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Spotting gravitational waves using ticks Adam Mann, Science Writer

In 2003, a group of Japanese astronomers studying Until this point, many researchers had considered the center of galaxy 3C 66B thought they’d spotted that it might one day be feasible to use to a pair of record-breaking supermassive black holes detect the presence or absence of gravitational waves. orbiting one another: each of them appeared to have But few knew that the field had matured to any sort of the mass of more than 27 billion suns (1). But a dis- practical applications. “I think that kind of woke peo- senting paper soon appeared, one that gleaned its ple up and made them realize this pulsar-timing thing ’ ” “ ’ insights from an unexpected source. A team watching isn t just a ruse, says Lommen. We re actually going ” a pulsar—a rapidly rotating that shoots to do something. out a beam of radiation like a cosmic lighthouse— Listening for Gravity knew that such a massive binary system would About four years later, Lommen, Jenet, and a few of be emitting powerful gravitational waves, which would their colleagues came together to form the North have interfered with the pulsar’s signal as they swept past American Nanohertz Observatory for Gravitational Waves (2). Hence, their observations suggested no such black (NANOGrav), a collaboration that aims to turn the local holes existed. Pulsar astrophysicist Andrea Lommen of universe into a gigantic gravity wave listening device. Franklin & Marshall College in Lancaster, Pennsylvania, Using the and Arecibo Observa- remembers a conversation with collaborator Rick Jenet, tory, the researchers monitor 54 pulsars in the hopes of an astronomer at the University of Texas at Brownsville. spotting minute variations in their beams that could “Rick called me and said ‘They don’thaveasourcethat indicate ripples in the fabric of space-time. Along with big because we’dseeit,’” recalls Lommen. similar projects around the world, NANOGrav could

Using the Green Bank Telescope and (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.

8878–8880 | PNAS | August 9, 2016 | vol. 113 | no. 32 www.pnas.org/cgi/doi/10.1073/pnas.1611117113 Downloaded by guest on September 24, 2021 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 In- terferometer Gravitational-Wave Observatory (LIGO), which announced its first 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 obser- vations 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 As an enormous passing gravitational wave sweeps past the Earth, it distorts signal remains an open question. the fabric of space-time. The pulsating signals from nearby pulsars are also altered, arriving slightly earlier or later than expected. By carefully tracking Wrinkles in Space-Time these deviations, researchers can reveal the gravitational wave’s presence. Pulsars were first discovered in 1967, when astronomers Image courtesy of B. Saxton (NRAO/AUI/NSF). and Antony Hewish detected reg- ular radio pulses coming from a spot on the night sky billion years and there remain many uncertainties in (5). It was eventually determined these were being emitted models explaining the underlying physics. Gravita- by the remnant core of a gigantic star that had gone su- tional waves from such a system would tell astrono- pernova, leaving behind a highly magnetized neutron star mers about the black holes’ size and speed, and whose beam was repeatedly sweeping past the Earth. optical telescopes could perform follow-up observa- Within a decade, other researchers had cataloged more tions to help them learn the details of the process. pulsars and begun realizing they might be used to test a Such a finding would require there to be a relatively part of Einstein’s , which states that close binary emitting powerful massive accelerating objects radiate gravitational waves (6). gravitational waves. So the NANOGrav team expects to “If you think of the pulsar as a clock, it has an internal see a more likely signal first: echoes from the era of rotation period, which is very stable,” says astrophysicist Maura McLaughlin of West Virginia University, another galaxy formation. Cosmological simulations suggest that member of the NANOGrav team. “If nothing else was galaxies started out small and then collided with one happening, we’d see its pulses bump, bump, bump— another to produce the larger spiral galaxies seen in the perfectly regular.” present day. Each collision would have also involved a A gravitational wave is a wrinkle in the fabric of space-time. Should one come between the Earth and We may see signals from early universe that a pulsar, the distance between them would shrink or would place constraints on the Big Bang. stretch and the pulsar’s pulse would arrive slightly sooner or later than expected. A single pulsar signal —Maura McLaughlin arriving off beat probably wouldn’t mean much. But if scientists saw the exact same shift in pulsars all over supermassive black hole merger and the sum total of the sky, it could indicate a passing gravitational their gravitational wave emissions “should produce an wave. The fluctuations are extremely tiny; over the overall crinkling of space-time,” says Lommen. course of five years the method would notice devi- This background would be a complex waveform ations of just a few nanoseconds from what was pattern but it would be embedded with important in- expected. So NANOGrav and other pulsar-timing formation. By studying it carefully, researchers could arrays only study highly stable millisecond pulsars, answer many open cosmological questions, such as which can rotate nearly a thousand times per second whether bigger galaxies consumed little ones over time and whose pulse arrivals can be predicted on or if galaxies of roughly the same size merged together. nanosecond timescales. Then there are the unexpected results that come “There are signals we would recognize as being from opening a new way of looking at the universe. “We like a pure tone, sort of like a tuning fork,” says Jenet. may see signals from early universe inflation that would “And then there are signals that sound more like if place constraints on the Big Bang,” says McLaughlin. Or, you’re listening to a radio with nothing on it; you’d he adds, they may see the absence or presence of cos- get that snow or white noise that sounds like rain.” mic strings, very dense and thin objects that some the- The pure tone waves would come from the after- orists believe evolved during the earliest fractions of math of a galactic collision. Nearly every galaxy is cosmic history. thought to have a supermassive black hole in its cen- ter. When two galaxies crash and merge their central Are We There Yet? black holes will orbit one another, produce gravita- Many researchers are hopeful NANOGrav will observe a tional waves, and eventually combine into a single signal soon. “It’s really just a matter of time and effort,” behemoth black hole. This sequence can take several said physicist Alan Weinstein of the California Institute of

Mann PNAS | August 9, 2016 | vol. 113 | no. 32 | 8879 Downloaded by guest on September 24, 2021 Technology, a LIGO team member who isn’t working on The NANOGrav collaboration disagreed, point- pulsar-timing arrays. The NANOGrav collaboration has ing out that the PPTA findings are based on moni- been adding about five new pulsars to their catalog per toring only four pulsars. “They did a careful job, year, improving their sensitivity steadily over time. but the astrophysics is complicated and we still But even after carefully accounting for all potential think detection could be within reach in the next interference (from, for example, the motions of the few years,” says McLaughlin. In March of this year, Earth around the sun) and clocking a pulsar’s rotation members of the team produced a paper concluding period out to 15 decimal places, the Parkes Pulsar that they have an 80% chance of finding gravita- Timing Array (PPTA) project, an Australian group tional waves within a decade (8). similar to NANOGrav, found no evidence of gravita- In either case, the field has come a long way since tional waves (7). The findings, released last year, its earliest days. In addition to NANOGrav and PPTA, suggested that models of the galactic gravitational there is also the European Pulsar Timing Array, and wave background needed to be revisited. all three collaborate and share data as part of an in- “It could be that when these supermassive black ternational consortium. Jenet says that 20 years ago, holes are merging they are producing less gravita- any limits on gravitational waves from pulsar timing tional waves than we think,” says astronomer Ryan were not much better than hand waving. “But now Shannon of the Commonwealth Scientific and Indus- we’ve gotten to a point where the theorists have to trial Research Organisation and ICRAR-Curtin Univer- go back and scratch their heads and say ‘Hmm...we sity, and a member of PPTA. “Or it could be there are didn’t see it here. So what’s wrong with our astro- fewer than people had predicted.” physical models?’”

1 Sudou H, Iguchi S, Murata Y, Taniguchi Y (2003) Orbital motion in the 3C 66B: Evidence for a supermassive black hole binary. Science 300(5623):1263–1265. 2 Jenet F, et al. (2004) Constraining the properties of the proposed supermassive black hole system in 3C66B: Limits from pulsar timing. Astrophys J 606(2):799–803. 3 Abbott BP, et al.; LIGO Scientific Collaboration and Virgo Collaboration (2016) Observation of gravitational waves from a merger. Phys Rev Lett 116(6):061102. 4 Abbott BP, et al.; LIGO Scientific Collaboration and Virgo Collaboration (2016) Properties of the black hole binary merger GW150914. Phys Rev Lett 116(24):241102. 5 Hewish A, et al. (1968) Observation of a rapidly pulsating radio source. Nature 217(5130):709–713. 6 Sazhin MV (1978) Opportunities for detecting ultralong gravitational waves. Sov Astron 22:36–38. 7 Shannon RM, et al. (2015) Gravitational waves from binary supermassive black holes missing in pulsar observations. Science 349(6255): 1522–1525. 8 Taylor S, et al. (2016) Are we there yet? Time to detection of nanohertz gravitational waves based on pulsar-timing array limits. Astrophys J Lett 819(1):L6.

8880 | www.pnas.org/cgi/doi/10.1073/pnas.1611117113 Mann Downloaded by guest on September 24, 2021