The entrance to the LIGO-Hanford facility in Washington State. Image credit: David Dickinson

Apr 24, 2015 Advanced LIGO to up the game in the hunt for gravitational waves

Sen—A fascinating and unique continent-spanning ‘observatory’ is set to open a new window on the universe later this year, when Advanced LIGO begins it first science run in the hunt for gravitational waves.

I had a chance to get a behind the scenes tour of the LIGO-Hanford facility in Washington State in 2014, and have been researching and following the LIGO story and the search for gravitational waves since a visit to the LIGO-Livingston facility in Louisiana near Baton Rouge in 2010.

LIGO stands for the Laser Interferometer Gravitational-Wave Observatory. Both LIGO-Livingston and LIGO-Hanford are now achieving locks in their advanced mode at ever higher sensitivities, which will culminate with their first effort to operate simultaneously in their advanced configuration. Current test runs for the system have reached three times previous sensitivity, and the new upgrade should eventually extend the range of the detector by a factor of ten. 

Advanced LIGO will begin its first science run in the fall of 2015. The European Space Agency’s Laser Interferometer Space Antenna (LISA) Pathfinder spacecraft is also set to launch on Oct. 2, 2015 from Kourou, French Guiana, and Japan is also at work on a gravitational wave detector of their own, named KAGRA.

This could usher in a powerful new tool to probe the cosmos, as Advanced LIGO promises to open up a new window on the Universe with the first direct detection of gravitational waves.

Einstein’s general theory of relativity predicted the existence gravitational waves distorting space-time almost a century ago. LIGO initially achieved ‘first lock’ in the summer of 2002, and looks for gravitational waves generated by such extra-galactic cataclysmic events as binary pulsar or black hole mergers and supernovae.

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LIGO engineers in the heart of one of the detectors. Image credit: NSF/LIGO

The existence of gravitational waves was indirectly confirmed in 1974 by observing the orbital decay of binary pulsar PSR 1913+16, which earned astronomers Russell Hulse and Joseph Taylor the Nobel Prize in Physics in 1993. Resonant mass detectors—known as ‘Weber bars’ after physics pioneer Joseph Weber—were also employed in the early attempts to detect of gravitational waves in the 1960s and 1970s.

Along with binary pulsar and black hole mergers, the Big Bang itself is another predicted source of gravitational waves. Already, the non-detection by LIGO has placed a lower constraint on primordial gravitational waves generated by the Big Bang.

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The comparative sensitivities of aLIGO, LIGO and LISA. Image credit: Wikimedia Commons/C. Moore, R. Cole and C. Berry

Advanced LIGO employs two geographically separated L-shaped detectors based over 3,000 kilometers apart. Each detector employs a 200 watt laser, which sends a beam through the pair of two kilometer-long arm cavities. The beam is measured to a high degree of precession. LIGO must account for everything that can shake the detector, from earthquakes to local traffic. The amount of motion imparted by a passing gravitational wave on the beam is tiny, less than the diameter of a proton. By using two detectors, researchers can not only reject local interference, but they can also triangulate the position and source of the event that generated the passing gravitational waves in the sky.

Many of the lessons learned in the development of LIGO helped in the construction of Advanced LIGO, which features a greater sensitivity and enhanced ability to isolate out interference. Whereas LIGO had a range of 15 megaparsecs (1 Mpc equals one million parsecs), Advanced LIGO is expected to see gravitational wave events out to over 150 Mpc distant at design sensitivity limits. For context, the Andromeda spiral galaxy—the closest major galaxy to our own Milky Way—is 780 kiloparsecs distant.

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The expanded range of Advanced LIGO. Image credit: atlasoftheuniverse.com

Though LIGO never detected gravitational waves through its first runs, blind injections were periodically placed in the data to demonstrate the ability for LIGO to tease out a potential signal from the background noise.

“The L1 detector now is providing multi-hour lock stretches at a sensitivity that appears to be about a factor of three times better than the best performance of the previous version of LIGO,” Dale Ingram, the Education and Outreach Coordinator at LIGO Hanford told Sen. “Intense commissioning will continue throughout the spring and summer, and LIGO plans to launch a data run in the fall with the two detectors comparable in sensitivity.”

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LIGO Livingston Data Analysis and EPO Scientist Amber Stuver notes that the direct detection of gravitational waves by LIGO by 2016 would also come in time for the centennial of Einstein’s prediction of their existence.

“We periodically turn it (LIGO) back on to test our work and are continually setting records of how far out we can detect our standard source (a neutron star binary merger),” Stuver told Sen. “That means that even while we are tinkering, aLIGO [Advanced LIGO] is more sensitive to gravitational waves than ever before.”

Advanced LIGO may also provide the first direct measurement of relic gravitational waves from the Big Bang. The recent controversial findings from the BICEP2 team involved indirect detection. Advanced LIGO will require a large amount of observation time to tease these out, as they probably lie at the very edge of its detection capability.

“BICEP2’s research target doesn’t overlap strongly with LIGO detector operations,” Ingram told Sen. “LIGO seeks signals from the 10 Hz into the KHz range.”

As with the hunt for the Higgs-Boson, the non-detection of gravitational waves by advanced LIGO, though not as Earth-shattering a story, would be even more bewildering to scientists.

“This is not expected,” Stuver said, “but it would still be VERY exciting once we knew for sure that everything was fine with our instrumentation. If LIGO truly has a null result (we find nothing at all, or much less than we expect), the theoretical community gets to take this new information about the Universe and run with it.”

And this all comes as Europe’s VIRGO and GEO 600 gravitational wave detectors are on the hunt as well. LIGO was originally envisioned as three detectors, and there is a project planned using hardware from LIGO as an eventual third detector based in India.

And Advanced LIGO may eventually evolve into even more sensitive versions over the next decade.

LISA Pathfinder also takes the hunt for gravitational waves in to space later this year, and should lay the groundwork for ESA’s LISA space-based gravitational wave observatory to make its way to the launch pad by 2034.

Advanced LIGO and the hunt for gravitational waves represents a major human endeavor on the forefront of modern physics, on par with the CERN detection of the Higgs-Boson and the ongoing quest for dark matter. Keep an eye on the Advanced LIGO story, as it may just provide the next big breakthrough!

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An aerial view of LIGO-Hanford. Image credit: NSF/LIGO

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Looking down one of the enormous arms of LIGO-Hanford. Image credit: David Dickinson

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