Another day, another detection of gravitational waves. Although it may seem like they're becoming mundane, it's worth remembering that observing these ripples in the very fabric of spacetime that are created by massive cataclysms in the very distant past, is one of the most important scientific finds in a century. The Laser Interferometer Gravitational Wave Observatory (LIGO) has just detected gravitational waves for the fourth time, but it wasn't alone this time: the signals were also measured by the Virgo detector in Italy, marking a new milestone in the observation of the Universe.

This latest event was observed on August 14 at 10:30:43 am UTC, and it was caused by a collision between two huge black holes about 1.8 billion light-years away. The two black holes involved in the mammoth merger had masses about 31 times and 25 times larger than the Sun, and the end result was a spinning black hole with about 53 times the mass of the Sun. That means about three solar masses were converted into the energy seen in the ripples.

Gravitational waves were first predicted by Albert Einstein over 100 years ago, but it took until September 2015 for them to be directly observed. LIGO went on to detect the phenomenon again in December that same year, and then again in January 2017.

Those events were all picked up by the twin LIGO detectors in Louisiana and Washington. Each of those facilities reflects laser beams down two 4-km (2.5 mi)-long tunnels, and by measuring the light as it exits, scientists can measure physical distortions as tiny as a fraction of a proton. This newest detection was backed up by the Virgo facility near Pisa, Italy, just two weeks after it joined the current observing run.

"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," says France Córdova, Director of the National Science Foundation. "This is an exciting milestone in the growing international scientific effort to unlock the extraordinary mysteries of our universe."

With Virgo joining the search from the other side of the planet, scientists can triangulate the source of the signals more precisely. The three-detector network shrinks 10-fold the section of sky that the waves originated from, allowing researchers to narrow down the search.

"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 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."

The current observing run ended on August 25, but the next is slated to begin in mid-2018. During that time, the scientists expect to be able to detect gravitational waves on a weekly basis.

"With this first joint detection by the Advanced LIGO and Virgo detectors, we have taken one step further into the gravitational-wave cosmos," says 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."

Source: National Science Foundation

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