Vasudevan Mukunth in The Wire:
The experiment that first directly detected gravitational waves in the spacetime continuum has repeated the feat, scientists announced at a meeting in San Diego on June 15. The achievement establishes the experiment, called LIGO, as the primary tool with which astrophysicists now observe the play of gravity around massive bodies in the universe. It also reposes faith in some of the sophisticated techniques developed by scientists to detect and study gravitational waves, and highlights the challenges in the road ahead.
“This finding confirms the fact that the first detection wasn’t an isolated event,” P. Ajith, leader of the astrophysical relativity group at the International Centre for Theoretical Sciences (ICTS), Bengaluru, told The Wire. “But more importantly, this is the beginning of serious gravitational astronomy.”
LIGO stands for Laser Interferometer Gravitational-wave Observatory. Its two identical detectors are located in Hanford, Washington, and Livingston, Louisiana. In the wee hours of December 26, 2015, they detected gravitational waves originating from a pair of black holes that were about to merge, about 1.3 billion lightyears away. Before the merger, in the inspiral phase, the black holes rapidly spiral in tight orbits around each other, their acceleration sending away ripples of gravitational energy that alternatively contract and expand spacetime (by minuscule amounts) as they move through it – much as a wave passing through a sheet of cloth would.