Some 130 million years ago, in a galaxy far away, the smoldering cores of two collapsed stars smashed into each other. The resulting explosion sent a burst of gamma rays streaming through space and rippled the very fabric of the universe.
On Aug. 17, those signals reached Earth — and sparked an astronomy revolution.
The distant collision created a “kilonova,” an astronomical marvel that scientists have never seen before. It was the first cosmic event in history to be witnessed via both traditional optical telescopes, which can observe electromagnetic radiation like gamma rays, and gravitational wave detectors, which sense the wrinkles in space-time produced by distant cataclysms. The detection, which involved thousands of researchers working at more than 70 laboratories and telescopes on every continent, heralds a new era in space research known as “multimessenger astrophysics.”
“It’s transformational,” said Julie McEnery, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Md., who was involved in the effort. “The era of gravitational wave astrophysics had dawned, but now it’s come of age. … We’re able to combine dramatically different ways of viewing the universe, and I think our level of understanding is going to leap forward as a result.”
The existence of gravitational waves was first theorized by Albert Einstein a century ago. But scientists had never sensed the waves until 2015, when a ripple produced by the merger of two distant black holes was picked up by two facilities of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Louisiana and Washington state. Since then, the collaboration has identified three more black hole collisions and has brought on a third gravitational wave detector near Pisa, Italy, to better pinpoint the sources of these minute distortions in space-time. Just this month, members of the LIGO team were awarded the Nobel Prize in physics for their achievement.
Yet because black holes emit no light or heat, past gravitational wave detections could not be paired with observations by conventional telescopes, which collect signals from what’s known as the electromagnetic spectrum. The scientists at LIGO and its European counterpart, Virgo, hoped to detect gravitational waves from a visible event, such as a binary star merger or a kilonova.
Kilonovas are swift, brilliant explosions that occur during the merger of neutron stars, which are ultradense remnants of collapsed stars that are composed almost entirely of neutrons, or uncharged particles.
Collisions between neutron stars are thought to be 1,000 times brighter than a typical nova, and they are the universe’s primary source of such elements as silver, platinum and gold. But much like gravitational waves, kilonovas have long been strictly theoretical. No scientist had ever seen one. Until this summer.
At 8:41 a.m. Eastern time on Aug. 17, a gravitational wave hit the Virgo detector in Italy and, 22 milliseconds later, set off the LIGO detector in Livingston, La. Three milliseconds after that, the distortion rippled through Hanford, Wash.
LIGO detects black hole mergers as quick chirps that last a fraction of a second. This signal lasted for 100 seconds, and it vibrated at higher frequencies. From the smaller amplitude of the signal, the researchers could tell this event involved less mass than the previously observed black hole collisions.
“When we detected this event, my feeling was, wow, we have hit the mother lode,” said Laura Cadonati, an astrophysicist at the Georgia Institute of Technology and LIGO representative.
Just 1.7 seconds after the initial gravitational wave detection, NASA’s Fermi Space Telescope registered a brief flash of gamma radiation coming from the constellation Hydra. Half an hour later, McEnery, the telescope’s project scientist, got an email from a colleague with the subject line, “WAKE UP.”
“It said, ‘This gamma ray burst has an interesting friend…. Buckle up,’” McEnery recalled.
Gamma ray bursts are the most energetic forms of light in the cosmos. Scientists had long predicted that a short burst would be associated with a neutron star merger. That violent collision shoots jets of radioactive matter into space, as though someone had smashed their palm on a tube of toothpaste with holes at both ends.
“We were beside ourselves,” McEnery said.
Gravitational waves travel at lightspeed. “Einstein predicts that gravity and photons move at the same speed … and [the signals] arrived within 2 seconds of each other, dramatically confirming that Einstein’s prediction is right,” McEnery said at a news conference on Monday. “While I’m not surprised that Einstein is right, it’s always nice to see him pass another test.”
Meanwhile, trigger alerts had gone out to LIGO collaborators at dozens of observatories around the globe. At Penn State University, phones began buzzing during a science operations team meeting for NASA’s Swift satellite. The 9:15 a.m. alert threw everything they had planned out the window, said Jamie Kennea, a Penn State astronomy professor. From low Earth orbit, the Swift satellite cycled through 750 points in the sky until it detected “a vast avalanche of data” in the form of ultraviolet rays coming from the neutron star merger. They were just in time: The UV emission disappeared in less than 24 hours.
Ryan Foley, an astronomer at the University of California at Santa Cruz, was walking around an amusement park when he got the urgent text from one of his collaborators. He abandoned his partner in front of the carousel, jumped on a bike and pedaled back to his office.
He and his colleagues stayed up all night, first waiting for the sun to set on their telescope in Chile, then sorting through the telescope’s images in search of a “transient” — an object in the sky that hadn’t been there before.
In the ninth image, postdoctoral researcher Charlie Kilpatrick saw it: a tiny new dot beside a galaxy known as NGC 4993, 130 million light-years away.
He notified the group through the messaging service Slack:
@foley found something
sending you a screenshot
Foley marveled at Kilpatrick’s measured tone in those messages. “Charlie is the first person, as far as we know, the first human to have ever seen optical photons from a gravitational wave event,” he said.
The event was named for the telescope that found it: Swope Supernova Survey 2017a.
Within 24 hours of the initial detection, it seemed as though half the telescopes in the world — and several more in space — were tilted toward SSS2017a, recalled Stephano Valenti, an astrophysicist at the University of California at Davis who took part in the optical search. “We were calling colleagues to talk, saying, ‘I cannot tell you why, but can you observe this object?’” he said. “Everyone was working together, sharing everything they had as soon the information was coming online. … I think this one was the most exciting week of my career.”
The neutron stars’ merger was not a well-kept secret. On Aug. 19, University of California at Santa Barbara astronomer Andy Howell tweeted, “Tonight is one of those nights where watching the astronomical observations roll in is better than any story any human has ever told.” He told The Washington Post on Friday that part of him regretted sending the tweet, after observers and the media connected his and other astronomers’ public hints to an event that set the world’s observatories buzzing. Members of the collaboration still had two months of painstaking work ahead of them, confirming and analyzing their data to make it ready for publication.
But Howell said he was motivated to mark the moment in scientific history. “I wanted to document what it felt like to find something completely new about the universe, that humans have never known,” Howell said.
Researchers collected data from the kilonova in every part of the electromagnetic spectrum. In the early hours the explosion appeared blue and featureless — the light signature of a very young, very hot new celestial body. But unlike supernovas, which can linger in the sky for months, the explosion turned red and faded. By separating light from the collision into its component parts, scientists could distinguish the characteristic signals of heavy elements like silver and gold coalescing in the cooling cloud of material. Both wedding rings and uranium bombs are elemental echoes of these merging neutron stars.
Scientists don’t know what happened in the wake of the explosion. Neutron stars are too faint to be seen from so far away, so researchers can’t tell if the merger produced one large neutron star, or if the bodies collapsed to form a black hole, which emits no light at all.
But after two months of analysis, the collaborators were ready to inform the world about what they have so far. Their results were announced Monday in more than a dozen papers in the journals Nature, Science and the Astrophysical Journal Letters.
The collaboration’s capstone paper in Astrophysical Journal Letters lists roughly 3,500 authors, approaching the record set in 2015 by 5,154 Large Hadron Collider physicists who estimated the mass of the Higgs boson. If gravitational wave research had already weakened the stereotype of a lone astronomer genius, the dawn of multi-messenger astrophysics dealt it a fatal blow.
“From this point onward,” Cadonati said, “the more we want to know, the more we need to work together.”
France A. Córdova, director of the National Science Foundation, which funds LIGO, compared traditional, visual astronomy to a silent film. The earliest gravitational wave detections added sound, but they were little more than strange noises echoing in the dark, she said. “We couldn’t pinpoint the location of the source.”
Now, for the first time, the soundtrack of the cosmos has synced up with what scientists can see. “It’s all the difference in the world,” she said. Astronomers will now seek new answers to old questions about the expansion rate of the universe, the properties of dark matter, and the birth and death of stars. And they will likely find themselves asking questions they had never considered before.
But the detection also serves as a confirmation of what scientists already believed. The events that unfolded in SSS17a hewed closely to theories about the merger of neutron stars based in nuclear physics, general relativity and research on the origins of elements.
“It’s really a triumph of science,” Foley said. “We as a civilization have essentially been confined to the Earth, and almost all the information we’ve ever received from the universe has been through light. Yet we were able to predict … things as extreme as two neutron stars colliding when even the idea of neutron stars is incredible.”
Now that astronomers can use not just light but also gravitational radiation to comprehend the cosmos, he said, “there’s a lot of amazing science that’s going to happen next.”