• Recently, LIGO Scientific Collaboration (LSC) has made the discovery of gravitational waves from a pair of neutron star-black hole (NS-BH) mergers.
  • The reverberations from these two objects were picked up using a global network of gravitational wave detectors, the most sensitive scientific instruments ever built.
  • Until now, the LIGO-Virgo Collaboration (LVC) was only able to observe collisions between pairs of black holes or neutron stars. The NS-BH merger is a hybrid collision.

Black Hole

  • A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space.
  • Gravitational waves are created when two black holes orbit each other and merge.
  • Neutron stars comprise one of the possible evolutionary end-points of high mass stars.
  • Once the core of the star has completely burned to iron, energy production stops and the core rapidly collapses, squeezing electrons and protons together to form neutrons and neutrinos.
  • A star supported by neutron degeneracy pressure is known as a ‘neutron star’, which may be seen as a pulsar if its magnetic field is favourably aligned with its spin axis.

Important points::

These are invisible ripples in space that form when:

  1. A star explodes in a supernova.
  2. Two big stars orbit each other.
  3. Two black holes merge.

Neutron star-Black hole (NS-BH) merges.

  • They travel at the speed of light (1,86,000 miles per second) and squeeze and stretch anything in their path.
  • As a gravitational wave travels through space-time, it causes it to stretch in one direction and compress in the other.
  • Any object that occupies that region of space-time also stretches and compresses as the wave passes over them, though very slightly, which can only be detected by specialized devices like LIGO.

Theory and Discovery:

  • These were proposed by Albert Einstein in his General Theory of Relativity, over a century ago.
  • However, the first gravitational wave was actually detected by LIGO only in 2015.
  • As the two compact and massive bodies orbit around each other, they come closer, and finally merge, due to the energy lost in the form of gravitational waves.
  • The Gravitational Waves signals are buried deep inside a lot of background noise. To search for the signals, scientists use a method called matched filtering.
  • In this method, various expected gravitational waveforms predicted by Einstein’s theory of relativity, are compared with the different chunks of data to produce a quantity that signifies how well the signal in the data (if any) matches with any one of the waveforms.
  • Whenever this match (in technical terms “signal-to-noise ratio” or SNR) is significant (larger than 8), an event is said to be detected.
  • Observing an event in multiple detectors separated by thousands of kilometers almost simultaneously gives scientists increased confidence that the signal is of astrophysical origin.

Importance of Discovery:

  • A neutron star has a surface and black hole does not. A neutron star is about 1.4-2 times the mass of the sun while the other black hole is much more massive. Widely unequal mergers have very interesting effects that can be detected.
  • Inferring from data as to how often they merge will also give us clues about their origin and how they were formed.
  • These observations help us understand the formation and relative abundance of such binaries.
  • Neutron stars are the densest objects in the Universe, so these findings can also help us understand the behaviour of matter at extreme densities.
  • Neutron stars are also the most precise ‘clocks’ in the Universe, if they emit extremely periodic pulses.
  • The discovery of pulsars going around Black Holes could help scientists probe effects under extreme gravity.

LIGO Scientific Collaboration (LSC):

  • LSC was founded in 1997 and currently made up of more than 1000 scientists from over 100 institutions and 18 countries worldwide.
  • It is a group of scientists focused on the direct detection of gravitational waves, using them to explore the fundamental physics of gravity, and developing the emerging field of gravitational wave science as a tool of astronomical discovery.
  • The LSC carries out the science of the LIGO Observatories, located in Hanford, Washington and Livingston, Louisiana as well as that of the GEO600 detector in Hannover, Germany.

LIGO-India Project

  • The LIGO-India observatory is scheduled for completion in 2024, and will be built in the Hingoli District of Maharashtra.
  • LIGO India is a planned advanced gravitational-wave observatory to be located in India as part of the worldwide network.
  • The LIGO project operates three gravitational-wave (GW) detectors.
  • Two are at Hanford in the State of Washington, north-western USA, and one is at Livingston in Louisiana, south-eastern USA.
  • The LIGO-India project is an international collaboration between the LIGO Laboratory and three lead institutions in the LIGO-India consortium: Institute of Plasma Research, Gandhinagar; IUCAA, Pune; and Raja Ramanna Centre for Advanced Technology, Indore.
  • It will significantly improve the sky localisation of these events.
  • This increases the chance of observation of these distant sources using electromagnetic telescopes, which will, in turn, give us a more precise measurement of how fast the universe is expanding.


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