LIGO/Virgo collaboration: the search for gravitational waves

Sara Coelho on the computing challenges of the collaboration and how EGI is supporting their groundbreaking work

Gravitational waves are tiny ripples of energy that propagate through the Universe. Researchers believe that gravitational waves are emitted by spinning stars in binary systems, black holes or massive stellar explosions. Albert Einstein predicted their existence in his general theory of relativity. Almost 100 years after, the Virgo and LIGO Scientific Collaborations announced the first confirmed observation of gravitational waves. A second detection of a gravitational wave event, thought to be originated by the coalescence of two stellar-mass black holes, was announced in January 2017, followed by two more in August.

The observations paved the way to the Physics Nobel Prize awarded in 2017 to Rainer Weiss, Kip Thorne and Barry Barish for their role in the detection of gravitational waves.
But the detection of gravitational waves is not a three-man achievement.

The LIGO Scientific Collaboration is a group of more than 1000 scientists from universities around the United States and in other 14 other countries. The two LIGO detectors are located in Hanford and Livingston in the US. The Virgo Collaboration consists of more than 250 physicists and engineers affiliated with European institutions such as CNRS, INFN, Nikhef, Wigner RCP, the POLGRAW group and the European Gravitational Observatory near Pisa. The two collaborations share scientific data and computing resources, technologies, computing teams and publications.

Looking for the ripple effect

Gravitational wave detectors are based on the concept of a Michelson interferometer. The system takes in the infrared light of a laser and splits it in two beams that are injected in two long arms (3km in Virgo, 4 km in LIGO), disposed at 90 degrees of one another. The laser beams are recombined at the end of the arms and reflected toward a photo-detector. If a gravitational wave passes through, it generates a small interference that can be spotted by the detector. And “small” in this context means really-really small: the amplitude of the interference is in the order of  10-20m, the size of what is left of a proton if you divide it into ten million parts. Just the fact that it is possible to detect such a difference is a massive technical achievement.

What happens to the data?

The two LIGO detectors send their raw and reduced data to a central data repository at Caltech, and store a local copy for redundancy. From Caltech, the reduced data is distributed across the LIGO Data Grid, a network of large dedicated computing clusters run by LIGO, and also the Condor Clusters federated in Open Science Grid – one of the major e-Infrastructures collaborating with EGI – and HPC resources from XSEDE.

Virgo data is collected at the European Gravitational Observatory site, where Virgo detector is located, but its final repositories are the CCIN2P3 computing centre in Lyon and the INFN-CNAF computing centres in Bologna. The lion share of the data analysis is performed by dedicated LIGO Data Grid clusters. Parts of the analyses are submitted as computational grid jobs both in the US (to Open Science Grid) and in Europe (to EGI). Continuous wave analyses are run thorough EGI via the Virgo Virtual Organisation (VO) mainly at CNAF.

The Virgo VO consumed collectively 40 Million CPU hours in 2015 and 2016.

The Virgo detector is located in Italy, within the site of the European Gravitational Observatory.

Special thanks to Michele Punturo, Virgo Data Analysis Software Computing Coordinator, and to Peter Couvares, LIGO Data Analysis Computing Manager, for their help with writing this article.

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