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This illustration shows the merger of two black holes and the gravitational waves that ripple outward as the black holes spiral toward each other. The black holes—which represent those detected by LIGO on Dec. 26, 2015—were 14 and 8 times the mass of the sun, until they merged, forming a single black hole 21 times the mass of the sun. In reality, the area near the black holes would appear highly warped, and the gravitational waves would be difficult to see directly.
Credit: LIGO/T. Pyle
LIGO Scientific Collaboration and the Virgo collaboration identify a
second gravitational wave event in the data from Advanced LIGO detectors
December 26, 2015 at 03:38:53 UTC, scientists observed gravitational
waves—ripples in the fabric of spacetime—for the second time.
gravitational waves were detected by both of the twin Laser
Interferometer Gravitational-Wave Observatory (LIGO) detectors, located
in Livingston, Louisiana, and Hanford, Washington, USA.
Observatories are funded by the National Science Foundation (NSF), and
were conceived, built, and are operated by Caltech and MIT. The
discovery, accepted for publication in the journal Physical Review Letters,
was made by the LIGO Scientific Collaboration (which includes the GEO
Collaboration and the Australian Consortium for Interferometric
Gravitational Astronomy) and the Virgo Collaboration using data from the
two LIGO detectors.
Gravitational 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—14 and 8 times the mass of the sun—to produce a single, more
massive spinning black hole that is 21 times the mass of the sun.
is very significant that these black holes were much less massive than
those observed in the first detection," says Gabriela Gonzalez, LIGO
Scientific Collaboration (LSC) spokesperson and professor of physics and
astronomy at Louisiana State University. "Because of their lighter
masses compared to the first detection, they spent more time—about one
second—in the sensitive band of the detectors. It is a promising start
to mapping the populations of black holes in our universe."
the merger, which occurred approximately 1.4 billion years ago, a
quantity of energy roughly equivalent to the mass of the sun was
converted into gravitational waves. The detected signal comes from the
last 27 orbits of the black holes before their merger. Based on the
arrival time of the signals—with the Livingston detector measuring the
waves 1.1 milliseconds before the Hanford detector—the position of the
source in the sky can be roughly determined.
"In the near future,
Virgo, the European interferometer, will join a growing network of
gravitational wave detectors, which work together with ground-based
telescopes that follow-up on the signals," notes Fulvio Ricci, the Virgo
Collaboration spokesperson, a physicist at Istituto Nazionale di Fisica
Nucleare (INFN) and professor at Sapienza University of Rome. "The
three interferometers together will permit a far better localization in
the sky of the signals."
The first detection of gravitational waves,
announced on February 11, 2016, 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.
discovery "has truly put the 'O' for Observatory in LIGO," says
Caltech's Albert Lazzarini, deputy director of the LIGO Laboratory.
"With detections of two strong events in the four months of our first
observing run, we can begin to make predictions about how often we might
be hearing gravitational waves in the future. LIGO is bringing us a new
way to observe some of the darkest yet most energetic events in our
"We are starting to get a glimpse of the kind of new
astrophysical information that can only come from gravitational wave
detectors," says MIT's David Shoemaker, who led the Advanced LIGO
detector construction program.
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.
"With the advent of Advanced LIGO, we
anticipated researchers would eventually succeed at detecting unexpected
phenomena, but these two detections thus far have surpassed our
expectations," says NSF Director France A. Córdova. "NSF's 40-year
investment in this foundational research is already yielding new
information about the nature of the dark universe."
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. The Virgo
detector is expected to join in the latter half of the upcoming
LIGO research is carried out by the LIGO Scientific
Collaboration (LSC), a group of more than 1,000 scientists from
universities around the United States and in 14 other countries. More
than 90 universities and research institutes in the LSC develop detector
technology and analyze data; approximately 250 students are strong
contributing members of the collaboration. The LSC detector network
includes the LIGO interferometers and the GEO600 detector.
research is carried out by the Virgo Collaboration, consisting of more
than 250 physicists and engineers belonging to 19 different European
research groups: 6 from Centre National de la Recherche Scientifique
(CNRS) in France; 8 from the Istituto Nazionale di Fisica Nucleare
(INFN) in Italy; 2 in The Netherlands with Nikhef; the MTA Wigner RCP in
Hungary; the POLGRAW group in Poland and the European Gravitational
Observatory (EGO), the laboratory hosting the Virgo detector near Pisa
The NSF provides most of the financial support for
Advanced LIGO. Funding organizations in Germany (Max Planck Society),
the U.K. (Science and Technology Facilities Council, STFC) and Australia
(Australian Research Council) also have made significant commitments to
Several of the key technologies that made Advanced
LIGO so much more sensitive have been developed and tested by the German
UK GEO collaboration. Significant computer resources have been
contributed by the AEI Hannover Atlas Cluster, the LIGO Laboratory,
Syracuse University, the ARCCA cluster at Cardiff University, the
University of Wisconsin-Milwaukee, and the Open Science Grid. Several
universities designed, built, and tested key components and techniques
for Advanced LIGO: The Australian National University, the University of
Adelaide, the University of Western Australia, the University of
Florida, Stanford University, Columbia University in the City of New
York, and Louisiana State University. The GEO team includes scientists
at the Max Planck Institute for Gravitational Physics (Albert Einstein
Institute, AEI), Leibniz Universität Hannover, along with partners at
the University of Glasgow, Cardiff University, the University of
Birmingham, other universities in the United Kingdom and Germany, and
the University of the Balearic Islands in Spain.