POLARBEAR project offers clues about origin of universe's cosmic growth spurt

Since the discovery in the late 1920s that the universe was expanding, step by step cosmologists have been working backward, building a picture of how the universe began and subsequently evolved.

In the 1980s, theorists proposed a seemingly outrageous step for the first fleeting fraction of an instant following the big bang, an enormous release of pent up energy that gave birth to the universe.

To solve discrepancies between the physics behind the big bang, as they then understood it, and the cosmos astronomers observed, they proposed that the universe underwent a powerful growth spurt, briefly expanding faster than light can travel.

Now, an international team of scientists say they have detected specific, subtle changes in the afterglow of the big bang that could well testify to the action of inflation on the cosmos shortly after it emerged from an enormous release of energy some 13.8 billion years ago.

This period of rapid inflation would have triggered waves that ruffle the very fabric of space-time itself. These gravity waves would reveal themselves through b-mode polarization in the afterglow, known as the cosmic microwave background.

In principle, the study of this primordial b-mode polarization could allow scientists to study the physics at play during a time when the four forces of nature humans see today were one unified force – something that occurred at energy levels no earthly particle accelerator could hope to achieve.

But several imposters – from dust to the ability of gravity to bend light – also introduce b-mode polarization in the cosmic microwave background, making the detection of polarization from primordial gravity waves difficult.

Using a new telescope in Chile's Atacama Desert, the team reports that it has detected this b-mode polarization and pinned it to cosmological sources with 97.2 percent confidence, ruling out dust as a significant influence on the results.

This alone is important because it will allow researchers to begin using b-mode polarization in the radiation from the cosmic microwave background to “tell us about all gravitational structure in the observable universe, and about the dark energy and dark matter that appear to dominate the energy density of the universe,” explains Kam Arnold, a researcher at the University of California at San Diego and a member of the team reporting its results in the current issue of the Astrophysical Journal.

Others go a bit farther.

“This isn't a smoking gun,” cautions Brian Koberlein, an astrophysicist at the Rochester Institute of Technology in New York, who is not a member of the research team reporting the results. Still, the work “demonstrates that there is an inflation signal because it can't all be dust.” 

Dust can be a big deal. In March, another team of scientists working with a radio-telescope in Antarctica announced with much fanfare that it had detected b-mode polarization from primordial gravity waves, providing independent evidence for the universe's inflationary period.

Since then, however, new maps of intervening dust produced by the European Space Agency's Planck Space Telescope have indicated that dust is more ubiquitous in the region of sky the team observed than they had estimated. The primordial b-mode signal is likely to be in there somewhere, mixed in with the dust's signal, other researchers say. But its detection is now far less certain.

“We generally thought that some of the signal was due to inflation,” Dr. Koberlein says. “But BICEP2 couldn't prove it, ” he adds, referring to the project responsible for the March announcement. “These guys are saying: There is a signal, it does look like inflation, let's move forward.”

“I think it's really good work,” he says.

The project delivering these latest results is dubbed POLARBEAR. The researchers used the first of a trio of telescopes designed to measure the intensity of incoming radiation and its polarization. The telescope, designed to observe the cosmos at millimeter and submillimeter wavelengths, is located at the James Ax Observatory on the Chajnantor Science Reserve, about 17,000 feet above sea level.

The dish-shaped telescopes are designed to zero in on a much smaller patch of the sky than was the telescope in Antarctica. The size regime is one where b-mode polarization is dominated by gravitational lensing from galaxy clusters between Earth and the cosmic microwave background; the effect from dust is far lower in this smaller field of view, Dr. Arnold explains.

When the team estimated the amount of dust in its field of view, they compared it with the Planck measurements and found the two agreed.

While 97.2 certainty percent sounds close to a done deal, for this kind of work it isn't, Koberlein says. This team is being far more cautious about its claims than the BICEP2 team was, he adds, knowing that other researchers will be going over the results with the proverbial fine-tooth comb.

With the addition of two more telescopes at the site next year, each with more sensitive detectors that can measure radiation over several bands of wavelengths simultaneously, the hunt for inflation-related b-mode signals will begin in earnest, in addition to exploring the universe's gravitational structure and doing a more-precise job of separating the sources from dust, Dr. Arnold writes in an e-mail.