of Gravitational Waves from a Binary Black Hole Merger

Abbott, B.P., et al.
(2016). Observation of Gravitational Waves from a Binary Black Hole   Merger. Physical
Review Letters, 116, 1-16. doi:             https://doi.org/10.1103/PhysRevLett.116.061102


article was written by B.P. Abbott et al., a team of scientists from the LIGO
Scientific Collaboration and the Virgo Collaboration, was published on the 11th
of February 2016. The article reveals two astrophysical breakthroughs; the
existence of gravitational waves and binary black hole systems. The existence
of gravitational waves has been heavily debated in the scientific community
since Albert Einstein theorized about it in 1916. He discovered that gravity
had wave solutions but realized that the wave amplitude would be very small and
very difficult to detect. In the decades since then, a black hole was widely
accepted to be a solution to Einstein’s gravitational equations in his work on
general relativity. However, the existence of black holes was proven with
observations via electromagnetic radiation but not through gravitational waves.
In the 1990’s, developments in technology and numerical relativity allowed for
the computational modelling of a binary black hole merger and the gravitational
waves it’d produce. Subsequently, this spurred investment into building interferometers
across the globe to detect gravitational waves.


On the 14th of September 2015, the Laser
Interferometer Gravitational-Wave Observatory (LIGO) in Hanford, Washington and
Livingston, Louisiana detected the gravitational wave signal titled, GW150914.
The detections were made within 10ms of each other. It was consistent with the
time a gravitational wave would take, travelling at the speed of light, to
travel between the two detection sites. The waveforms detected were compared to
each other ensure that they were both caused by the same event and not
something local to each detection site. Both signals matched. The distinctive
characteristics of GW150914 led scientists to predict that it was caused by a
binary black hole merger. The orbital frequency of the waveform of 75 Hz, which
is half the gravitational wave frequency, led the team to deduce that the
stellar objects responsible for the signal needed to be very close, compact and
heavy. A binary neutron star system would not have the required mass to produce
GW150914. While a black hole-neutron star binary would have the necessary mass,
it would not have generated the same orbital frequency. Thus, a binary black
hole was hypothesized to be only possible cause. Reconstructed waveforms using computer
calculations of binary black hole mergers were compared to the signals generated
by GW150914, and they matched up to 99.9%. The black holes responsible for
GW150914 had a mass of 36 and 29 times that of our sun respectively. They’re located
approximately 1.4 billion light years away from our solar system. In addition,
the power radiated from GW150914 was about 50 times greater than the total
power radiated by light by all the stars in the observable universe.


            Despite solid evidence, the team scrutinized
the data with great rigour to assess the possibility of a secondary explanation
for GW150914. A statistical analysis of GW150914 was carried out and 16 days of
additional data between the 12th of September to the 20th
of October 2015 was analyzed. Two types of searches were conducted on this data,
– the generic transient search and the binary coalescence search – with each
yielding a string of possible causes for GW150914. The statistical significance
of each potential cause from both searches was assessed. The generic transient
search and the binary coalescence search had a calculated false alarm
probability of GW150914 being caused by a gravitational wave to be an event 1
in 8400 years and 1 in 203 000 years respectively. The scientists concluded
that with a statistical significance of 5.1s and a confidence level of 99.99994% that
GW150914 was caused by a gravitational wave from a binary black hole merger.
This marked the first time in history that a gravitational wave was recorded
and a binary black hole merger was observed.


            This research article, written by
the team of scientists at LIGO and Virgo, revolutionized an avant-garde branch
of physics – gravitational-wave astronomy. Prior to the detection of GW150914,
observations of the universe were limited by light emissions along the
electromagnetic spectrum. This paper pioneered a new field by demonstrating
that gravity can be used as a lens to decode the mysteries of the universe. This
is shown in the article as they scientist deduce the cause for GW150914 by
using the waveform detected. Their diligent work on understanding the signal and
conscientious assessment of secondary causes for the signal is clearly
articulated in the article. 


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