For the first time, three detectors have tracked the gravitational waves emitted by a merger of two black holes -- a critical new capability that allows scientists to more closely locate a gravitational wave's birthplace in space. Gravitational waves are ripples in space and time created when two massive, compact objects such as black holes merge.
The new detections were made on August 14, 2017 by the LIGO Scientific Collaboration's two gravitational-wave detectors in the United States and by the Virgo Gravitational-Wave Observatory in Italy. It is the first joint detection of gravitational waves by the LIGO and Virgo detectors. It also is the fourth announced detection of a binary black-hole system and the first significant gravitational-wave signal recorded by the Virgo detector.
The detection is especially important because it highlights the scientific potential of a three-detector network of gravitational-wave detectors. "The Virgo Gravitational-Wave Observatory joining forces with LIGO is a huge moment in astronomy," said Chad Hanna, assistant professor of physics and of astronomy & astrophysics and Freed Early Career Professor at Penn State, who has served as co-chair of LIGO's Compact Binary Coalescence Group, which has detected all the gravitational waves discovered thus far.
"It is our hope to one day detect gravitational waves and to simultaneously observe the source of the gravitational waves with conventional telescopes so that we might learn even more about what causes the gravitational waves. In order to do that, we need to know where to look. LIGO and Virgo together allow us to pinpoint the gravitational wave source in the sky far better than before, which will dramatically improve our chances of capturing the gravitational wave source with other telescopes."
The new ability to observe gravitational waves with three detectors -- two at LIGO plus one at Virgo -- opens the door to additional scientific knowledge.
"Virgo adds new capability to the LIGO network. Thanks to Virgo we can now test a key prediction of Einstein's general relativity, namely the polarization property of the wave," said B. S. Sathyaprakash, the Elsbach Professor of Physics and Professor of Astronomy and Astrophysics at Penn State. "The two LIGO detectors alone are not able to infer the polarization property of gravitational waves because they alone could not fix the position of the source in the sky. The addition of Virgo as a third detector helps in localizing the source and hence allows us to infer the wave's polarization. As far as we can tell, Einstein is still right."
France Córdova, director of the U.S. National Science Foundation and a former head of the Penn State Department of Astronomy and Astrophysics, said "Little more than a year and a half ago, NSF announced that its Laser Gravitational-Wave Observatory had made the first-ever detection of gravitational waves resulting from the collision of two black holes in a galaxy a billion light-years away. Today, we are delighted to announce the first discovery made in partnership between the Virgo Gravitational-Wave Observatory and the LIGO Scientific Collaboration, the first time a gravitational-wave detection was observed by these observatories, located thousands of miles apart. This is an exciting milestone in the growing international scientific effort to unlock the extraordinary mysteries of our Universe."
LIGO is funded by NSF and operated by Caltech and MIT, which conceived and built the project.