On January 4, 2017, at 11:11:58 CET, scientists observed gravitational waves--ripples in the fabric of spacetime--for the third time.

The twin Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors--located in Livingston, Louisiana, and Hanford, Washington--detected the gravitational wave event, named GW170104. The LIGO Scientific Collaboration (LSC) and the Virgo Collaboration published a report describing the discovery and its implications on June 1, 2017 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 two historic detections, the final moments of a black hole merger produced the gravitational waves that LIGO observed on January 4, 2017.

LIGO's first detection, on September 14, 2015, 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 and most recent detection lies in the middle, resulting from a merger between two black holes, more than 31 and 19 times the mass of the sun, respectively. The merger produced a single, more massive black hole that is slightly less than 49 times the mass of the sun, and transformed the remaining mass into gravitational energy.

"The observation and interpretation of yet another LIGO signal, GW170104, confirms the success of our theoretical program to model binary black holes," 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. 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.

"For the third LIGO signal we could gather some evidence that at least one black hole might be rotating in a direction misaligned with the overall orbital motion--a spin configuration favored by some astrophysical formation scenarios of binary black holes" Buonanno added, noting that her team made substantial improvements to their methodology throughout 2016, in between LIGO's observing runs.

The newly detected merger occurred approximately 3 billion years ago, making it more than twice as old (and more than twice as distant) as the first two events, which occurred 1.3 and 1.4 billion years ago, respectively. Based on the arrival time of the signals--the Hanford detector measured the waves 3 milliseconds before the Livingston detector--researchers can roughly determine the position of the source in the sky.

The first detection of gravitational waves, 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 latest detection adds to the diversity of black hole merger events we've been able to observe directly," said Peter Shawhan, an associate professor of physics at UMD and an LSC principal investigator who serves as Data Analysis Committee Chair for the LSC. "We're conducting a census of black holes in binary systems in our universe and we expect to discover other types of signals too. Besides just counting them, we can learn from their properties how they were born and how they evolved."

The researchers also looked for an effect called dispersion, which is known to occur when light waves in a physical medium such as glass travel at different speeds depending on their wavelength. This is how a prism creates a rainbow, for example. Einstein's general theory of relativity forbids dispersion from happening in gravitational waves as they propagate from their source to Earth. LIGO did not find evidence for this effect in GW170104.

"Even for this new event, which is about two times farther away than our first two detections, we could not find any evidence that gravitational waves disperse as they travel in the fabric of space-time," Buonanno said.

"Einstein still seems to have been right about the true nature of gravity," Shawhan added.

The new detection occurred during LIGO's second observing run, which began November 30, 2016, and will continue through August 2017. The LIGO-Virgo team is continuing to search the latest LIGO data for signs of space-time ripples from the far reaches of the cosmos. They are also working on substantial technical upgrades for LIGO's third run, scheduled to begin in late 2018, with help from the nearly completed Virgo detector in Italy collecting data simultaneously.

The research paper, "GW170104: Observation of a 50-Solar-Mass Binary Black Hole Coalescence at Redshift 0.2," by the LIGO Scientific Collaboration and the Virgo Collaboration, was published online June 1, 2017 in the journal Physical Review Letters.

The LIGO Laboratory is funded by the National Science Foundation (NSF), and operated by Caltech and MIT, which conceived and built the Observatory. The NSF led in financial support for the Advanced LIGO project with funding organizations in Germany (MPG), the U.K. (STFC) and Australia (ARC) making significant commitments to the project. More than 1,000 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. LIGO partners with the Virgo Collaboration, which is supported by Centre National de la Recherche Scientifique (CNRS), Istituto Nazionale di Fisica Nucleare (INFN) and Nikhef, as well as Virgo's host institution, the European Gravitational Observatory, a consortium that includes 280 additional scientists throughout Europe. Additional partners are listed at: http://ligo. .

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The College of Computer, Mathematical, and Natural Sciences at the University of Maryland educates more than 7,000 future scientific leaders in its undergraduate and graduate programs each year. The college's 10 departments and more than a dozen interdisciplinary research centers foster scientific discovery with annual sponsored research funding exceeding $150 million. </s> Aerial view of the Virgo site showing the Mode-Cleaner building, the Central building, the three kilometer-long west arm and the beginning of the north arm. The other buildings include offices, workshops, computer rooms and the control room of the interferometer. Credit: The Virgo collaboration/CCO 1.0

In August, detectors on two continents recorded gravitational wave signals from a pair of black holes colliding. This discovery, announced today, is the first observation of gravitational waves by three different detectors, marking a new era of greater insights and improved localization of cosmic events now available through globally networked gravitational-wave observatories.

The collision was observed Aug. 14 at 10:30:43 a.m. Coordinated Universal Time (UTC) using the two National Science Foundation (NSF)-funded Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors located in Livingston, Louisiana, and Hanford, Washington, and the Virgo detector, funded by CNRS and INFN and located near Pisa, Italy.

The detection by the LIGO Scientific Collaboration (LSC) and the Virgo collaboration is the first confirmed gravitational wave signal recorded by the Virgo detector. A paper about the event, a collision designated GW170814, has been accepted for publication in the journal Physical Review Letters.

"Little more than a year and a half ago, NSF announced that its Laser Interferometer Gravitational Wave Observatory had made the first-ever detection of gravitational waves, which resulted from the collision of two black holes in a galaxy a billion light-years away," said NSF Director France Córdova. "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 operates two detector sites -- one near Hanford in eastern Washington, and another near Livingston, Louisiana. The Livingston detector site is pictured here. Credit: LIGO Collaboration

The detected gravitational waves—ripples in space and time—were emitted during the final moments of the merger of two black holes, one with a mass about 31 times that of our sun, the other about 25 times the mass of the sun. The event, located about 1.8 billion light-years away resulted in a spinning black hole with about 53 times the mass of our sun—that means about three solar masses were converted into gravitational-wave energy during the coalescence.

"This is just the beginning of observations with the network enabled by Virgo and LIGO working together," says LSC spokesperson David Shoemaker of the Massachusetts Institute of Technology (MIT). "With the next observing run planned for fall 2018, we can expect such detections weekly or even more often."

LIGO has transitioned into a second-generation gravitational-wave detector, known as Advanced LIGO, that consists of two identical interferometers. Beginning operations in September 2015, Advanced LIGO has conducted two observing runs. The second observing run, "O2," began Nov. 30, 2016, and ended Aug. 25, 2017.

The Virgo detector, also now a second-generation detector, joined the O2 run Aug. 1, 2017 at 10 a.m. UTC. The real-time detection Aug. 14 was triggered with data from all three LIGO and Virgo instruments.

View of the LIGO detector in Hanford, Washington. LIGO research is carried out by the LIGO Scientific Collaboration, a group of more than 1,000 scientists from universities around the US and 14 other countries. Credit: LIGO Laboratory

"It is wonderful to see a first gravitational-wave signal in our brand new Advanced Virgo detector only two weeks after it officially started taking data," says Jo van den Brand of Nikhef and Vrije Universiteit Amsterdam, spokesperson of the Virgo collaboration. "That's a great reward after all the work done in the Advanced Virgo project to upgrade the instrument over the past six years."

When an event is detected by a three-detector network, the area in the sky likely to contain the source shrinks significantly, improving distance accuracy. The sky region for GW170814 has a size of only 60 square degrees, more than 10 times smaller than the size using data available from the two LIGO interferometers alone.

"Being able to identify a smaller search region is important, because many compact object mergers—for example those involving neutron stars—are expected to produce broadband electromagnetic emissions in addition to gravitational waves," says Georgia Tech's Laura Cadonati, deputy spokesperson for the LIGO Scientific Collaboration. "This precision pointing information enabled 25 partner facilities to perform follow-up observations based on the LIGO-Virgo detection, but no counterpart was identified—as expected for black holes."

"With this first joint detection by the Advanced LIGO and Virgo detectors, we have taken one step further into the gravitational-wave cosmos," says Caltech's David H. Reitze, executive director of the LIGO Laboratory. "Virgo brings a powerful new capability to detect and better locate gravitational-wave sources, one that will undoubtedly lead to exciting and unanticipated results in the future."

Explore further New gravitational wave data analysis now underway </s> In February of 2016, scientists at the Laser Interferometer Gravitational-wave Observatory (LIGO) announced the first-ever detection of gravitational waves (GWs). Since then, multiple events have been detected, providing insight into a cosmic phenomena that was predicted over a century ago by Einstein’s Theory of General Relativity.

A little over a year ago, LIGO was taken offline so that upgrades could be made to its instruments, which would allow for detections to take place “weekly or even more often.” After completing the upgrades on April 1st, the observatory went back online and performed as expected, detecting two probable gravitational wave events in the space of two weeks.

LIGO announced the first of the two new GW events on April 8th, which was followed by a second announcement on April 12th. The signals were detected thanks to the three-facility collaboration between LIGO and the Virgo Observatory in Italy, and both are believed to have been the result of a pair of black holes merging.

Thanks to upgrades made to both LIGO and Virgo, this scientific collaboration has been able to increase the sensitivity of its instruments by about 40%. For their third observation run (dubbed O3), the astronomical community also benefited from a new public alert system, where the LIGO team sends out alerts the moment detections are made so that observatories around the world can point their telescopes at the source.

By observing the source in different wavelengths (optical, X-ray, ultraviolet, radio, etc.), scientists hope to learn more about what causes GW events and about the dynamics behind them. For these latest detections, a team of scientists from Penn State University – led by Chad Hanna, an associate professor of physics, astronomy and astrophysics – played a vital role.

As Cody Messick, a graduate student in physics at Penn State and member of the LIGO team, explained:

“Penn State is part of a small team of LIGO scientists that analyze the data in almost real-time. We are constantly comparing the data to hundreds of thousands of different possible gravitational waves and upload any significant candidates to a database as soon as possible. Although there are several different teams all performing similar analyses, the analysis ran by the Penn State team uploaded the candidates that were made public for both of these detections.”

Over the past nine months, Messick has been responsible for ensuring that newly-uploaded GW candidates contain information from all detectors running at the time of detection. This helps astronomers localize signals by narrowing down the predicted area of the sky that the signal is predicted to have come from.

LIGO public alerts also include a sky-map that shows the possible location of the source in the sky, the time of the event, and what kind of event it is believed to be. LIGO has also said that in the future, announcements of candidate events will be followed by more detailed information once they have had a chance to properly vet and study them.

As Ryan Magee, a graduate student in physics at Penn State and member of the LIGO team, put it:

“These are near real-time detections of gravitational waves produced from two probable black holes colliding. We detected the first signal within about 20 seconds of its arrival to earth. We can set up automatic alerts to get phone calls and texts when a significant candidate is identified. I thought I was getting a spam phone call at first!”

So far, astronomers have deduced that GW events can be the result of binary black hole mergers, a merger between a black hole and a neutron star, or a binary neutron star merger. Each of these events produce gravitational waves with very different signals, which allows astronomers to determine the cause.

In this case, the events are believed to be the result of binary black hole mergers, which will be tested with follow-up observations in the coming weeks and months. Surabhi Sachdev, a Eberly Postdoctoral Research Fellow in physics at Penn State and member of the LIGO team, explained the importance of these latest events:

“This is the first LIGO observation that was made public right away in an automated fashion. This is the new LIGO policy starting with this observing run. Events are instantly made public automatically. After human vetting, a confirmation or retraction is issued within hours.”

With the increased sensitivity of their detectors, the LIGO team hopes to not only make more detections, but detect a greater variety of signals. So far, events have been detected that were the result of mergers between two black holes or neutron stars. It is hoped that in the near future, the team might detect a signal produced by the merger of a black hole and a neutron star.

Whatever form the next events take, you can expect that we will be hearing about it! The public can keep track of public alerts at https://gracedb.ligo.org/latest/, or you can download the alert app at Gravitational Wave Events iPhone App.
LIGO and Virgo observatories detect the gravitational wave signal from two merging black holes.