Astronomers have detected gravitational waves coming from the collision of black holes somewhere in the universe—again.
The detection, announced Wednesday, marks the fourth time in less than two years that scientists have observed the cosmic phenomenon. The Laser Interferometer Gravitational-Wave Observatory, or LIGO, announced the first-ever detection in February 2016. That news came a century after Albert Einstein predicted the existence of gravitational waves in 1916 as part of his general theory of relativity. A second detection was made public in June 2016 and a third in June of this year.
In all four detections, the gravitational waves were produced by the merger of two black holes more massive than our sun. In the final moments of the collision, gravitational waves fanned out in all directions, traveling at the speed of light. As they spread out through the universe, the waves expanded and contracted the fabric of space and time, like concentric rings rippling across a pond.
When the ripples reached Earth, they were detected by twin observatories in Washington state and Louisiana, known together as LIGO, and a third gravitational-wave detector in Pisa, Italy, known as Virgo. Virgo joined LIGO in its observations at the beginning of August, and the three observatories detected the most recent set of waves two weeks later.
The collision of black holes is a monstrously energetic event. But by the time gravitational waves travel across space and reach Earth, their distortions of space-time are faint. Spotting them requires extremely sensitive technology, like the instruments inside the LIGO and Virgo observatories.
The LIGO observatories, separated by nearly 2,000 miles to avoid false detections from earthly sources, are called laser interferometers. The observatories each have an L-shaped two-and-a-half-mile-long tube, with mirrors at the ends of each arm. Scientists shoot laser light into the tube and let it bounce around, traveling back and forth down the length of the tube, between the mirrors. When gravitational waves reach Earth, they stretch and shrink the arms of the tube, which changes the distance the light travels between the mirrors. The size of this distortion is about one-thousandth the width of a proton—unfathomably small, but able to be detected by LIGO and similar technology at Virgo.
Astronomers say the latest waves came from about 1.8 billion light-years away from Earth. They don’t have the technology to determine where exactly in the universe the collision occurred, but they can use mathematical models to estimate the masses of the objects involved. The latest detection involved two black holes about 25 and 31 times the mass of our sun. Their merger produced a new black hole with a mass about 53 times that of the sun. Before LIGO’s first detection of gravitational waves, astronomers thought stellar-mass black holes couldn’t get bigger than about 10 solar masses. All four detections have involved black holes that measured 20 to 30 times the mass of the sun. Their collisions produced even more massive black holes. With every new observation, astronomers are getting more evidence for a new category of black holes, somewhere in between stellar-mass black holes and supermassive black holes, which can have millions or billions solar masses.
The newest observation actually may have been disappointing for some scientists and astronomy fans. Back in August, as LIGO and Virgo finished up a months-long observation run, rumors circulated that scientists had detected gravitational waves from a collision between a black hole and a neutron star, or between two neutron stars. A neutron star is the small core of a dead star, a cluster of uncharged particles left behind after the outer layers of the star were ejected into space during a supernova. Neutron stars are about the size of a city but have about the same mass as our sun. They’re also the densest known objects in the universe; a teaspoon of material from one would weigh about 10 million tons. The detection of gravitational waves coming from a neutron-star collision would open a new chapter in gravitational-wave astronomy, itself a brand-new field. Unlike black holes, neutron stars emit beams of light in visible wavelengths. Astronomers could point powerful telescopes toward the source of gravitational waves and see what they find.
LIGO and Virgo will kick off another months-long observation run in fall 2018. Virgo’s instruments aren’t as sensitive as LIGO’s, but three detectors are certainly better than two. Astronomers hope their combined power eventually could help them determine the sources of gravitational waves with more accuracy. During that run, astronomers say they expect to detect gravitational waves every week—“or even more often.”