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First detection: GW150914

GW150914 waveform

The processed waveform of GW150914. Credit: LIGO Scientific Collaboration Link to high-resolution image.

On at 9:51 GMT on 14th September 2015, the two LIGO detectors picked up the first ever direct signature of gravitational waves, ripples in the fabric of spacetime originating 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 gravitational waves were produced during the final fraction of a second of the merger of two black holes to produce a single, more massive, spinning black hole. Collisions of two black holes in this way had been predicted but never observed.

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 Hemisphere.

According to general relativity, a pair of black holes orbiting around each other lose energy through the emission of gravitational waves, causing them to gradually approach each other over billions of years, and then much more quickly in the final minutes. During the final fraction of a second, the two black holes collide into each other at nearly half the speed of light to form a single more massive black hole, converting a portion of the combined black holes’ mass to energy following Einstein’s famous formula E=mc2. This energy is emitted as a final, strong burst of gravitational waves.

GW150914 localization

Possible location of GW150914. Credit: LIGO/Roy Williams & CDS Strasbourg. Link to high-resolution image.

Detailed models of the merging of black holes have been developed by Cardiff scientists, using detailed numerical relativity simulations of Einstein's theory of General Relativity. Using these models, the LIGO team estimates that the black holes for this event were about 29 and 36 times the mass of the Sun. Energy equivalent to around 3 times the mass of the Sun was converted into gravitational waves in a fraction of a second, with a peak power output greater than all the stars in the observable Universe. It is these gravitational waves that LIGO detected.

The discovery, accepted for publication in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration (which includes the GEO Collaboration and the Australian Consortium for Interferometric Gravitational Astronomy) and the Virgo Collaboration using data from the two LIGO detectors. For data relating to GW150914, please see the LIGO Open Science Centre.

The search for events in the LIGO data requires sophisticated search algorithms and considerable computing power. Cardiff scientists played a key role in the development of the algorthims that search for the merging of compact objects, particularly black hole binaries. Cardiff University, funded by the Science and Technology Facilities Council, also provides some of the computational resources used in the searches.