False alarm raises integrity of LIGO report

Since beginning operations in 2002, the more than 800 scientists working on the Laser Interferometer Gravitational-Wave Observatory have been anxious to observe tiny variations in interferometric light signals detected from space that could be indicative of gravitational waves—ripples or fluctuations in the space-time continuum predicted by Einstein’s general theory of relativity that are thought to be emitted by neutron stars, black holes, or supernovae.

Since beginning operations in 2002, the more than 800 scientists working on the Laser Interferometer Gravitational-Wave Observatory (LIGO; www.ligo.org) have been anxious to observe tiny variations in interferometric light signals detected from space that could be indicative of gravitational waves—ripples or fluctuations in the space-time continuum predicted by Einstein’s general theory of relativity that are thought to be emitted by neutron stars, black holes, or supernovae.

On Sept. 16, 2010, both the LIGO interferometers in Washington and Louisiana, as well as the Italian-French Virgo interferometer (www.virgo.infn.it), picked up a signal dubbed the “Big Dog” from the constellation Canis Major that had all the earmarks of a neutron star orbiting a black hole. The signal sweeps upward in frequency (chirp) as the stars spiral into one another, approaching merger. The loudness of the signal was consistent with one coming from a galaxy 60 to 180 million light years from ours. The LIGO scientists sent alerts to partners operating robotic optical telescopes in the southern hemisphere (ROTSE, TAROT, Skymapper, Zadko) and the Swift x-ray space telescope, all of which took images of the sky on subsequent days in the hope of capturing an optical or x-ray “afterglow.”

But nearly six months later, at a joint LIGO-Virgo meeting on Mar. 14, 2011, it was revealed by LIGO top management that the signal observed was a false alarm—a blind injection designed to specifically test the detection capabilities of LIGO and Virgo. The project scientists knew that the signal could be a blind injection from the beginning, but they also knew to act like it was the real thing. Although the Big Dog was not a real event, the ability of all three interferometers to detect the tiny signal raises the prospects for detection of a “real” event in the near future. Advanced LIGO detectors now under construction will be operating shortly after 2015 and—when achieving their designed sensitivity—are expected to detect tens of signals per year.

Contact Gabriela Gonzalez atgonzalez@lsu.edu.

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