Masao Sako, an associate professor of physics and astronomy in the School of Arts & Sciences (SAS), was on vacation with his family when he got the news. The Laser Interferometer Gravitational-wave Observatory, or LIGO, had made a fifth detection of gravitational waves, which expand and contract the fabric of the universe.

While the previous four detections were a result of merging black holes, these waves originated from the merger of two neutron stars—collapsed stars that form the densest form of observable matter in the universe—an event that had never before been observed. Unlike black holes, which do not emit light, neutron stars are more likely to give out an optical signal when they merge, making them easier to locate.

Sako, a member of the Dark Energy Survey, immediately called Dillon Brout, one of his grad students, who was working to get information from LIGO in real time. Brout was producing maps predicting roughly where the source of the waves should be in the sky in order to help his collaborators find it using the Dark Energy Camera, or DECam, the most powerful survey instrument of its kind.

Scientists on the Dark Energy Survey joined forces for this effort with a team of astronomers based at the Harvard-Smithsonian Center for Astrophysics in Massachusetts, or CfA, working with observatories around the world to bolster the original data from DECam. Their images captured the flaring and fading over time of a kilonova, an explosion similar to a supernova, but on a smaller scale, which occurs when neutron stars crash into each other, creating heavy radioactive elements.

This particular kilonova occurred 130 million years ago in a nearby galaxy, generating the first gravitational waves to ever be traced back to a visible source. Unlike supernovae, which can often remain visible in the sky for months after the initial explosion, this kilonova lasted just a short time before vanishing.

“It’s unlike anything else that we’ve seen,” Sako says. “Ordinary stellar explosions usually get bright on timescales of a week and take months for it to fade. This thing pretty much disappeared in two weeks.”

Because of the event’s relative proximity—about 130 light years away—scientists were able, within a few hours of receiving the notice from LIGO/Virgo, to point telescopes in the direction of the event and get a clear picture of the light.

In addition to providing scientists with a new way to probe the physics of mysterious, compact objects such as neutron stars and black holes, this event also provides a new and unique way to measure the present expansion rate of the universe.

In 2006, Bhuvnesh Jain, the Walter H. and Leonore C. Annenberg Professor in the Natural Sciences at SAS, collaborated on a theoretical paper published in Physical Review D pointing out that an “optical counterpart” to gravitational waves would allow scientists to measure expansion rate of the universe and, therefore, test for dark energy. 

“One of the papers to be released Monday is indeed an estimate of the expansion rate using this one event,” Jain says. “We had not dreamed that in just over a decade after our theory paper, not just gravity waves, but the accompanying light would be detected. The expansion rate measurements are not accurate yet, but with 10 or more such events it will get very interesting.”

DECam has the capability to observe kilonovae as much as 10 times the distance of this one, which will play an important role in future detections of gravitational waves from merging neutron stars, allowing scientists to locate and image sources at even greater distances. This new kind of measurement will help the Dark Energy Survey uncover more about dark energy, the mysterious force accelerating the expansion of the universe.

“Marrying optical data with gravitational wave data,” Brout says, “marks the beginning of a new era in astrophysics. It provides a unique opportunity to weigh in on the current debate over the expansion rate of the universe and may even tell us about the nature of dark energy. We’re directly testing our theories of the universe in a completely new and independent way. This first discovery is already beyond our wildest expectations, and it’s hard to imagine what we’ll learn next.”

A paper describing the DECam discovery of the optical counterpart was recently accepted for publication in The Astrophysical Journal.