Scientists have, for the first time, detected gravitational waves ripples in the fabric of space-time produced by the collision of a neutron star and a black hole (NS-BH). On January 5, 2021, the Advanced LIGO detector in Louisiana, US, and the Advanced Virgo detector in Italy, picked up the inward spiral movement of a neutron star that met its end when a black hole swallowed it whole. It was also detected that when the neutron star collided and entered into the black hole, gravitational ripples spread outward through the cosmos, reaching Earth. After some days, a second signal was picked up by both detectors coming from the final orbits and smashing together of another neutron star and black hole pair. Previous gravitational wave detections have spotted black holes colliding, and neutron stars merging but not one of each. This discovery was published in the Astrophysical Journal Letters on June 29, 2021.
Gravitational Waves
Gravitational waves are produced when celestial objects collide, and the ensuing energy creates ripples in the fabric of space-time which carry all the way to detectors on Earth. Albert Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity. In 2015, LIGO physically sensed the distortions in space-time caused by passing gravitational waves, generated by two colliding black holes nearly 1.3 billion light years away. The strongest gravitational waves are produced by catastrophic events such as colliding black holes, the collapse of stellar cores (supernovae), coalescing neutron stars or white dwarf stars, the slightly wobbly rotation of neutron stars that are not perfect spheres, and possibly due to the remnants of gravitational radiation created by the birth of the Universe.
Salient Features of the Discovery
Some of the salient features of the discovery are as follows:
- These detections confirm that there are populations of binary systems consisting of a neutron star and a black hole.
- These astrophysical systems can help answer many big questions about the universe, from star formation and stellar evolution to the expansion fate of our Universe.
- Until now, the LIGO-Virgo collaboration (LVC) of gravitational waves detectors have only been able to observe collisions between pairs of black holes or neutron stars.
- Gravitational waves travel with the speed of light. It means that the scientists observed mergers that happened about 1 billion years ago well before life appeared on Earth!
Laser Interferometer Gravitational-wave Observatory (LIGO)
World’s largest gravitational wave observatory, LIGO is a marvel of engineering. It consists of two enormous laser interferometers, located thousands of kilometres apart. LIGO exploits the physical properties of light and of space itself to detect and understand the origins of gravitational waves.
Variability of Solar IRradiance and Gravity Oscillations (VIRGO)
Virgo is a giant laser interferometer, designed to detect gravitational waves. It has been designed and built by collaboration of the French Centre National de la Recherche Scientifique (CNRS) and the Italian Istituto Nazionale di Fisica Nucleare (INFN).
Laser Interferometer Gravitational-wave Observatory, India (LIGO-India)
LIGO-India is a collaboration between the LIGO Laboratory (operated by Caltech and MIT) and three Indian institutes—the Raja Ramanna Centre for Advanced Technology (RRCAT), Indore; the Institute for Plasma Research (IPR), Ahmedabad; and the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune. LIGO-India is a planned advanced gravitational-wave observatory to be in India as part of the worldwide network, whose concept proposal is now under active consideration in India and the USA.
Significance of the Discovery
This discovery of gravitational waves will open windows to many interesting facts. For instance, it was found that a neutron star has a surface, but a black hole does not have a surface. A neutron star is about 1.4–2 times the mass of the sun while the 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. 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. The discovery helps us understand the formation and relative abundance of such binaries. These findings may enable us to understand the behaviour of matter at extreme densities because Neutron stars are the densest objects in the universe. Moreover, these stars are also the most precise ‘clocks’ in the universe, provided they emit extremely periodic pulses. The discovery of pulsars going around black holes may prove helpful for scientists in probing effects under extreme gravity.
Neutron Star and Black Hole Collision
According to Director of Inter-University Centre for Astronomy and Astrophysics (IUCAA), Professor Somak Raychaudhury, this development was expected for a long time but was not confirmed. So, a new analysis was done to reconfirm this discovery which has now been published in the international journal.
Dr Shasvath Kapadia from the International Centre for Theoretical Sciences (ICTS) in Bengaluru says, “Inferring from data as to how often they merge will also give us clues about their origin and how they were formed.” He helped with the estimation of the NS-BH merger rate, using a method he co-developed.
Technique Used
The technique, used to detect the signal, is called ‘matched filtering,’ developed at IUCAA in the 1990s by Sanjeev Dhurandhar and others. This was also used for the first discovery of gravitational, waves. In this technique, 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 kilometres, almost simultaneously gives scientists increased confidence that the signal is of astrophysical origin, which is the case for both events.
Scientists at the Laser Interferometer Gravitational-wave Observatory, India (LIGO-India) say that it is a hybrid collision since the scientists have been primarily detecting binary black hole mergers and binary neutron star mergers (until now).
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