Gravitational Waves Detected a Fourth Time
University of Maryland physicists contribute to identification of fourth gravitational wave event using data from Advanced LIGO and Virgo detectors
On August 14, 2017, at 10:30:43 UTC, scientists observed gravitational waves—ripples in the fabric of spacetime—for the fourth time.
The twin Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors—located in Livingston, Louisiana, and Hanford, Washington—detected the gravitational wave event, named GW170814. For the first time, the Virgo detector located near Pisa, Italy, also contributed to the observation. The Virgo Collaboration and the LIGO Scientific Collaboration (LSC) announced the discovery and its implications on Wednesday, September 27, 2017, at a press conference in Torino, Italy. A research paper describing the findings has been accepted and will appear in the journal Physical Review Letters.
Gravitational waves carry information about their origins and about the nature of gravity that cannot otherwise be obtained. Physicists on the LIGO and Virgo teams concluded that, as with LIGO’s first three historic detections, the final moments of a black hole merger produced the gravitational waves that LIGO and Virgo observed on August 14, 2017.
The black holes that merged to produce GW170814 were about 30 and 25 times the mass of the sun, respectively. The merger produced a single, more massive black hole that is about 53 times the mass of the sun and transformed the remaining mass into gravitational energy.
“We listened to yet another gravitational wave emitted by merging black holes, and this time the joint LIGO/Virgo detector network allowed a more accurate measurement of the position and distance of the source,” said Alessandra Buonanno, a UMD College Park Professor of Physics and LSC principal investigator who also has an appointment as Director at the Max Planck Institute for Gravitational Physics in Potsdam, Germany. “This is because three ears can listen even more accurately than two ears.”
Based on the arrival times of the GW170814 signals—the Livingston detector measured the waves 8 milliseconds before the Hanford detector and 14 milliseconds before the Virgo detector—researchers can more accurately determine the position of the source in the sky. Prior detections using the two LIGO detectors alone could only narrow down the position of the event to within about 2.8 percent of the total sky area as viewed from Earth. The Virgo detector, which came online August 1, 2017, helped reduce this candidate area to roughly 0.15 percent of the total sky area.
“Even though the Advanced Virgo detector is currently less sensitive than LIGO, it made a big difference for our understanding of this new event,” said Peter Shawhan, a professor of physics at UMD and an LSC principal investigator who serves as Data Analysis Committee Chair for the LSC. “Besides knowing its location in the sky better than any of our earlier events, we are able to compare the data from the three detectors to check whether gravitational waves are polarized in the way that Einstein’s theory predicts.”
UMD physicists contributed to the analysis and interpretation of GW170814 in multiple ways.
Buonanno has led the effort to develop highly accurate models of gravitational waves that black holes would generate in the final process of orbiting and colliding with each other. While analyzing GW170814, Buonanno’s team and other members of the LIGO and Virgo collaborations used these waveform models to localize the source in the sky and identify it as a pair of orbiting black holes.
“With more observatories in the network, we are able to extract information on the orientation of the binary’s orbit and pursue new tests of general relativity,” Buonanno added.
Shawhan and his students at UMD have worked with other LIGO and Virgo team members to establish a program to quickly share information about each gravitational wave event candidate, including sky location, with astronomers. This will enable astronomers to look for the event with their telescopes and other instruments. Although no counterpart to GW170814 has been reported, Shawhan noted that “the improved capabilities demonstrated by the three-detector network bode well for identifying counterparts to future gravitational-wave events.”
The first detection of gravitational waves, observed on September 14, 2015, and announced on February 11, 2016, 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.
The first event resulted from a merger of two black holes about 36 and 29 times the mass of the sun. In contrast, the black holes that created the second event were relative flyweights, tipping the scales at 14 and eight times the mass of the sun. The third detection sat in the middle, resulting from a merger between two black holes, more than 31 and 19 times the mass of the sun, respectively.
The newly detected merger occurred approximately 1.7 billion years ago, putting it at about the same distance as the first two events, which occurred 1.3 and 1.4 billion years ago, respectively. The third event occurred about 3 billion years ago, making it more than twice as old (and more than twice as distant) as the other three events.
The research paper, “GW170814: A three-detector observation of gravitational waves from a binary black hole coalescence,” by the LIGO Scientific Collaboration and the Virgo Collaboration, was published online on October 6, 2017 in Physical Review Letters.
About LIGO and Virgo
LIGO is funded by the National Science Foundation (NSF), and operated by Caltech and MIT, which conceived and built the project. Financial support for the Advanced LIGO project was led by NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council) making significant commitments and contributions to the project. More than 1,200 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. Additional partners are listed at http://ligo.org/partners.php. The Virgo collaboration consists of more than 280 physicists and engineers belonging to 20 different European research groups: six from Centre National de la Recherche Scientifique (CNRS) in France; eight from the Istituto Nazionale di Fisica Nucleare (INFN) in Italy; two in The Netherlands with Nikhef; the MTA Wigner RCP in Hungary; the POLGRAW group in Poland; Spain with the University of Valencia; and EGO, the laboratory hosting the Virgo detector near Pisa in Italy.
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