In 2003, a group of Japanese astronomers studying the center of galaxy 3C 66B thought they’d spotted a pair of record-breaking supermassive black holes orbiting one another: each of them appeared to have the mass of more than 27 billion suns (1). But a dissenting paper soon appeared, one that gleaned its insights from an unexpected source. A team watching a pulsar—a rapidly rotating neutron star that shoots out a beam of radiation like a cosmic lighthouse—knew that such a massive black hole binary system would be emitting powerful gravitational waves, which would have interfered with the pulsar’s signal as they swept past (2). Hence, their observations suggested no such black holes existed. Pulsar astrophysicist Andrea Lommen of Franklin & Marshall College in Lancaster, Pennsylvania, remembers a conversation with collaborator Rick Jenet, an astronomer at the University of Texas at Brownsville. “Rick called me and said ‘They don’t have a source that big because we’d see it,’” recalls Lommen.

Using the Green Bank Telescope and Arecibo Observatory (pictured), the NANOGrav project aims to monitor pulsars in the hopes of spotting minute variations in their beams, suggesting ripples in the fabric of space-time. Image courtesy of Shutterstock/Dennis van de Water.

Until this point, many researchers had considered that it might one day be feasible to use pulsars to detect the presence or absence of gravitational waves. But few knew that the field had matured to any sort of practical applications. “I think that kind of woke people up and made them realize this pulsar-timing thing isn’t just a ruse,” says Lommen. “We’re actually going to do something.”

Listening for Gravity About four years later, Lommen, Jenet, and a few of their colleagues came together to form the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), a collaboration that aims to turn the local universe into a gigantic gravity wave listening device. Using the Green Bank Telescope and Arecibo Observatory, the researchers monitor 54 pulsars in the hopes of spotting minute variations in their beams that could indicate ripples in the fabric of space-time. Along with similar projects around the world, NANOGrav could one day reveal important information about the dynamics of black holes, the formation of galaxies, and potentially even more exotic phenomena. Like electromagnetic waves, gravitational waves come in a spectrum of different frequencies. So these efforts are complementary to those of the Laser Interferometer Gravitational-Wave Observatory (LIGO), which announced its first gravitational wave detection in February of this year and another in June, but which is sensitive to waves 11 orders-of-magnitude smaller than the ones NANOGrav intends to see (3, 4). The observations require extreme precision, and it’s been difficult to find enough useful pulsars and to develop the detection methods. The NANOGrav team thinks they could spot gravitational waves in the next 5–10 years, although exactly when they’ll finally capture their elusive signal remains an open question. As an enormous passing gravitational wave sweeps past the Earth, it distorts the fabric of space-time. The pulsating signals from nearby pulsars are also altered, arriving slightly earlier or later than expected. By carefully tracking these deviations, researchers can reveal the gravitational wave's presence. Image courtesy of B. Saxton (NRAO/AUI/NSF).

Wrinkles in Space-Time Pulsars were first discovered in 1967, when astronomers Jocelyn Bell Burnell and Antony Hewish detected regular radio pulses coming from a spot on the night sky (5). It was eventually determined these were being emitted by the remnant core of a gigantic star that had gone supernova, leaving behind a highly magnetized neutron star whose beam was repeatedly sweeping past the Earth. Within a decade, other researchers had cataloged more pulsars and begun realizing they might be used to test a part of Einstein’s Theory of Relativity, which states that massive accelerating objects radiate gravitational waves (6). “If you think of the pulsar as a clock, it has an internal rotation period, which is very stable,” says astrophysicist Maura McLaughlin of West Virginia University, another member of the NANOGrav team. “If nothing else was happening, we’d see its pulses bump, bump, bump—perfectly regular.” A gravitational wave is a wrinkle in the fabric of space-time. Should one come between the Earth and a pulsar, the distance between them would shrink or stretch and the pulsar’s pulse would arrive slightly sooner or later than expected. A single pulsar signal arriving off beat probably wouldn’t mean much. But if scientists saw the exact same shift in pulsars all over the sky, it could indicate a passing gravitational wave. The fluctuations are extremely tiny; over the course of five years the method would notice deviations of just a few nanoseconds from what was expected. So NANOGrav and other pulsar-timing arrays only study highly stable millisecond pulsars, which can rotate nearly a thousand times per second and whose pulse arrivals can be predicted on nanosecond timescales. “There are signals we would recognize as being like a pure tone, sort of like a tuning fork,” says Jenet. “And then there are signals that sound more like if you’re listening to a radio with nothing on it; you’d get that snow or white noise that sounds like rain.” The pure tone waves would come from the aftermath of a galactic collision. Nearly every galaxy is thought to have a supermassive black hole in its center. When two galaxies crash and merge their central black holes will orbit one another, produce gravitational waves, and eventually combine into a single behemoth black hole. This sequence can take several billion years and there remain many uncertainties in models explaining the underlying physics. Gravitational waves from such a system would tell astronomers about the black holes’ size and speed, and optical telescopes could perform follow-up observations to help them learn the details of the process. Such a finding would require there to be a relatively close supermassive black hole binary emitting powerful gravitational waves. So the NANOGrav team expects to see a more likely signal first: echoes from the era of galaxy formation. Cosmological simulations suggest that galaxies started out small and then collided with one another to produce the larger spiral galaxies seen in the present day. Each collision would have also involved a We may see signals from early universe inflation that would place constraints on the Big Bang.—Maura McLaughlin supermassive black hole merger and the sum total of their gravitational wave emissions “should produce an overall crinkling of space-time,” says Lommen. This background would be a complex waveform pattern but it would be embedded with important information. By studying it carefully, researchers could answer many open cosmological questions, such as whether bigger galaxies consumed little ones over time or if galaxies of roughly the same size merged together. Then there are the unexpected results that come from opening a new way of looking at the universe. “We may see signals from early universe inflation that would place constraints on the Big Bang,” says McLaughlin. Or, he adds, they may see the absence or presence of cosmic strings, very dense and thin objects that some theorists believe evolved during the earliest fractions of cosmic history.