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. </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> One of LIGO’s gravitational wave detectors Christian Offenberg / Alamy Stock Photo

LIGO is back at it. Having just turned back on after months of upgrades on 1 April, the Laser Interferometer Gravitational-Wave Observatory has already spotted another pair of black holes colliding.

Gravitational waves are ripples in space-time that occur whenever massive objects move, like the wake behind a boat travelling across a lake. LIGO announced the first-ever observations of gravitational waves in 2016 and has now spotted a total of 12 gravitational signatures of pairs of enormous objects smashing together.

Now that the twin detectors in Livingston, Louisiana, and Hanford, Washington, have been upgraded, we expect to see about one set of gravitational waves per week. And just one week after the detectors were turned back on, that expectation is already becoming reality with the detection of gravitational waves from a pair of merging black holes almost 5 billion light years away. LIGO made the find in collaboration with the Virgo gravitational wave observatory in Italy.

Advertisement

LIGO and Virgo haven’t released any information on the sizes of the two black holes. But based on the strength of the signal from so far away, they are probably relatively large like several of the earlier detections, perhaps around 30 times the mass of the sun, says astronomer Derek Fox at Pennsylvania State University. “Just being big and loud and right at the beginning of the run is exciting,” he says.

Now, these marvels of general relativity have become almost commonplace, so much so that you can sign up to be alerted to each new detection via an iPhone app. The point of the public alerts is to let astronomers around the world know where to point their telescopes for follow-up observations, says Fox.

That will come in handy during this new set of observations, which is expected to last a year or more. “This observing run is supposed to bring a real harvest of black holes and also probably a few neutron stars,” says Fox. “Some of us have been looking forward to this new era for a long time.”
LIGO and Virgo detect a gravitational wave from a galaxy 6 billion light-years away in the direction of Cassiopeia, with further details being released on April 10.