It was the tiny wobble that shook science. On September 14, 2015, the billion-dollar, multi-decade mega-experiment LIGO (the Laser Interferometer Gravitational-Wave Observatory) felt gravitational waves rippling through Earth, confirming one of the thorniest predictions of Einstein’s theory of general relativity and inaugurating the era of gravitational wave astronomy.
But from that very first Nobel Prize-winning moment, researchers were already thinking about the next big thing: zeroing in on a burst of light from a cosmic smashup that produced the gravitational waves. Finding a light counterpart for a gravitational wave source would instantly fix the newborn science of gravitational wave astronomy to a firm observational foundation and give astrophysicists a multi-dimensional new look at the sky.
Before LIGO’s new-and-improved detectors booted up in 2015, dozens of telescope teams were already lined up to chase down every LIGO find. All signed confidentiality agreements, promising to keep their targets secret. For years, their attempts turned up nothing, even as LIGO racked up three more detections.
But now, thanks to a new gravitational wave detection that’s unlike any of the previous four, they have finally done it. The discovery was announced this morning at the National Press Club in Washington, D.C., and will be detailed in papers in Physical Review Letters, Nature, Astrophysical Journal Letters, and Science.
In the wake of the discovery, astronomers have mobilized some 70 ground- and space-based observatories to examine the source of the gravitational waves, which they think is a neutron star collision in a galaxy 130 million light years from Earth. Combining radio, optical, infrared, gamma-ray, X-ray, and gravitational wave observations, astrophysicists have created the most complete description ever of a neutron star merger, giving resounding closure to a whole constellation of astronomical mysteries and marking a major advance in the way we learn about our universe.
The morning of August 17, Edo Berger, a professor of astronomy at Harvard and the leader of one of the LIGO follow-up teams, was midway through a committee meeting when both his office and cell phones started ringing. He ignored them at first, but the calls kept coming. Finally, Berger picked up his cell phone. The screen was dense with alerts. First, one from the Fermi Gamma-ray Space Telescope: its gamma-ray burst detector had picked up a two-second blast of gamma rays. That wasn’t unusual; Fermi snags a new gamma ray burst almost daily. But then came a rare alert from LIGO. Earlier that morning, the detector in Hanford, Washington, had detected gravitational waves. Berger checked the time stamps: the events were only two seconds apart.
That’s when Berger kicked everybody out of his office.
Before LIGO, almost everything we knew about the universe came to us in the form of light or other electromagnetic waves. The sky was a silent movie, a riotous rainbow reel of radio waves, visible light, X-rays, gamma-rays, and everything in between. But the movie was missing a soundtrack.
Gravitational waves are that lost soundtrack. Gravitational waves are ripples in space-time that radiate out from massive objects when they change speed or direction. They carry information about some of the most awesome events in the universe—black holes colliding, dead star cores crashing into each other, mega-stars going supernova—events about which light provides only limited clues. The gravitational wave energy spilling out in the final moments of a black hole merger can easily surpass the light energy of every star in every galaxy in the sky.
The challenge, though, is lining the soundtrack up with the picture, for despite its exquisite sensitivity, LIGO is quite poor at “localizing” gravitational wave sources—that is, at pinning down their sky coordinates and distance from Earth. LIGO’s yawning search areas span hundreds of square degrees, making it difficult for telescopes to cover them sensitively. Even if a transient light source does turn up in the search zone, it’s practically impossible to be sure it’s not just a coincidence.
Researchers made significant headway on the localization problem in August, when LIGO teamed up with Virgo, a freshly-upgraded gravitational wave observatory in Italy. LIGO and Virgo made their first joint gravitational wave detection on August 14. The search area for the August 14 event was about ten times smaller than it had been for LIGO’s solo detections, but still, astronomers could not find an electromagnetic counterpart for the gravitational waves.
That wasn’t actually so surprising, given the source of the gravitational waves: two black holes merging into one. Though some subspecies of black hole mergers may give off electromagnetic light, most are probably as dark as their progenitors.
The alerts on Berger’s phone suggested that the new detection was something different. All four of the previous gravitational events were split-second blips: when black holes swallow each other up, it’s a tidy, one-gulp affair. The new find rumbled on for more than a minute, hinting that it might be something else, something messier. Plus, the strength of the gravitational wave signal meant that the colliding objects were relatively lightweight, with a combined mass of about three Suns. That pointed away from a black hole merger and toward something LIGO had never seen before: the collision of two neutron stars.
A neutron star is not really a star at all. It is the bare stellar corpse left behind by a supernova explosion, transformed by cascading gravitational collapse into something as dense as an atomic nucleus. (Astronomers like to boast that one teaspoon of neutron star stuff would weigh billions of tons.) “When we call them stars, we’re short-selling how strange and exotic they are,” says Robert Owen, a physicist at Oberlin College in Ohio who models black hole collisions as part of the Simulating eXtreme Spacetimes collaboration. “They are essentially a single atom the size of a star.”
There could be billions of neutron stars in the Milky Way alone, many of them orbiting each other in binary pairs. Over billions of years, those pairs gradually spiral in toward each other until finally the two neutron stars collide, merging into either a black hole or a new, more massive neutron star. Astronomers can’t actually see the neutron stars merging, but theorists predict that telescopes should be able to detect telltale aftereffects: a short burst of gamma rays and a brief, relatively dim flash of light called a “kilonova,” powered by the radioactive decay of shrapnel from the smashup. This shrapnel could also account for the heavy elements, like gold and platinum, in the universe.
Astronomers had some evidence linking short gamma-ray bursts and kilonovae with neutron star mergers, Berger says, but the evidence was circumstantial. For years, researchers had been waiting for an opportunity to solidify the links. Berger saw the alerts on his phone and knew that this might be that chance: If astronomers could find a kilonova that matched the Fermi gamma-ray burst and the gravitational wave detection from LIGO and Virgo, that would clinch it. The three observations together would be a sort of astronomical Rosetta Stone.
Before Berger and his team could start searching for that kilonova, though, they had to know where to look. And the first LIGO alert provided no information at all on where astronomers might point their telescopes.
The problem was that only one of LIGO’s two detectors had picked up the signal. Though it is described as a single observatory, LIGO is actually made up of two instruments, spaced 2,000 miles apart. One half of LIGO sits in Livingston, Louisiana, while its identical twin is in Hanford, Washington. The redundant design helps physicists distinguish gravitational waves from other things that shake the Earth, like semi trucks and seismic motion. It also helps them locate gravitational wave sources.
While the Hanford detector reported the vibrations, in Louisiana, a “noise event”—rumbles from a passing vehicle, maybe—had caused signal-detection software to ignore the gravitational wave vibration. Acting quickly on the Hanford detection, LIGO scientists salvaged the Louisiana data, rooted out the signal, and correlated their findings with data from Virgo.
Virgo, meanwhile, barely saw it, suggesting that the gravitational waves were coming from somewhere inside its blind spot. Working backwards, researchers used that non-detection to generate a Virgo search zone, which they combined with the LIGO data to zero in on a 30-square-degree patch of sky around the constellation Hydra.
Hydra was set to rise over Chile, home to some of the most sensitive telescopes on Earth, at about 8 pm Eastern Time. Half a dozen teams scrambled to get ready. Berger’s group lined up the Dark Energy Camera, which is mounted on the Victor M. Blanco four-meter Telescope at the Cerro Tololo Inter-American Observatory. Because of its wide field of view, the camera would be able to cover the whole search area in just ten snapshots.
Meanwhile, other groups, including the DLT40 survey at Cerro Tololo and 1M2H project at the Las Campanas Observatory, searched the area galaxy by galaxy. They were all after for the same thing: some bright light source that hadn’t been there the night before. “It was like being in the emergency room,” Berger says. “Very tense, very exciting.”
Less than an hour later, Berger recalls, they had it: a burst of light coming from a galaxy called NGC 4993, 130 million light years away from Earth.
“That was an astounding moment,” Berger recalls. “We were all silent for a few minutes. We took a few minutes to absorb the shock of seeing this and then get the plan into action.”
The other teams had homed in on it, too, and within the span of less than an hour they had all transmitted the precise coordinates of the kilonova to the rest of the LIGO follow-up network. Three more groups reported seeing it later that night. It was the first time astronomers had ever matched up electromagnetic waves with gravitational waves.
Since then, physicists have combined the gravitational wave signal with observations from some 70 different observatories, covering the electromagnetic spectrum from bottom (radio waves) to top (gamma rays) to piece together an unprecedented picture of the neutron star collision. “Studying black holes tells us how space-time behaves in its most extreme limits, and studying neutron stars tells us how matter behaves in its most extreme limits,” Owen says. Physicists have used data from the event to verify their working models of neutron stars, ruling out some wilder variants, and to confirm Einstein’s prediction that gravitational waves should travel at the speed of light.
They have also parsed the gravitational wave readings to reveal a remarkably detailed narrative of the neutron stars’ final moments, says Wynn Ho, an associate professor of mathematics and physics and astronomy at the University of Southampton, where he studies gravitational wave sources and is part of the LIGO Scientific Collaboration. At the moment LIGO first felt the gravitational waves, the neutron stars were circling each other about 15 times per second. For the next minute or so, they danced closer and closer, faster and faster. With less than a second left to go, they were whipping around each other 100 times each second.
At this point, gravity started to distort the shape of each neutron star, Ho says, raising up “tides” of star-stuff in the gap between the stars. The two stars touched and, finally, merged.
What did the cataclysm leave behind? If the remnant of the merger is spinning fast enough, it may be able to stave off gravitational cave-in and persist as a neutron star for a while. But ultimately, as it spins down, the star will give way and collapse into a black hole, Ho says. LIGO scientists tried but were unable to pick up the gravitational wave signature of a new, fast-spinning neutron star, but that doesn’t necessarily mean that merger immediately blinked into a black hole, he says.
Meanwhile, researchers have also confirmed the signatures of heavy elements like lead and gold near the kilonova, providing the best evidence yet that Earth’s store of these metals really does come from neutron star mergers. Berger also hopes to study the gamma ray burst, the nearest one of its kind with an established distance measurement, to learn more about how gamma rays jet out from a neutron star merger.
“It’s extremely rare to have one singular event that answers so many questions, that has so many firsts,” Berger says. “I’m trying to savor this moment, because I don’t think it happens very often.”