How Do You See Gravitational Waves?

Photo: Steffen Richter/Harvard University

On Monday, a team of astronomers announced some very big, potentially Nobel Prize-winning news: the first sighting of gravitational waves that filled the very early universe. 

The detection, which was made by the BICEP2 experiment in Antarctica, still needs to be confirmed by other experiments. But if it is, the signal could provide a window into "inflation"—a brief but explosive period when the universe is thought to have ballooned enormously in size. "This is literally a window back to almost the beginning of time itself," physicist Lawrence Krauss told Wired magazine.  

As a technology-minded physics buff, I inevitably wind up asking two very basic questions whenever a long-sought discovery takes place. First, why didn't we see this before? And, second, why are we seeing it now?

The answer to the first question is pretty much what you'd expect: the signal is quite hard to see. Like a number of ongoing experiments, BICEP2 hunted for evidence of primordial gravitational waves in the cosmic microwave background (CMB), a haze of light that was given off when the universe was just 380 000 years old. After some 13.8 billion years of cosmic expansion, the wavelength of this relic radiation has been stretched so that the photons now reaching Earth sit in the microwave band of the electromagnetic spectrum.

Astronomers have been studying the CMB since it was discovered in 1963. And over the decades, they've designed a range of detectors to map its properties. These have been lofted into the air by balloons, mounted on mountaintops, and launched into space. And most have been designed to do what amounts to taking the temperature of the CMB. The small differences in intensity from spot to spot correspond to variations in temperature and density in the early universe. This data can be used to determine a range of properties about the universe's composition, including the density of dark energy and the amount of ordinary matter.

BICEP2 and other experiments have been searching for something different and considerably more elusive: evidence of gravitational waves that stretched and compressed space itself. They aren't looking for the gravitational waves themselves, but rather the imprint these waves have left on the CMB. This shows up as a slight "twist" or "swirl" in the polarization of photons. This kind of polarization is called the B-mode. 

Most of the photons emanated by the CMB have randomly oriented polarizations. And there are other sources of polarization in the CMB. In the end, astronomers were looking at "B-mode" fluctuations on the order of one 10 millionth of a degree in the haze of 2.73-Kelvin CMB radiation, says cosmologist Andrew Jaffe of Imperial College London. The much-studied temperature variations are a much bigger signal, roughly 1/100 000th of a degree. To add to the difficulty, confounding sources like galaxies can produce polarization patterns that look like those created by primordial gravitational waves. 

Given all of that, the results owe a debt to "very very careful data analysis," says BICEP2 team member Kent Irwin, a professor of Stanford University and at the SLAC National Accelerator Laboratory.

But Irwin says, something is also different this time around—a set of superconducting circuitry that has dramatically improved the sensitivity of the experiments. "New generations of hardware have totally enabled this measurement," he says.

A snapshot of part of BICEP2’s antenna network, which is used to separately detect the x- and y-polarization of light. (Image: BICEP2)

BICEP2's detectors, which were fabricated at NASA's Jet Propulsion Laboratory in Pasadena, Calif., employ a few key technologies that weren't present in the previous generation of B-mode hunters. 

BICEP's first incarnation, which ran until 2008, used semiconductor-based bolometers. Bolometers heat up when hit with radiation, and this heating shows up as a change in electrical resistance that can then be sensed by read-out circuitry and processed. But the semiconductor versions of these devices proved difficult to manufacture into large arrays, which is what astronomers needed in order to scan the sky faster and drive down statistical noise. A key issue was the wiring. To keep the detector noise down, the transistors were cooled to less than a degree above absolute zero, but at some point, the signals had to be taken out of the telescope to room-temperature electronics. The more sensors you have, Irwin says, the more wires and thus more heat-conducting elements that can sap the cooling power of your cryogenic equipment.

For BICEP2, the bolometers were made from superconducting elements that can be manufactured all in one go. Much of the processing electronics was also made from superconducting circuity and moved into the detector. To cut down on the wiring, the pixels were made to share wires and were read out one at a time. With this set-up, the team was able to construct a system containing 512 superconducting sensors, compared to BICEP1's 98 semiconducting ones.

To map the CMB with this array, each of BICEP2's pixels takes in incoming radio waves through two interleaved phased array antennas. One picks up the signal from photons with an x component to their polarization, and the other is sensitive to the y component. The resulting electromagnetic signals are then sent down two superconducting transmission lines. Each line terminates in a small resistive device containing what's called a "transition edge sensor". The current is converted into heat, and the heat alters the conductivity of the circuitry. The result is read out and processed by a superconducting integrated circuit.

Microwave radiation enters from the left into a “resistive meander”. The heat deposited is read out by a titanium transition-edge sensor (TES) [blue rectangle on right]. A second, aluminum TES [white rectangle] is used to improve the response. (Image: BICEP2)

The sensors are called "transition edge sensors" because they operate right at the transition zone where a material goes from being normally conducting (resistive) to superconducting (lacking resistance). Transition edge sensors ride this edge, self-regulating along the way. When their temperature goes down, their resistance also decreases. This lets more electricity flow across them, which raises the temperature and thus the resistance. The increase in resistance reduces the power dissipated, lowering the temperature.

"It's an exquisitely sensitive instrument," says Irwin, who worked to develop them while a graduate student at Stanford some 20 years ago. "You can't actually get more sensitive." At this point, the sensitivity of such detectors is limited by the shot noise of the incoming photons: even if the temperature of a spot on the sky stays perfectly stable, the number of photons hitting the detector will fluctuate over time due to statistics simply because there's a finite number of them.

Nowadays, most of the B-mode hunting experiments, which include Polarbear in Chile and the South Pole Telescope (which sits right next to BICEP2 and can be seen in the background in the photo at the top of this story) incorporate similar superconducting detectors. The Planck space telescope launched less than a year before BICEP2 started operations, but it carries semiconductor-based polarization sensors because space-bound engineering specs have to be locked in quite early.

Irwin says that BICEP2 likely found the gravitational wave signal first because it was optimized for this observation and began a bit earlier than its competitors. The South Pole Telescope saw the first evidence of B-mode signals in CMB light last year. But these signals turned out to be due to the bending of CMB light by intervening massive objects. BICEP2, which has a smaller telescope and lower resolution, is optimized to look over larger angles, where the gravitational wave signal is expected to be strongest. Given all the clutter in the sky, sometimes it helps to have some fuzzy glasses.

If the signal holds up, we'll likely be able to extract even more information from it in the not-too-distant future. BICEP2 looked at light of just a single microwave frequency: 150 GHz. The next step for these kinds of detectors will be to adjust the electronics so they're sensitive to more colors, says Matt Dobbs of McGill University in Montreal, who has worked on the South Pole Telescope's B-mode detector.

That will be a "gigantic jump", Dobbs says, that will double or triple the number of photons the detectors can accept, improving sensitivity. It will also allow astronomers to perform some basic spectral analysis that will help them separate out relatively near sources of polarized light from those coming from the CMB. In the end, this technology could help us learn even more about the universe's earliest moments.

This post was updated on 19 March.