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OSG helps LIGO confirm Einstein’s theory

Speed read
  • Gravitational waves, predicted by Einstein in 1916, finally detected by NSF-funded Laser Interferometer Gravitational-Wave Observatory (LIGO).
  • Open Science Grid (OSG) resources play a key role in LIGO analysis.
  • LIGO taps Extreme Science and Engineering Discovery Environment (XSEDE) supercomputers Comet and Stampede. 

Just over a century ago, Albert Einstein posed the idea of gravitational waves in his general theory of relativity. Long predicted, the power of the Open Science Grid (OSG) is being used by scientists with the LIGO Scientific Collaboration (LSC) to finally observe these ripples in the fabric of spacetime.

LIGO makes history. On September 14, 2015, Laser Interferometer Gravitational-Wave Observatory (LIGO) observed ripples in the fabric of spacetime. This video narrative tells the story of the science behind that important detection. Courtesy Caltech.

Led by support from the US National Science Foundation (NSF), the Laser Interferometer Gravitational-Wave Observatory (LIGO) consists of two installations within the United States — one in Hanford, Washington and the other in Livingston, Louisiana. By triangulating the two detectors with a point in deep space, researchers can use temporal differences to determine the source of a gravitational wave.

LIGO's first data run of its advanced gravitational wave detector began in September 2015 and ran through January 12, 2016. Both detectors heard gravitational waves for the first time on September 14, 2015. OSG and Extreme Science and Engineering Discovery Environment (XSEDE) -based computing resources are helping ensure LIGO isn't deaf to these signals.

<strong>Bowling for Einstein.</strong> Gravitational waves are caused when a massive object distorts spacetime around it like a bowling ball dropped on a trampoline. If a marble were circling around the bowling ball on the dimpled trampoline, it would fall inward, toward the bowling ball, like a rock in space circling a planet. Courtesy NASA.

The waves LIGO detected originated when two supermassive black holes collided far off in the distant universe. To picture this scenario, imagine the two black holes as two very large bowling balls moving at near light speed in ever-tighter circles around one another on a very thick trampoline.

As these two objects whip up the stellar stew around them, they eventually collide and explode with tremendous force (3 solar masses worth, say LIGO scientists.)

The thick trampoline in this metaphor is analagous to spacetime: Indented with the mass of the objects, and drawing nearby objects closer and closer. The shock waves from the collision rippled out and were finally detected by LIGO when they touched our little blue marble about 1.3 billion years later.

LIGO uses fixed amounts of time on supercomputers Comet at the San Diego Supercomputer Center (SDSC) and Stampede at the Texas Advanced Computing Center (TACC) via NSF-funded eXtreme Digital allocations. While Stampede looks and behaves very much like a traditional supercomputer resource, Comet has a new virtualization-based interface that eliminates the need to submit to a batch system. OSG provides this through a virtual machine (VM) image, and then LIGO simply uses the OSG environment.

Long ago, in a galaxy far away . . . A computer simulation shows the collision of two black holes, a tremendously powerful event detected for the first time ever by LIGO. This simulation was created by the multi-university SXS (Simulating eXtreme Spacetimes) project. Courtesy SXS.

“Normally our computing is done on small clusters at LIGO and our partners,” said Peter Couvares, data analysis computing manager for the Advanced LIGO project at Caltech, “but OSG allows more flexibility as we add in systems that aren't part of our traditional systems. The combination of OSG and the LIGO data grid is very powerful.”

Thus far, LIGO has consumed almost four million hours on OSG — 628,602 hours were on Comet and 430,960 on Stampede resources. OSG's Brian Bockelman  of the University of Nebraska-Lincoln and Edgar Fajardo from the SDSC used HTCondor software to help LIGO implement their Pegasus workflow on 16 clusters at universities and national labs across the US.

During a data analysis run on the OSG, the LIGO team performs modeled searches to look for events like the merger of neutron binary stars or black holes. This type of search is computationally expensive since it requires a search of 100,000 different models. The capacity to split up many of the search filters across their grid is where OSG shines.

“The parallel nature of the OSG is what's valuable,” said Couvares. “It is well suited to a high throughput environment. LIGO will always need dedicated in-house computing for low latency searches that need to be done quickly, but now we have the potential elasticity of OSG.”

Einstein never lived to see the vindication of his theories. Thanks to the OSG and XSEDE supercomputing resources provided by the NSF, we live in a time where we can validate theories that at times even led a genius like Einstein to question himself.

Read more about OSG's involvement in the LIGO discovery here.

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