About 130 million light-years from Earth, the relics of two exploded stars neared the end of a spiraling, dyadic dance around each other. The dance partners were incredibly dense neutron stars: Just a teaspoonful of their neutron-rich star stuff has a mass of about 1 billion metric tons.
Over time, the stars drifted toward each other and picked up speed. Just before collision, each orbit took fractions of a second. Then came the big finale: a final merger that sent ripples through the fabric of space-time and the astrophysics community. On August 17, 2017, scientists at the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States detected the gravitational waves that emanated from that cosmic collision.
The event turned out to be the “cosmic gift that keeps on giving,” says astrophysicist Samaya Nissanke, of Radboud University Nijmegen in the Netherlands. Observations of the merger ushered in a long list of firsts: The event was the first direct sighting of two neutron stars slamming together, the first confirmation that such collisions produce vast amounts of heavy elements, such as gold, and the first time scientists identified both gravitational waves and light coming from the same cosmic event. The only previously detected gravitational waves had originated from merging black holes, which scientists don’t expect to produce light.
Those discoveries led 44 of the world’s top neutron star experts to gather in November at a three-day workshop hosted by the Flatiron Institute’s Center for Computational Astrophysics (CCA) to discuss what scientists know — and don’t know — about neutron star mergers.
The meeting was the first time experts from around the world were able to assemble following the public announcement of the neutron star merger a mere month before. Flatiron’s expertise, facilities and resources enabled the meeting to come together that quickly, says meeting organizer Matteo Cantiello, an associate research scientist at CCA. “We were able to rapidly attract people faster than other groups could, to talk about where we stand in terms of understanding this phenomenon that we’ve never seen before.”
The early fall meeting buzzed with excitement: The neutron star merger marked the beginning of the era of ‘multi-messenger astronomy,’ in which both gravitational waves and light can reveal insights into the same event.
“This event brought us into a whole new regime of understanding,” says Jennifer Barnes, an astrophysicist at Columbia University. “For me, at least, this was the first time I was exposed to some new ideas and interpretations of the event. The meeting highlighted just how many open questions there are. It helped clarify the landscape of uncertainties and questions and ongoing debates.”
In the immediate aftermath of LIGO’s detection of the neutron star merger, scientists paired the measurements with data from Advanced Virgo in Italy (another hunter of gravitational waves). The combined data allowed scientists to triangulate where in the sky the gravitational waves originated. Within 11 hours of the initial gravitational wave observations, astronomers spotted the afterglow of the neutron star merger in a galaxy about 130 million light-years from Earth. “This was a stupendous event for astronomers,” astrophysicist James Lattimer of Stony Brook University said at the Flatiron meeting.
The first day of the meeting at the Flatiron Institute focused on the discovery and the scientific theories it quashed or confirmed. Observations suggest, for instance, that the neutron star merger spewed heavy elements such as silver, platinum and uranium into space, including the equivalent of 10 Earth masses’ worth of gold. That abundance of heavy elements points to a previously proposed mechanism called the r-process, in which neutrons cram into an atom’s nucleus faster than the atom can undergo radioactive decay. Scientists believe the r-process is responsible for much of the gold on Earth. If neutron star mergers are rare, the gold in a galaxy might be patchy, rather than evenly distributed.
Other observations agreed with theoretical predictions as well, such as that neutron star mergers generate the bright flashes of gamma rays that puzzled scientists for decades. Theoretical astrophysicists, it seemed, had gotten a lot of things right.
“The first day of the meeting, there was a feeling like there’s nothing to be done here. We can just pat our backs and head home,” Cantiello says. That feeling didn’t last, though. “The day after, there were a lot of interesting talks that changed the mood. There was a lot of controversy about how much we understand about aspects of this event.”
The uncertainty built over the second day’s presentations, culminating in an open discussion that evening. Brian Metzger, an astrophysicist at Columbia University, led the group through a list of significant questions about neutron star mergers, each accompanied by a list of possible answers. After each question, the scientists in attendance voted by a show of hands on which answer they thought was correct. “This is known to lead to correct scientific hypotheses,” joked one of the participants.
Some of the questions had a definite answer. Everyone agreed, for instance, that the event spotted in August was a merger of two neutron stars, rather than something else, like the merger of a neutron star and a black hole.
Other questions, though, lacked clear-cut answers. The observed gravitational waves generated by the merger reached peak intensity 1.7 seconds before the burst of gamma rays. Because both gravitational waves and gamma rays travel at the speed of light, the time gap was a surprise.
“It’s an interesting question as to why we had such a delay,” says astrophysicist Bruno Giacomazzo of the University of Trento in Italy. “It’s not something we expected, and it’s not something we can confidently explain right now. We still have a lot of things we don’t understand.”
That lack of certainty isn’t a bad thing, Cantiello says. “At the end of the meeting, people were happy because there seems to be an understanding both that scientists have done an amazing job and that there is more work to be done,” he says. “People want there to be stuff to be done. People have a passion for astrophysical puzzles. If everything is understood, you have to move on to another problem.”