Year in review: Gravitational waves offer new cosmic views

Soon astronomers might detect black hole collisions as often as once a day

illustration of gravitational waves

MAKING WAVES  The first gravitational wave signal detected by LIGO came from the merger of two black holes spiraling inward, as depicted in this numerical simulation.

© S. Ossokine, A. Buonanno (Max Planck Institute for Gravitational Physics), Simulating eXtreme Spacetime project, D. Steinhauser (Airborne Hydro Mapping GmbH)

The secrets gleaned from the universe’s most mysterious giants are incongruously subtle when witnessed at Earth: Detectors budge by a tiny fraction of a proton’s breadth, outputting a feeble, birdlike chirp.

For centuries, astronomers have peered out into the universe almost exclusively by observing its light. But 2016’s announcement of the first detection of gravitational waves, produced 1.3 billion years ago in the collision of two monstrous black holes, has given scientists a whole new way of observing the heavens.

The waves tore through the cosmos at the speed of light and arrived at Earth just in time for the start-up of the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, which measured the minute stretching and squeezing of space. With a second detection already recorded and more expected in 2017, scientists hope to uncover new details about elusive black holes and their pairings. Soon, as more detectors come online, scientists will even be able to pinpoint where gravitational waves originate and inspect the sky for the aftermath of the cataclysms that caused them.

“This is a great success story of science,” says astrophysicist Avi Loeb of Harvard University, who was not involved in the detection. It’s the kind of major discovery that comes along only once in a few decades, he says.

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SPACETIME RIPPLES  Scientists announced the direct detection of gravitational waves this year. Here’s everything you need to know about these quivers in spacetime.H. Thompson, E. Otwell, LIGO, SXS, NASA

On February 11, LIGO scientists announced the discovery at a news conference in Washington, D.C., and in a paper published in Physical Review Letters. Since publication, the paper has garnered around 100 citations a month, evidence of a newly intensified focus on the waves. Some physicists had dedicated entire careers to finding the spacetime tremors, which will be a boon for researchers for decades if not centuries to come.

The patterns of ripples appeared nearly simultaneously in LIGO’s two enormous L-shaped detectors—in Hanford, Wash., and Livingston, La.,—on September 14, 2015. The signal closely matched that expected from a pair of black holes that spiral around one another, getting closer and closer before merging into one. At the early stages of their do-si-do, the two black holes were about 35 and 30 times the mass of the sun. The behemoths melded together into a black hole 62 times the sun’s mass, releasing three suns’ masses worth of energy (SN: 3/5/16, p. 6; SN: 7/9/16, p. 8). When scientists converted the gravitational waves into sound waves, the waves produced something like the everyday chirp of a bird, quickly rising in pitch and volume before cutting off. The sound felt like a plaintive question, as if the universe was asking, “Hello? Is anyone there?” This time, the answer was yes.

Taken on its own, the discovery was a blockbuster—confirming Einstein’s prediction that spacetime can ripple, providing an intimate new glimpse of black holes and verifying astrophysicists’ calculations for how two black holes can fuse into one. But the detection’s landmark status is largely because of its future promise. LIGO is expected to usher in a new era of astronomy, in which gravitational wave detections could become commonplace. Black holes, previously dark to humankind, will regularly communicate their coalescences to Earth.

In pursuit of this new type of astronomy, scientists have been chasing gravitational waves for decades. After such a long search, it was “incredibly gratifying,” says David Shoemaker, leader of LIGO’s efforts at MIT, “to wake up in the morning and know in my bones” that gravitational waves had finally been detected.

Almost as soon as LIGO’s updated detectors were turned on, the gravitational waves rippled by, slightly altering the length of LIGO’s ultrasensitive detectors. “We flipped the switch and said, ‘OK, we’re going to start running,’ and boom,” says LIGO laboratory executive director David Reitze of Caltech. That quick detection raised hopes among astrophysicists who daydream of datasets with tens or hundreds of such events.

With each new coalescence, scientists will learn more about how common black hole pairs are, as well as the properties of black holes and the dying stars that collapsed into oblivion to create them. “What we’re really learning about when we study these black holes is the stars that were their progenitors,” says LIGO member Daniel Holz of the University of Chicago. “From the stars, we then are learning about the early universe.”

Scientists hope to reconstruct how pairs of black holes find one another in the lonely universe. There are two main competing theories: Two stars could be born together like twins, with each later collapsing into a black hole, or the black holes could meet up later in life, in dense systems where many black holes and stars interact (SN Online: 6/19/16).

Proving that the detection was no fluke, LIGO scientists reported June 15 that they had spotted the quivers of a smaller pair of merging black holes (SN: 7/9/16, p. 8). LIGO shut down for upgrades following the two detections, but restarted again in November. Further improvements to the LIGO detectors will boost their sensitivity, allowing them to catch even fainter ripples. When those upgrades are complete — perhaps by 2019 — scientists could glimpse black hole mergers as frequently as once a day.

With the first detections, physicists used LIGO’s data to confirm Einstein’s general theory of relativity in a more extreme environment than ever before. “That’s a triumph,” says Loeb. But future detections will add even more precision to tests of general relativity. Any deviation from expectations could signal some way in which Einstein’s theory breaks down. The equations of general relativity also suggest that black holes have no “hair,” or distinguishing characteristics aside from mass, electric charge and angular momentum. But this leads to a conundrum about what happens to information swallowed up by the black hole (SN: 10/3/15, p. 10). In the future, scientists could use gravitational waves to test whether the no-hair theorem is true.

The discovery “injected a lot of momentum in the field,” says Emanuele Berti, an astrophysicist at the University of Mississippi in Oxford.

Another gravitational wave detector, Virgo, in Italy, is undergoing upgrades and should be switched on in 2017 (SN: 3/5/16, p. 24). The trio of detectors — Virgo, plus LIGO’s two—will give scientists the ability to locate the sources of gravitational waves on the sky. The government of India is also taking steps toward creating a gravitational wave observatory. And related projects are garnering more attention: Results announced in June from the European Space Agency’s LISA Pathfinder satellite demonstrated the technological capabilities needed to search for gravitational waves not from the ground but from space (SN Online: 6/7/16).

If researchers can triangulate the source of the waves, they can point telescopes in that direction to spot any luminous aftermath. Such a signal would be unexpected for shadowy black holes, but they aren’t the only source. Scientists expect to find undulations from smashups of neutron stars, which might produce detectable light. If luck is on LIGO’s side and a star explodes within the Milky Way, LIGO may be able to spot its gravitational fallout, too.

Combining gravitational waves with other messengers from space, including various wavelengths of light and particles such as neutrinos, will create a diverse toolkit for observing the cosmos. Scientists may even find unforeseen sources of gravitational waves, says Loeb. “There is a chance that our imagination is limited.”

Physics writer Emily Conover has a Ph.D. in physics from the University of Chicago. She is a two-time winner of the D.C. Science Writers’ Association Newsbrief award.

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