Observationof Gravitational Waves from a Binary Black Hole MergerAbbott, B.P., et al.

(2016). Observation of Gravitational Waves from a Binary Black Hole   Merger. PhysicalReview Letters, 116, 1-16. doi:             https://doi.org/10.1103/PhysRevLett.116.

061102             Thisarticle was written by B.P. Abbott et al., a team of scientists from the LIGOScientific Collaboration and the Virgo Collaboration, was published on the 11thof February 2016. The article reveals two astrophysical breakthroughs; theexistence of gravitational waves and binary black hole systems. The existenceof gravitational waves has been heavily debated in the scientific communitysince Albert Einstein theorized about it in 1916. He discovered that gravityhad wave solutions but realized that the wave amplitude would be very small andvery difficult to detect. In the decades since then, a black hole was widelyaccepted to be a solution to Einstein’s gravitational equations in his work ongeneral relativity.

However, the existence of black holes was proven withobservations via electromagnetic radiation but not through gravitational waves.In the 1990’s, developments in technology and numerical relativity allowed forthe computational modelling of a binary black hole merger and the gravitationalwaves it’d produce. Subsequently, this spurred investment into building interferometersacross the globe to detect gravitational waves.  On the 14th of September 2015, the LaserInterferometer Gravitational-Wave Observatory (LIGO) in Hanford, Washington andLivingston, Louisiana detected the gravitational wave signal titled, GW150914.

The detections were made within 10ms of each other. It was consistent with thetime a gravitational wave would take, travelling at the speed of light, totravel between the two detection sites. The waveforms detected were compared toeach other ensure that they were both caused by the same event and notsomething local to each detection site. Both signals matched. The distinctivecharacteristics of GW150914 led scientists to predict that it was caused by abinary black hole merger. The orbital frequency of the waveform of 75 Hz, whichis half the gravitational wave frequency, led the team to deduce that thestellar objects responsible for the signal needed to be very close, compact andheavy. A binary neutron star system would not have the required mass to produceGW150914.

While a black hole-neutron star binary would have the necessary mass,it would not have generated the same orbital frequency. Thus, a binary blackhole was hypothesized to be only possible cause. Reconstructed waveforms using computercalculations of binary black hole mergers were compared to the signals generatedby GW150914, and they matched up to 99.9%.

The black holes responsible forGW150914 had a mass of 36 and 29 times that of our sun respectively. They’re locatedapproximately 1.4 billion light years away from our solar system. In addition,the power radiated from GW150914 was about 50 times greater than the totalpower radiated by light by all the stars in the observable universe.

             Despite solid evidence, the team scrutinizedthe data with great rigour to assess the possibility of a secondary explanationfor GW150914. A statistical analysis of GW150914 was carried out and 16 days ofadditional data between the 12th of September to the 20thof October 2015 was analyzed. Two types of searches were conducted on this data,– the generic transient search and the binary coalescence search – with eachyielding a string of possible causes for GW150914. The statistical significanceof each potential cause from both searches was assessed.

The generic transientsearch and the binary coalescence search had a calculated false alarmprobability of GW150914 being caused by a gravitational wave to be an event 1in 8400 years and 1 in 203 000 years respectively. The scientists concludedthat with a statistical significance of 5.1s and a confidence level of 99.99994% thatGW150914 was caused by a gravitational wave from a binary black hole merger.This marked the first time in history that a gravitational wave was recordedand a binary black hole merger was observed.

             This research article, written bythe team of scientists at LIGO and Virgo, revolutionized an avant-garde branchof physics – gravitational-wave astronomy. Prior to the detection of GW150914,observations of the universe were limited by light emissions along theelectromagnetic spectrum. This paper pioneered a new field by demonstratingthat gravity can be used as a lens to decode the mysteries of the universe. Thisis shown in the article as they scientist deduce the cause for GW150914 byusing the waveform detected.

Their diligent work on understanding the signal andconscientious assessment of secondary causes for the signal is clearlyarticulated in the article. 

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