LIGO detects colliding black holes for third time

UChicago scientists: Results help unveil diversity of black holes in the universe

The Laser Interferometer Gravitational-Wave Observatory has made a third detection of gravitational waves, providing the latest confirmation that a new window in astronomy has opened. As was the case with the first two detections, the waves—ripples in spacetime—were generated when two black holes collided to form a larger black hole.

The latest findings by the LIGO observatory, described in a new paper accepted for publication in Physical Review Letters, builds upon the landmark discovery in 2015 of gravitational waves, which Albert Einstein predicted a century earlier in his theory of general relativity. 

“The UChicago LIGO group has played an important role in this latest discovery, including helping to discern what emitted the gravitational waves, testing whether Einstein’s theory of general relativity was correct, and exploring whether electromagnetic radiation—such as visible light, radio, or X-rays—were also emanated by the black hole collision,” said Daniel Holz, associate professor in Physics and Astronomy & Astrophysics, and head of UChicago’s LIGO group.

The new detection occurred during LIGO’s current observing run, which began Nov. 30, 2016, and will continue through the summer. The newfound black hole formed by the merger has a mass about 49 times that of our sun. The discovery fills in a gap between the systems previously detected by LIGO, with masses of 62 and 21 times that of our sun for the first and second detections, respectively.

“We continue to learn more about this population of heavy stellar-mass black holes, with masses over 20 solar masses, that LIGO has discovered,” said LIGO collaborator Ben Farr, a McCormick Fellow at UChicago’s Enrico Fermi Institute. “LIGO is making the most direct and pristine observations of black holes that have ever been made, and we’re taking large strides in our understanding of how and where these black holes are formed.”

LIGO made the first direct observation of gravitational waves in September 2015 during its first observing run. The second detection was made in December 2015, and the third detection, called GW170104, was made on Jan. 4, 2017.

In all three cases, each of the twin detectors of LIGO observed gravitational waves from the tremendously energetic mergers of black hole pairs. The collisions produce more power than is radiated by all of the stars in all of the galaxies in the entire observable universe. The recent detection is the farthest one yet, with the black holes located about 3 billion light-years away. The black holes in the first and second detections were located 1.3 billion and 1.4 billion light-years away, respectively.

"It is truly remarkable that, 100 years after the formulation of general relativity, we are now directly observing some of the most interesting predictions of this theory,” said LIGO collaborator Robert Wald, the Charles H. Swift Distinguished Service Professor in Physics at UChicago. “LIGO has opened an entirely new window on our ability to observe phenomena involving strong gravitational fields, and we can look forward to its providing us with many further observations of great astrophysical and cosmological significance in the coming years.”

‘Looks like Einstein was right’

The LIGO Scientific Collaboration is an international collaboration whose observations are carried out by twin detectors—one in Hanford, Wash., and the other in Livingston, La.—operated by California Institute of Technology and Massachusetts Institute of Technology with funding from the National Science Foundation.

The discoveries from LIGO are once again putting Albert Einstein’s theories to the test. For example, the researchers looked for an effect called dispersion, in which light waves in a physical medium travel at different speeds depending on their wavelength—the same way a prism creates a rainbow.

Einstein’s general theory of relativity forbids dispersion from happening in gravitational waves as they propagate from their source to Earth, and LIGO’s latest detection is consistent with this prediction.

"It looks like Einstein was right—even for this new event, which is about two times farther away than our first detection," said Laura Cadonati, associate professor of physics at Georgia Institute of Technology and deputy spokesperson for the LIGO Scientific Collaboration. "We can see no deviation from the predictions of general relativity, and this greater distance helps us to make that statement with more confidence."

The LIGO team working with the Virgo Collaboration is continuing to search the latest LIGO data for signs of space-time ripples from the far reaches of the cosmos. They also are working on technical upgrades for LIGO’s next run, scheduled to begin in late 2018, during which the detectors’ sensitivity will be improved.

“With the detection of GW170104, we are taking another important step toward gravitational-wave astronomy,” Holz said. “We now have three solid detections, and these provide our first hints about the diversity of black hole systems in the universe.”

LIGO is funded by the National Science Foundation. More than 1,000 scientists from around the world participate in the effort through the LIGO Scientific Collaboration and Virgo Collaboration.