First scientific results from LIGO

First scientific results from LIGO (the Laser Interferometer Gravitational-Wave Observatory). Essentially a giant strain gauge to measure the local distortion in spacetime of a passing gravitational wave, LIGO has detectors in Hanford, Washington, and Livingston, Louisiana. (For more on LIGO’s operation, see Physics Today, October 1999, page 44 .) The ripples in spacetime radiated, for example, by the collapsing inspiral of two neutron stars are predicted to produce a strain in LIGO of perhaps one part in 10²⁰, which would change the distance between mirrors some 4 km apart by about 10⁻¹⁸ meters, a displacement 1000 times smaller than a proton. At the April APS meeting, the LIGO team reported its first official results from the initial science run, conducted over 17 days in the late summer of 2002. Gary Sanders (Caltech) and Erik Katsavounidis (MIT) reported that, as expected, no gravitational-wave events were seen, but new upper limits were set on three of the four prime source categories. For coalescing binaries, no more than 164 events per year are expected from the Milky Way or an equivalent galaxy. The limit for both known and unknown burst sources, at a strain of 10⁻¹⁷, is 1.4 events per day. Using one pulsar as a test case, LIGO has demonstrated sensitivity to pulsar sources at a strain amplitude of 10⁻²², within a factor of 100 of that expected from the Crab pulsar. Finally, LIGO’s limit on stochastic waves that could have arisen in the early universe—expressed as their contribution to the energy density needed to close the universe—has Ω_GW less than 72.4, a bit higher than the current best limit of 60. All of these limits are expected to improve dramatically after data from the recently concluded second science run, with its tenfold increase in sensitivity, are analyzed.