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 with 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-size 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 “multi-messenger” 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 multi-messenger 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 multi-messenger 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.”
The Golden Opportunity
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 marshaled 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 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 catalogue 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 with 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. Meanwhile other teams had spotted the dot and were embroiled in follow-ups of their own. The rapid confirmation and spectra from Foley’s team, however, clinched provenance for them. “We had the first image of this, and we have the first identification of the source in this image,” Simon says. “Because we obtained both of those so early, we were also able to get the first spectrum for this merger—which no one else in Chile was able to do that first night—and then we issued the first announcement to the rest of the community.”
Those early spectral observations proved vital for subsequent analysis and solving several mysteries. They showed the merger’s leftovers rapidly cooling, fading from a brilliant sapphire blue to a dim ruby in the sky. These readings were verified over the next few weeks of observation as the visible dot faded, its afterglow shifting and peaking in cooler, longer-wavelength infrared light. The general pattern of colors, cooling and expansion hews close to what was predicted years earlier by a number of theorists working independently of each other, most notably Brian Metzger of Columbia University and Dan Kasen of the University of California, Berkeley.
In short, Metzger explains, what astronomers have seen from the merger’s aftermath is something called a “kilonova”: an intense outburst of luminosity created by the ejection and radioactive decay of white-hot, neutron-rich material from the neutron stars. As the material expands and cools, most of its neutrons are captured by the nuclei of iron and other heavy elements left over as ashes from the neutron star’s formative supernova explosion, creating even heavier elements. “Over the course of about one second, as the ejecta are capturing these neutrons and expanding through space, one of these mergers will form the lower half of the periodic table—gold, platinum, uranium and so on,” Metzger says. Near its conclusion, the kilonova’s light dramatically shifts to infrared as the neutrons cascading through the ejecta forge the heaviest elements, which efficiently absorb visible light.
Measuring the kilonova’s spectral evolution, in turn, allows astronomers to estimate the amount of different elements it has produced. Edo Berger, who studies kilonovae at the Harvard–Smithsonian Center for Astrophysics and oversaw many of the most ambitious follow-up observations of the merger, the event produced roughly 16,000 Earth masses worth of heavy elements. “That’s everything—gold and platinum and uranium as well as all the weird ones you see just as letters on the periodic table and don’t know their names,” he says. “As for the breakdown? For that, I don’t think we have exact answers yet.”
Some theorists have suggested only a few tens of Earth masses of gold were made in the merger. Metzger, for his part, pegs the merger’s gold output at roughly 100 Earth masses, with about three times more platinum and 10 times less uranium. In any case, when paired with updated statistical estimates of how often these mergers must occur, based in part on the latest detection, “you get a high enough rate per galaxy per year to build up the elements that form our own solar system and the abundances we see in other stars,” Metzger says. “All that stuff we see, you can explain through these mergers. There may be other ways to make heavy elements, but you don’t seem to need them.” On average, he says, probably only one neutron star merger occurs in the Milky Way every 10,000 years.
The Far Frontier
What’s more, studying exactly how a merger’s kilonova evolves can convey crucial information about how the collision unfolded. For instance, the light from this merger’s initial emission was bluer than expected, suggesting to Metzger and others the kilonova was being viewed at an angle rather than face-on. In this scenario the early blue emission would come from a spherical shell or equatorial band of relatively neutron-poor material blown out from the neutron stars at perhaps 10 percent light-speed. The later, redder emission would emerge from very neutron-rich material ejected at two to three times higher speeds from the neutron stars’ poles as they collided, like toothpaste squirted from a tube.
Paired with detailed x-ray and radio observations, this scenario helps explain the curious nature of the gamma-ray burst associated with the merger—the closest GRB ever seen, but also one of the faintest. Short GRBs are thought to be bipolar jets of intense radiation spun up and ejected at nearly light-speed by churning magnetic fields within colliding neutron stars as they coalesce and collapse into a black hole. Viewed face-on—down the barrel of the GRB gun, so to speak—they are extremely bright. This is the case with the majority of such bursts that astronomers witness in the distant universe. But if they are tilted or inclined from our perspective they would appear rather dim and would only be detectable if they were relatively close, within several hundred million light-years.
Using the wealth of data available from multi-messenger astronomy, then, astronomers could eventually determine the viewing angles of many kilonovae throughout the observable universe, making each one a more potent marker for measuring large-scale cosmic structure and evolution. This could allow scientists to better confront a mystery arguably deeper than the origin of the heavy elements: the baffling fact that the universe is not merely expanding, but accelerating at an ever-increasing rate under the influence of a kind of cosmos-spanning anti-gravity known as dark energy.
Cosmologists hope to better understand dark energy by precisely measuring its effect on the universe, tracking objects in ever-more-distant regions of the universe to see how far away they are, and how fast they are moving, caught up in dark energy’s accelerating flow. But to do this they need reliable “standard candles,” objects with known brightness that can be used to calibrate this vast, sweeping view of spacetime. Daniel Holz, an astrophysicist and LIGO collaborator at the University of Chicago, has demonstrated how merging neutron stars could contribute to this effort. His work shows the strength of this latest merger’s gravitational waves and the emissions of its kilonova can be used to calculate the local universe’s expansion rate. Limited to just one merger, the technique yields a value with significant uncertainties, albeit still in the ballpark of the expansion rate obtained from other methods. But in coming years—as gravitational wave observatories and a new generation of large telescopes on the ground and in space work together to identify hundreds or even thousands of neutron star collisions per year—those estimates will markedly improve.
“What all this means is that the gravitational waves from these mergers measured by LIGO and Virgo are complementary with modeling of kilonovae that suggests their inclination, their viewing angle, by their spectral evolution from blue to red,” says Richard O’Shaughnessy, an astrophysicist and LIGO team member at Rochester Institute of Technology. “That is a powerful synergy. If we know the inclination, we can know the distance, and that helps us with cosmology. What has been done here is a prototype for what we will be doing regularly in the future.”
“If you think about it, the universe is sort of a cosmic particle collider, with neutron stars as the particles,” O’Shaughnessy says. “It throws them together, and we now have the opportunity to see what comes out. We are going to see so many of these in the coming years—how many, I can’t tell you, but people already describe it as a ‘rain.’ This event is a Rosetta stone, giving us real data to connect disparate threads of astrophysics that previously only existed in the mind of theorists or as bits in a supercomputer simulation. It allows us to understand the cosmic abundance of heavy elements. It allows us to probe the squishiness of nuclear matter at extreme densities. It allows us to measure the expansion of the universe. These synergies set the agenda for all of high-energy astrophysics for decades to come, and are built on decades of investment. We are now reaping the reward, a mountain of gold 10 or a hundred times the mass of the Earth, that the universe just gave us.”