LIGO Does it Again! Second Black Hole Merger Recorded

It’s a good day. This morning, scientists from the Laser Interferometer Gravitational-Wave Observatory (LIGO) Scientific Collaboration and Virgo Collaboration announced that late on the evening of December 25, 2015, LIGO detectors observed gravitational waves from a black hole merger that happened 1.4 billion years ago. They made the announcement from the 228th American Astronomical Society meeting, and the work was published today in Physical Review Letters.

Numerical simulations of the gravitational waves emitted by the inspiral and merger of two black holes. The colored contours around each black hole represent the amplitude of the gravitational radiation; the purple and blue lines represent the trajectories of the black holes, and the green arrows represent their spins.
Image Credit: NASA/Ames Research Center/C. Henze. Public Domain.

Predicted by Einstein’s general theory of relativity, gravitational waves are ripples in the fabric of space that travel at the speed of light. They are created by intense, massive events like supernovae and colliding black holes, and they carry information that we can’t observe in any other way. Gravitational waves provide a whole new way of seeing the universe.

This announcement reflects just the second gravitational wave signal ever observed. Like the first one, the December 25th signal came from the merging of two black holes. The black holes were approximately 14 and 8 times the mass of the sun. As they spiraled toward each other in their last few moments of being single, they produced the ripples that were detected Christmas day by LIGO’s two gravitational wave observatories, one in Livingston, Louisiana and the other in Hanford, Washington.
On merging, the two black holes formed a spinning black hole about 21 times the mass of the sun. If you’re following the math, you’ll realize that doesn’t quite work out. But this missing mass is key. It reflects the energy converted into the gravitational waves, remember that mass and energy are related by E=mc2.

The signal included another cool result—a glimpse into how one of the black holes was spinning before the merger. On a scale from 0 to 1, with 0 describing a black hole that is not spinning at all and 1 describing one that is spinning at its maximum, one of the black holes scored greater than 0.2. We don’t know very much about how black holes like these usually spin, what happens at a spin of 1, why this black hole was spinning, or even which of the two black holes was spinning—but that is all part of the adventure! Hopefully more detections will fill in some of the missing pieces soon.

Many scientists thought we’d never be able to detect the tiny effects of gravitational waves here on Earth, but believers have been working toward this since 1960s. LIGO has now done it twice in four months. The experiment looks for gravitational waves using laser interferometry (you can read about the design here). LIGO opened its doors to gravitational waves in 2002, but none made an appearance until a $200 million Advanced LIGO upgrade dramatically increased LIGO’s sensitivity and the volume of space it can see last fall.

The groundbreaking first detection was announced in February, and came from the merger of two black holes that were each more than 25 times the mass of the sun. “With detections of two strong events in the four months of our first observing run, we can begin to make predictions about how often we might be hearing gravitational waves in the future. LIGO is bringing us a new way to observe some of the darkest yet most energetic events in our universe,” Albert Lazzarini, deputy director of the LIGO Laboratory, said in a statement.

These detections have opened the door to a whole new way of exploring our universe. LIGO is not currently taking data—work is being done to increase its sensitivity even more—but it will start a 6-month data collecting run in September. In early 2017 an upgraded interferometer run by the European Virgo collaboration will come online in Italy. LIGO-like interferometers should be operating in Japan and India in the next ten years. This network of instruments will enable scientists to pinpoint the source of future gravitational wave signals even more precisely. Space-based gravitational wave detectors are also in the works. Things are just getting started!

For more on gravitational waves, check out these related stories from Physics Central:

Gravitational Waves

The First Detection

LIGO: What You Need to Know

LIGO Live! Q&A With Lynn Cominsky

The Truth About Gravitational Waves (Podcast)

Gravitational Wave Dress

LISA Pathfinder: The Freest Fall

Kendra Redmond

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