Gravitational Waves Detected for the Second Time

June 15, 2016 | Joanne Kennell

Two black holes, moments before colliding and merging
Photo credit: Numerical Simulations: S. Ossokine and A. Buonanno, Max Planck Institute for Gravitational Physics, and the Simulating eXtreme Spacetime (SXS) project. Scientific Visualization: T. Dietrich and R. Haas, Max Planck Institute for Gravitational Physics.

It is just as monumental as the first.

On December 26, 2015 at 03:38:53 UTC (December 25, 2015 at 22:38:53 EDT), scientists once again observed gravitational waves — dynamic ripples in the fabric of spacetime. The event, named GW151226, was detected by the twin Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington.

This second detection is just as monumental as the first because as more waves are uncovered, the nature and origin of gravity begins to unfold.

"The excitement around the first of anything is obviously enormous, but this one is really on par," Chad Hanna, assistant professor of physics at Penn State, told Rebecca Boyle of The Atlantic. "The thing with having only one is, while we’re certain that was a gravitational wave, you don’t know if you happened to get lucky. When we got the second one, we’re all breathing a sigh of relief."

According to the researchers, the gravitational waves were the result of the last 27 orbits of a cataclysmic black hole merger. The event fused together two black holes 14 and 8 times the mass of the sun, producing a single, massive spinning black hole and spitting out a sun’s worth of mass in the form of gravitational energy.

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Since the Livingston detector measured the waves 1.1 milliseconds before the Hanford detector, the researchers were able to determine that the merger occurred approximately 1.4 billion years ago.

In comparison, the first gravitational wave detection was the consequence of the merger of two black holes, 36 and 29 times the mass of the sun, 1.3 billion years ago.

"It's fabulous that our waveform models have pulled out from the noise such a weak but incredibly valuable gravitational wave signal," said Alessandra Buonanno, a University of Maryland College Park Professor of Physics and LIGO Scientific Collaboration principal investigator, said in the press release.

"GW151226 perfectly matches our theoretical predictions for how two black holes move around each other for several tens of orbits and ultimately merge," Buonanno added. "Remarkably, we could also infer that at least one of the two black holes in the binary was spinning."

Excitingly, now that two black hole mergers have been observed, physicists can start to surmise about how often black holes are born and die, and how many sun-sized black holes there are. "It is a promising start to mapping the populations of black holes in our universe," said Gabriela Gonzalez, LSC spokesperson and professor of physics and astronomy at Louisiana State University, in the release.

But it doesn’t end with black holes. Scientists want to use LIGO to study neutron stars — celestial objects of very small radius and very high density — to measure the centimeter-high “mountains” that form on their surfaces and make them spin as pulsars.

LIGO will complete another survey of the sky this fall after improvements to the detector’s sensitivity are complete. The researchers hope that after the upgrade, the detector will be able to scan twice as much volume as the first survey.

The discovery has been accepted for publication in the journal Physical Review Letters.

Read next: Three Spacecrafts to Be Launched in Orbit Around Earth to Detect Gravitational Waves

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