Gravitational waves detected from 2nd pair of colliding black holes

An international team of scientists – including physicists at the University of Wisconsin-Milwaukee – has detected gravitational waves for the second time.

The gravitational waves were observed Dec. 26 by the twin Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in Livingston, La., and Hanford, Wash.

The first detection of these waves, announced in February, was a milestone in physics and astronomy; it confirmed a major prediction of Albert Einstein’s 1915 general theory of relativity and marked the beginning of the new field of gravitational-wave astronomy.

“This second detection tells us that the first detection wasn’t some sort of lucky break,” said Patrick Brady, director of the Leonard E. Parker Center for Gravitation, Cosmology and Astrophysics at UWM. “Gravitational-wave astronomy has truly started.”

Gravitational waves are ripples in the fabric of space-time that are caused by the movement of massive objects in space. The waves carry information about their origins and about the nature of gravity that cannot otherwise be obtained, and physicists have concluded that these gravitational waves were produced during the final moments of the merger of two black holes about 1.4 billion years ago. The collision of the two black holes created a single, more massive spinning black hole that is 21 times the mass of the sun.

The discovery, which will be described in an article accepted for publication in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration, a group of more than 1,000 scientists (including approximately 250 students) from universities in the United States and other countries, and the Virgo Collaboration, consisting of more than 250 scientists from 19 European research groups.

Along with Brady, UWM’s team includes faculty members Jolien Creighton, Xavier Siemens, and Alan Wiseman and 26 other scientists and students. UWM also contributed significant computer resources to the data analysis that identified both events.

It is a promising start to mapping the populations of black holes in our universe, Brady said. Because black holes are not visible even with the most powerful telescopes, scientists know relatively little about them.

“Black holes are formed when massive stars die,” he said. “But we know very little about how many black holes are out there, how massive they are, or how fast they spin. This discovery is a major step toward finding the answers to these questions.”

The first black hole merger detected was unexpectedly massive, Creighton said. Until then, there was no information that proved that mergers could exist at so large a mass.

“The signal of the first one stood out from the noise much more than this one,” he said. “This one is still a gold-plated event; it just required more sophistication to detect it in the data.”

The detected signal comes from the last 55 orbits of the black holes, before their merger. Based on the arrival time of the signals – with the observatory in Livingston, La., measuring the waves 1.1 milliseconds before the observatory in Hanford, Wash. – the position of the source in the sky can be roughly determined.

Both discoveries were made possible by the enhanced capabilities of Advanced LIGO, a major upgrade that increases the sensitivity of the instruments compared to the first generation LIGO detectors, enabling a large increase in the volume of the universe probed.

Advanced LIGO’s next data-taking run will begin this fall. By then, further improvements in detector sensitivity are expected to allow LIGO to reach as much as 1.5 to 2 times more of the volume of the universe. Another detector operated by the Virgo Collaboration is expected to join LIGO in the latter half of the upcoming observing run.

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