The gravitational-wave event GW-150914 observed by the LIGO (source)
The first ever direct detection of gravitational waves was announced on 11th of February. Gravitational waves are ripples in space-time that occur under certain circumstances, for example when two very compact celestial objects, like neutron stars or black holes merge. The detection was made, by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), about a century after Albert Einstein predicted their existence. At the time of detection the VIRGO detector, also constructed to detect gravitational waves was being upgraded, whereas the GEO 600, was not in observational mode.
LIGO is comprised of four distinct facilities across the United States: two gravitational wave detectors (the interferometers) and two university research centers. The interferometers are located in isolated areas of Washington and Louisiana, separated by about 3,000Km. A gravitational wave travelling through space causes distance to shrink and stretch. The detectors use laser light as a stopwatch to measure this effect. If one of the two interferometers is stretched or shrunk, the beam of light will travel longer or shorter, respectively, compared to the beam of light in the other interferometer, thus creating a signal. The strength of the signal is measured by a dimensionless quantity known as strain, which quantifies the change in the length of the interferometer divided by its initial length.
According to the scientists that made the detection the two compact objects that merged and created the signal, couldn’t be a pair of neutron stars as they don’t have the required mass, nor a black hole neutron star binary, since in this case mass would be too large, and the frequency would be much lower than that of the detected signal. Therefore, the detected signal has been generated by the merging of two black holes. Based on their estimations, one of the black holes was about 36 times the mass of the Sun and the other was about 29 solar masses. The merged black hole weighs 62 solar masses and the merger took place between 600 million and 1.8 billion years ago. This is the first robust confirmation that binary black holes merge. The detection of gravitational waves also provides confirmation that heavy stellar mass black holes exist (with masses larger than 25 solar masses) and that binary black holes form in nature.
As Mansi Kasliwal, an astronomer at Caltech, said: “It’s the very real dawn of a new era”.
Sources: Nature, American Physical Society, astrobites
Video: Nature
Publications: Abbott et al. 2016a, Abbott et al. 2016b