New Source of Gravitational Waves Discovered

• 02 Jul 2021

Why in News

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

• 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.

Key Points:

• About the Gravitational Waves:
  
  • These are invisible ripples in space that form when:
    
    • A star explodes in a supernova.
    • Two big stars orbit each other.
    • 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.
• Detection Technique:
  
  • 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.
  • LIGO Observatories: 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.
  • Other Observatories:
    
    • VIRGO: Virgo is located near Pisa in Italy. The Virgo Collaboration is currently composed of approximately 650 members from 119 institutions in 14 different countries including Belgium, France, Germany, Hungary, Italy, the Netherlands, Poland, and Spain.
    • The Kamioka Gravitational Wave Detector (KAGRA): The KAGRA detector is located in Kamioka, Gifu, Japan. The host institute is the Institute of Cosmic Ray Researches (ICRR) at the University of Tokyo.
      
      • This interferometer is underground and uses cryogenic mirrors. It has 3 km arms.

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.

Source: IE