionth of a trillionth of a trillionth of a second after the big bang. But how to find any evidence of that vanishingly fast event 13.8 billion years ago?
It was the study of the universe's microwave background that led cosmologists to take inflation seriously, Kamionkowski says.
Theorists had reasoned that if the universe began in an enormous release of energy, some sort of afterglow should still be visible, and in 1964, Arno Penzias and Robert Wilson, who worked at Bell Telephone Laboratories, discovered this afterglow as a hiss in their microwave antenna.
Since then, scientists have gathered precise maps of this microwave background and the distribution of its temperature across the sky. They found that subtle changes in temperature reflect subtle variations in density that, over time, would become galaxies, galaxy clusters, and larger structures in the cosmos.
The maps display enough uniformity across the sky to support inflation as the mechanism for flattening and smoothing the universe at its largest scales, but enough subtle variation to hint at quantum fluctuations during inflation that would lead to regions of different density.
In looking to corroborate inflation, however, Dr. Kovac examined not the temperatures of the cosmic microwave background but its polarization patterns.
Previous work had suggested that inflation's unique polarization pattern should leave an imprint on the cosmic microwave background. But it was unclear if that was strong enough to be detected, says Lloyd Knox, an astronomer at the University of California at Davis who focuses his work on the early universe.
Kovac's team, as well as others, had detected one mode of polarization, which can be linked to inflation but which also has other sources. But using BICEP2, a microwave telescope at the South Pole, his team detected the telltale twist of so-called B-mode polarization, which comes only as a byproduct of inflation.
Theory holds that B-mode polarization is imparted by gravity waves – undulations in the fabric of space-time that were triggered by the roiling expansion of an inflating universe.
It took Kovac's team three years of painstaking efforts to analyze the data, but its first detection of gravity waves from the universe's inflationary period could have huge implications for science.
Some theorists have been seeking to show that the four forces of nature evident today are merely low-energy manifestations of one single force that briefly reigned at the dawn of the cosmos. Researchers have shown that three of the four forces of nature – electromagnetism, the strong force (which binds atomic nuclei), and the weak force (which governs radioactive decay) – were one during inflation and based firmly on quantum physics. But the fourth, gravity, seems locked in classical physics and Einstein's general theory of relativity. Theorists have been working on ways to bring gravity into the quantum-physics fold.
The observation of gravity waves from inflation hints that gravity might have once acted on the quantum scales that characterized inflation.
"The waves that result, that are frozen on cosmological scales, are observable as a pattern today," Kovac says.
Whether the link could ever be proven is another matter. The energy levels during inflation were 10 trillion times higher than the energies that the Large Hadron Collider – the most powerful particle collider on earth – can reach.
Although the results back the general notion of cosmic inflation, theorists have devised a range of variants – many of which now have been ruled out by these results. Among those remaining, there are still some discrepancies between the new results and some of the models, notes Albert Stebbins, an astrophysicist at the Fermi National Accelerator Laboratory in Batavia, Ill.
Still, "this result sounds like it's not going to go away," he says.
More work needs to be done to see how well these results, and those expected from other experiments aimed at mapping B-mode polarization over wider swaths of the sky than the BICEP2 team explored, compare with the remaining variants on inflation, Dr. Stebbins says. "It sounds like this is pretty good."