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When searching for clues about the physics of the early universe, one has to aim high—and a balloon-borne telescope array known as Spider, set to soar over Antarctica this winter, may succeed where a highly publicized ground-based experiment fell short.
Physicists working on that earthbound experiment, known as BICEP2, announced several months ago that they had found evidence for primordial gravitational waves—ripples in the fabric of space itself, dating back to the universe’s earliest moments. The finding made headlines around the world. After further analysis, however, they admitted their data was inconclusive. The signal they detected could just as likely have been caused by interstellar dust as by the much-sought-after gravitational waves. Scientists working with Spider believe its detectors may settle the matter. (Spider’s six telescopes gave the experiment its name. Splayed out in a circle in an early design, they suggested a six-legged creature. The design has since evolved, but the name stuck.)
As with BICEP2, the detectors on board Spider will measure the cosmic microwave background, or CMB, created when the universe was less than half a million years old. Embedded within the CMB, however, may be a telltale signal from an even earlier epoch: the fleeting period of cosmic inflation, when the universe grew exponentially during the first tiny fraction of a second after the big bang. According to general relativity—Einstein’s theory of gravity—a slew of gravitational waves should have been released during this period of inflation. These waves should distort the CMB, changing the polarization of individual microwaves (that is, changing their orientation of their constituent electric and magnetic fields); the resulting patterns of subtle swirls are called “B-modes.”
There is a problem, however. Tiny grains of interstellar dust, aligned with our galaxy’s magnetic field, radiate at about the same frequency and can create a similar pattern of microwave swirls. Earth’s atmosphere adds to the trouble because it degrades both signals to the point where they are hard to tell apart. Spider, about the size of a large SUV, is intended to rise above this difficulty. It will be carried high, into the stratosphere, by a giant helium balloon. Observing from such a height “buys you an enormous amount of sensitivity,” explains Jeffrey Filippini, a physicist at the California Institute of Technology and the head of the team designing Spider’s microwave receivers. “A day in flight is as good as a month on the ground.”
Spider also has a second advantage. Whereas BICEP2 measured the CMB at only a single frequency, Spider will collect data at two frequencies—and the ratio of these two signals, Filippini says, can also help distinguish the effects of dust from a possible gravitational wave imprint.
Caltech is one of about a dozen institutions from the U.S., U.K., Canada and South Africa that are collaborating on Spider, which is set to fly in late December. Many of the researchers now working on Spider, including Filippini, are also part of the BICEP2 collaboration.
In fact, Spider and BICEP2 are just two of the many experiments, some in operation and some in the planning stages, that will be joining in the hunt for primordial gravitational waves in the months ahead, including the South Pole Telescope and the Atacama Cosmology Telescope. Data from the Planck spacecraft, which has measured the amount of dust over the entire sky, have also been vital in helping to calibrate other studies that cover smaller regions of the sky. Meanwhile, physicists using the POLARBEAR telescope array in Chile recently announced that they had found B-mode polarization in the CMB. These B-modes, however, are thought to be the product not of gravitational waves but of another phenomenon called gravitational lensing—a distortion of the CMB due to intervening matter such as galaxy clusters, whose gravity could pull microwaves into a B-mode swirl. POLARBEAR may provide valuable information about the first structures that formed in the universe, but it was not designed to probe the era of inflation.
The theory of cosmological inflation, developed in the early 1980s, is currently the leading theory for explaining how tiny quantum fluctuations at the beginning of time were magnified into the giant structures that make up today’s universe. Quantum noise should also be imprinted onto the gravitational waves created by inflation, and the waves, too, should get blown up to cosmological scales. Just as the CMB can be thought of as the echo of the big bang, these gravitational waves could be read as the gravitational echoes of the rapid expansion brought about by inflation. If Spider detects a gravitational wave signal in the CMB, it will be seen as “very strong evidence for the inflationary picture,” Filippini says. On the other hand, failure to detect the waves may not rule inflation out, since the theory is very flexible. “Theorists are clever, and whatever we observe I’m sure there will be multiple interpretations,” he says.