Free Astronomy Magazine March-April 2016

COSMOLOGY MARCH-APRIL 2016 5 LIGO opens new window on the universe with observa- tion of gravitational waves from colliding black holes. ONAL F or the first time, scientists have observed ripples in the fabric of spacetime called grav- itational waves, arriving at the Earth from a cataclysmic event in the distant universe. This confirms a major prediction of Albert Einstein’s 1915 general theory of relativity and opens an unprecedented new window onto the cosmos. Gravitational waves carry information about their dramatic origins and about the nature of gravity that cannot otherwise be obtained. Physicists have concluded that the detected grav- itational waves were produced during the final fraction of a second of the merger of two black holes to produce a single, more massive spin- ning black hole. This collision of two black holes had been predicted but never observed. The gravitational waves were detected on Sep- tember 14, 2015 at 5:51 a.m. Eastern Daylight Time (09:51 UTC) by both of the twin Laser In- terferometer Gravitational-wave Observatory (LIGO) detectors, located in Livingston, Louisi- ana, and Hanford, Washington, USA. The LIGO Observatories are funded by the Na- tional Science Foundation (NSF), and were con- ceived, built, and are operated by Caltech and MIT. The discovery, accepted for publication in the journal Physical Review Letters , was made by the LIGO Scientific Collaboration (which in- cludes the GEO Collaboration and the Austra- lian Consortium for Interferometric Gravita- tional Astronomy) and the Virgo Collaboration using data from the two LIGO detectors. Based on the observed signals, LIGO scientists estimate that the black holes for this event were about 29 and 36 times the mass of the Sun, and the event took place 1.3 billion years ago. About 3 times the mass of the Sun was converted into gravitational waves in a fraction of a second— with a peak power output about 50 times that of the whole visible universe. By looking at the time of arrival of the signals—the detector in Livingston recorded the event 7 milliseconds before the detector in Hanford—scientists can say that the source was located in the Southern