22 May 2020

Merger of black holes3 min read


Source: The Hindu

Manifest pedagogy: Black holes remain the mysterious forces in the universe. With the advancement in space science, man is able to understand different phenomena related to these black holes. This topic is important for upsc prelims as a new phenomenon is observed for the first time in binary black holes.

In news: The gravitational wave observatories at LIGO scientific collaboration have detected a merger of two unequal-mass black holes.

Placing it in syllabus: Space


  • What are Black Holes?
  • Recent detection by LIGO
  • About LIGO


What are Black Holes?

  • 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.
  • This can happen when a star is dying.
  • As no light can get out, they are invisible.
  • Space telescopes with special tools can help find black holes. 
  • Black holes can be big or small, like as small as just one atom or have mass up to 20 times more than the mass of the sun (stellar). 
  • Stellar black holes are made when the center of a very big star falls in upon itself, or collapses. When this happens, it causes a supernova.


  • Laser Interferometer Gravitational-wave Observatory (LIGO) is the world’s largest gravitational wave observatory.
  • It comprises two enormous laser interferometers operating in unison: one at the LIGO Livingston Observatory in Livingston, Louisiana and other at the LIGO Hanford Observatory, located near Richland, Washington, about 3000 kilometers apart.
  • It exploits the physical properties of light and of space itself to detect and understand the origins of gravitational waves (GW).
  • Three things distinguish LIGO from a stereotypical astronomical observatory: 
    • LIGO is blind, which means it does not see electromagnetic radiation as gravitational waves are not part of the electromagnetic spectrum. 
    • It is not round – Since LIGO doesn’t need to collect light from stars, it doesn’t need to be round or dish-shaped like optical telescope mirrors. It can also detect gravitational waves coming from any direction.
    • It cannot point at a particular part of the sky.


  • LIGO-India, or INDIGO, is a planned collaborative project between the LIGO Laboratory and the Indian Initiative in Gravitational-wave Observations (IndIGO) to create a gravitational-wave detector in India. 
  • It is a collaboration between LIGO Laboratory and the LIGO-India consortium: Institute of Plasma Research, Gandhinagar; IUCAA (Inter-University Centre for Astronomy and Astrophysics), Pune and Raja Ramanna Centre for Advanced Technology, Indore.
  • A site near Aundha Nagnath in the Hingoli district of Maharashtra has been selected.

Recent detection by LIGO:

  • For the first time since LIGO started functioning, the gravitational wave observatories have detected a merger of two unequal-mass black holes.

  • Nearly all the systems observed so far have comprised black holes of nearly equal masses.
  • This trend was broken by the observation of the merger on April 12, 2019.
  • The signal, named GW190412, was detected by the Advanced Virgo and the two Advanced LIGO detectors.
  • The black holes that merged had masses equal to 30 and 8 times the mass of the sun.
  • The actual merger took place at a distance of 2.5 billion light years away. 
  • Such mass differences are predicted by theory to produce subtle differences in the gravitational-wave signal.
  • These variations, which appear as higher “harmonics” in the waveform, have been observed for the first time in this event.

Merger of black holes – of equal and unequal masses:

  • As two binary black holes merged together, emission was at a frequency three times the orbital frequency. 
  • This emission is negligible when binaries contain equal masses.
  • The asymmetry in the masses made the feeble higher harmonic component better ‘heard’, leading to its unambiguous detection. 
  • The existence of higher harmonics was itself a prediction of General Relativity.
  • The spin of the more massive black hole can be determined from the extra features in the signal waveform. 
  • The spin of the heavier black hole plays a more prominent role in the dynamics of the binary.
  • These features make it possible to infer a more accurate determination of the distance from the event, the spin or angular momentum of the more massive black hole and the orientation of the whole event with respect to viewers on Earth.

Mould your thoughts: What is LIGO? How is the merger of two binary blackholes of varied masses different from that of binaries of same masses?

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