LIGO Detects Second Black Hole Collision

The gravitational wave observatory has detected a second event, heralding a new era in astrophysics.

This image depicts two black holes in the moments before a catastrophic collision.
S. Ossokine / A. Buonanno /  T. Dietrich / R. Haas / Max Planck Institute for Gravitational Physics / the Simulating eXtreme Spacetime (SXS) project 

The day after Christmas last year, the cosmos quietly gifted scientists with gravitational waves — ripples in the fabric of spacetime – produced in a collision between two stellar-mass black holes. It’s the second event detected by physicists working with the Laser Interferometer Gravitational-Wave Observatory (LIGO), and it suggests that after some fine-tuning LIGO will be spotting dozens of these events each year.

The first discovery, detected last September and announced this February, was an astounding achievement for science, another piece of evidence proving Einstein’s general theory of relativity correct and, moreover, throwing open a new window on the cosmos. It ushered in an era where astronomers can use gravitational waves to detect black holes previously hidden from view. At the time, LIGO Executive Director David Reitze called it a “scientific moonshot.”

A Second Landing

And the second event is no less remarkable. The two black holes, which weighed in at roughly 7.5 and 14.2 times the mass of the sun, had been circling each other for eons before they finally crashed together 1.4 billion years ago. The bang was so furious — with energy equivalent to the entire mass of the sun — that it sent shudders across the universe.

As those gravitational waves rippled through the universe, they stretched and squeezed the fabric of space-time by an exceedingly tiny amount. At the LIGO detectors, for example, they distorted space-time by as much as three parts in 1022. “That's like changing the distance between the earth and the sun by a fraction of an atomic diameter,” says Gabriela Gonzalez (Louisiana State University), chair of the LIGO collaboration, in a much-anticipated press conference yesterday at the American Astronomical Society meeting in San Diego.

In this schematic of LIGO, a beamsplitter sends light along two paths perpendicular to each other. Each beam bounces between two mirrors, one of which allows a fraction of the light through. When the two transmitted beams meet and interfere, they’ll cancel each other out — if the length of the path they’ve each traveled has remained constant. But if a gravitational wave passes through, it’ll warp spacetime and change that distance, creating an interference pattern.
S&T: Leah Tiscione

LIGO can detect such a miniscule distortion with the help of two laser beams that bounce between mirrors at the opposite ends of 4-kilometer-long vacuum pipes. As the gravitational waves pass through the detector, they cause the length of one of the pipes to shorten or expand, forcing that pipe’s laser beam to arrive a fraction of a second before or after the other pipe’s.

In December, this allowed scientists to detect a wave whose oscillation began at 35 cycles per second (hertz) and rapidly increased to 450 hertz. When converted to an audible sound, the wave makes an unmistakable chirp. “This is what we call gravity's music,” says Gonzalez. And the first chirp took the world by storm.

But the two chirps detected so far are not quite identical, revealing vast differences between the two fender benders that produced them. The first signal LIGO spied radiated outward from two surprisingly heavy stellar-mass black holes. As such the chirp was loud but short. It lasted only 0.2 seconds and allowed scientists to glimpse the last 10 cycles of the black holes’ orbits before their collision. But the latest signal radiated outward from two lighter black holes. This chirp was weak, barely visible above the noise, but long. It lasted a full second and allowed scientists to witness the last 55 orbits of the black holes’ inspiral.

“The first LIGO detection was a whopping huge signal from improbably large, merging black holes,” says Chiara Mingarelli, a gravitational wave astronomer from the California Institute of Technology in Pasadena, who is not a part of the LIGO collaboration. “This second signal tells us more about the [typical] population of black holes in the local universe.”

The latest result also suggests that one of the black holes was spinning at roughly 20 percent of the maximum theoretical spin rate for a black hole. “A measurement of the binary black hole’s spin can serve as an indicator, or tracer, of how the binary came to be and how it evolved,” says Mingarelli. “A binary that formed in a relatively isolated system is expected to have aligned spins, whereas if the binary formed in a dense environment, one would expect the spins to be misaligned.” Thus, future detections should relay the black holes’ surroundings and hint as to how they formed.

A New Era

Perhaps more importantly, the second observation shows that LIGO has only just begun. “The future is going to be full of binary black hole mergers from LIGO,” says Reitze. “We're going to see a lot more of these as we go forward.”

The next run, which begins in the fall, will increase its sensitivity by 15 to 25 percent. Given that the last four-month run resulted in two mergers, Reitze expects that the added sensitivity will result in six to eight mergers over the six-month run. That’s important because researchers would like to collect enough findings to study the frequency of black hole collisions across the universe. (The team also observed an additional event in the first run that was too weak to confirm.)

Physicists hope that LIGO will eventually detect gravitational waves from other, fainter, types of cosmic collisions, like those between neutron stars. These events are expected to both trigger gravitational waves and emit electromagnetic radiation. A detection of both could allow astronomers to better understand such wild events. But first, physicists need to pinpoint them on the sky. More gravitational-wave detectors coming online in the coming years, including one in Italy and two in India, will help scientists do just that.

But before we get ahead of ourselves, the second detection alone is still an event to marvel at. “With this, we can tell you now that the era of gravitational wave astronomy has begun,” says Gonzalez.