Third detection of gravitational waves confirms new population of black holes

MILWAUKEE _ University of Wisconsin-Milwaukee physicists and their Laser Interferometer Gravitational-wave Observatory (LIGO) colleagues around the world on Thursday announced their third detection of gravitational waves.

The first detection of gravitational waves in September 2015 confirmed a major prediction of Albert Einstein’s 1915 general theory of relativity, and a second detection in December 2016 confirmed that the first wasn’t a fluke.

This third detection of ripples in the fabric of space-time confirms the existence of a new class of black holes with large masses that had gone undetected before LIGO. Analysis of the wave may also contribute to our understanding of how binary black holes – pairs of black holes in orbit around each other – are created.

As was the case with the first two detections, physicists at UW-Milwaukee’s Center for Gravitation, Cosmology and Astrophysics played a key role in confirming and characterizing waves generated when two black holes merged to form a larger black hole. The UWM team led the development of a network of supercomputers used to analyze the huge amount of data coming from the LIGO instruments.

“Black holes like this can only be detected through the ripples in space-time produced as they merge,” says Patrick Brady, director of the Leonard E Parker Center for Gravitation, Cosmology and Astrophysics at the University of Wisconsin-Milwaukee and senior member of the LIGO Scientific Collaboration. “This observation reveals a little more about the types of black holes that form and merge in our universe.”

The newfound black hole, formed by the merger of two black holes, has a mass about 49 times that of our sun. This fills in a gap between the masses of the two merged black holes detected previously by LIGO, which had solar masses of 62 (first detection) and 21 (second detection).

The recent detection is the most distant yet, with the black holes located about 3 billion light-years away. (The black holes in the first and second detections are located 1.3 billion and 1.4 billion light-years away, respectively.)

The new detection occurred during LIGO’s current observing run, which began Nov. 30, 2016, and will continue through the summer. LIGO is an international project with members across the globe. Its observations are carried out by twin detectors — one in Hanford, Washington, and the other in Livingston, Louisiana — operated by Caltech and MIT with funding from the National Science Foundation.

The newest observation also provides clues about the directions in which the black holes are spinning, and this offers clues about how the pair formed.

As pairs of black holes spiral around each other, they also spin on their own axes — like the Earth spinning on its axis as it orbits around the sun. Binary black holes whose axes are aligned likely formed together; binaries with unaligned axis may have fallen into orbit together after they formed. The new LIGO data suggest that the spin of at least one of the black holes may have been tilted. More observations with LIGO are needed to say anything definitive about the spins of binary black holes, but these early data offer exciting clues.

“With this detection, we are starting to unravel the mysteries of binary black hole formation,” says Jolien Creighton, a professor at UWM and co-chair of the science working group responsible for identifying and interpreting the signal in the LIGO data.

LIGO is part of a planned global network of gravitational-wave detectors that will include the European Virgo detector.

Over the next few years, LIGO and Virgo will continue to alternate observing runs with periods of detector improvements that will bring better measurements of binary black holes and other new insights into the universe. Brady and his colleagues expect those refinements to offer a new window on less massive but no less exotic objects, such as neutron stars.

“Neutron stars are basically massive nuclei, and we have no idea how matter behaves at that density,” Creighton said. “Neutron stars provide the only physical laboratory we have to study matter of this sort.”

Jo van den Brand, the Virgo Collaboration spokesperson, a physicist at the Dutch National Institute for Subatomic Physics (Nikhef) and professor at VU University in Amsterdam, said: “The LIGO instruments have reached impressive sensitivities. We expect that by this summer Virgo, the European interferometer, will expand the network of detectors, helping us to better localize the signals.”

David Shoemaker, the newly elected spokesperson for the LIGO Scientific Collaboration (LSC), added: “It is remarkable that humans can put together a story, and test it, for such strange and extreme events that took place billions of years ago and billions of light-years distant from us. The entire LIGO and Virgo scientific collaborations worked to put all these pieces together.”