Cosmic collision sparks revolution
Scientists detect gravitational waves from new nova
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 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
We have hit the mother lode.” Laura Cadonati, an astrophysicist
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, or 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.
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. Wedding rings and uranium bombs are elemental echoes of these merging neutron stars.
For millennia, the two dead stars circled each other approaching the speed of light, shaking off gravitational waves, which in turn pulled them closer together. When the husks smashed together, dinosaurs walked the planet. The shock wave from stars’ collision finally reached Earth in August.
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.
This kilonova was so bright that it could have been observed even by amateurs with tiny telescopes. In the future, LIGO will alert the whole world to potential detectors, allowing citizen scientists to join professional astronomers in the global search for light from the universe’s most dramatic cataclysms.
France 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.