Spacetime ripples from a stellar cataclysm in a distant galaxy help explain the cosmic origins of gold, and chart the course for a new age of “multimessenger” astronomy
Ushering in the beginning of a new era in astronomy and physics, scientists on Monday announced they have for the first time detected the spacetime ripples known as gravitational waves from the collision of two neutron stars. Streaming in from the sky over the Indian Ocean on August 17, the waves registered at the twin detecting stations of the U. S.-based Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), as well as a European detector called Virgo located in Italy. This is the fifth time in the last two years that scientists have confirmed spotting such waves, a phenomenon that Einstein first predicted more than a century ago—and that led to this year’s Nobel Prize in Physics for three of LIGO’s leaders.
All of the previously detected gravitational waves, however, came from merging pairs of black holes. These objects are so dense that light cannot escape their grasp, making such mergers essentially invisible to normal telescopes despite the prodigious gravitational waves they generate in the final moments of their incredibly violent death spirals. Without a much-larger network of gravitational wave observatories, astronomers cannot pin down the precise locations of merging black holes, let alone deeply investigate them.
But neutron-star mergers begin with objects that in comparison to black holes can be featherweights. A neutron star is the highly compressed core of an expired massive star, and is formed in the aftermath of a supernova explosion. Its gravitational field is strong enough to squeeze and break down an entire sun’s worth of matter into a city-sized orb of neutrons, making it less a true “star” and more an atomic nucleus as big as Manhattan. But a neutron star’s gravity is still too weak to trap light. So the flash from two of them slamming together can escape into the cosmos, producing not just gravitational waves but also one of the universe’s most brilliant fireworks displays for anyone who cares to look.
In this case, after the initial chirp of gravitational waves signaling the onset of the merger, the “fireworks” consisted of a two-second-long gamma ray burst (GRB) followed by a weeks-long, multi-wavelength afterglow—and “anyone” proved to be nearly every astronomer and physicist on Earth who had found out about the event. Julie McEnery, project scientist for the Fermi Gamma-ray Space Telescope, which spotted the GRB, called August 17 “the most exciting morning of the nine-year Fermi mission.”
The astronomers working with the LIGO and Virgo physicists had been sworn to secrecy. But the sheer volume of follow-up observations around the world unavoidably spawned public rumors, now confirmed, about a global campaign to track the collision and its aftermath. The resulting frenzy of new observations and theories is the most potent example yet of “multimessenger” astronomy, an emerging field in which light, gravitational waves and subatomic particles emitted from astrophysical cataclysms are collected and studied in unison.
In an overwhelmingly mammoth series of papers published simultaneously across several journals, researchers are linking the latest event to a vast range of phenomena and providing fresh insights on everything from fundamental nuclear physics to the large-scale evolution of the universe. Among other things, the merger gave observers a front-row seat at the birth of a black hole, which the colliding neutron stars likely produced. The discovery that most glitters, though, is smoking-gun evidence that neutron star mergers—rather than run-of-the-mill supernovae—are the cosmic crucibles that forge the universe’s heavy elements: substances including uranium, platinum and gold.
So it looks as if the radioactive pile in a nuclear reactor, the catalytic converter in your car, and yes, the precious metal in your wedding band may all come from the smashed-up innards of the universe’s smallest, densest and most exotic stars—or at least whatever fraction can escape without falling into a merger’s resulting black hole. The result could solve an ongoing debate over the cosmic origins of heavy elements that has possessed theorists for more than half a century. The bulk of the universe’s hydrogen and helium was produced in the first moments after the big bang, and most of the lighter elements—oxygen, carbon, nitrogen and so on—were formed from nuclear fusion in stars. But the origin of the heaviest elements had been a lingering question until now.
“We have hit the mother lode!” says Laura Cadonati, an astrophysicist at Georgia Institute of Technology and LIGO’s deputy spokesperson. “This is really the first time we have multimessenger detection of a single astrophysical event, where gravitational waves are telling us the story of what happened before the cataclysm and the electromagnetic emissions are telling us what happened after.” Although presently inconclusive, Cadonati says, analyses of the event’s gravitational waves could eventually reveal details of how matter “sloshes around” within neutron stars as they merge, giving researchers a new way to study these bizarre objects and learn just how big they can get before collapsing into a black hole. Relatedly, Cadonati notes, there was a mysterious gap of about two seconds between the end of the gravitational-wave chirp and the onset of the GRB—an interval, perhaps, in which the structural integrity of the combined neutron stars briefly resisted the inevitable collapse.
For many researchers the breakthrough has been a long time coming. “My dream has come true,” says Szabolcs Marka, an astrophysicist and LIGO team member at Columbia University who was an early proponent of multimessenger astronomy in the late 1990s. Back then, he recalls, he was seen as “that crazy guy” trying to prepare for follow-up observations on gravitational waves—a phenomenon that was then still decades away from direct detection. “Now, I and others feel vindicated,” Marka says. “We have studied this system of colliding neutron stars in a very diverse set of messengers. We have seen it in gravitational waves, in gamma rays, in ultraviolet, visible and infrared light, and in x-rays and radio waves.… This is the revolution—the evolution—of astronomy that I first hoped for 20 years ago.”
France Córdova, director of the National Science Foundation, or NSF (the U. S. federal agency that supplied the bulk of LIGO’s funding), calls the observatory’s latest achievement a “historic moment in science” that could not have come without decades of sustained governmental support for a variety of astrophysical observatories. “The detection of gravitational waves, from the first short chirp heard round the world to this latest, longer chirp, not only validates the kind of high-risk, high-reward investments that the NSF makes, but also spurs us to want to do more,” Córdova says. “My hope is that the NSF will continue to support innovators and innovations that will transform knowledge, and inspire many generations to come.”
After the initial detections of the merger’s gravitational waves and its subsequent GRB (the latter of which was immediately observed by the Fermi and Integral space telescopes), the race was on to find the collision’s source—and hopefully its afterglow—in the sky. Within hours multiple teams had marshalled available telescopes to stare at the region where LIGO’s and Virgo’s scientists had calculated the source must be: a swath of the heavens spanning 31 square degrees and containing hundreds of galaxies. (Using LIGO alone, Cadonati says, the search would have been like “looking for the glimmer of a gold ring in the Pacific.” With the addition of a third data point from Virgo, she says, the researchers could properly triangulate the source’s position, reducing the search to something more like seeking “a gold ring somewhere in the Mediterranean.”)
The bulk of the observations took place at observatories in Chile as soon as the sun had set and the crucial region of sky drifted up over the horizon, with different teams adopting an assortment of search strategies. Some simply “tiled” the region with observations, moving methodically from one side to the other; others targeted subsets of galaxies that theories suggested would be most likely to host a neutron star merger. In short order, the targeting strategy won out.
First to actually see the optical afterglow was Charles Kilpatrick, a postdoctoral researcher at the University of California, Santa Cruz. He was sitting at his desk and sorting through images of selected galaxies at the behest of one of his coworkers at Santa Cruz, the astronomer Ryan Foley, who had helped organized the campaign. In the ninth image he examined, hastily taken and transmitted by colleagues half a world away using the meter-wide SWOPE telescope at the Las Campanas Observatory in Chile, he saw it: a bright blue dot embedded in a giant elliptical galaxy, a 10-billion-year-old swarm of old, red stars about 120 million light years away, nameless save for catalog designations. Such galaxies are thought to be the main cosmic homes for neutron-star mergers due to their advanced age, stellar density and relative lack of recent star formation. A side-by-side comparison of earlier images of that same galaxy showed no such dot; it was something new and recent. “It very slowly dawned on me what a momentous occasion this was,” Kilpatrick recalls, “but I had tunnel vision at the time, just trying to work as quickly as possible.”
Kilpatrick notified other team members including Josh Simon, a Carnegie Observatories astronomer who rapidly obtained a confirmation image with one of the larger 6.5-meter twin Magellan telescopes in Chile. The blue dot was there, too. Over the course of an hour, Simon followed-up by measuring the dot’s spectrum—the various colors of light it emitted—in a pair of five-minute exposures. Those spectra could prove useful for further study, he reasoned, or if nothing else they could serve to ensure the blip was not an ordinary supernova or some other cosmic imposter.
