The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale observatory to detect cosmic gravitational waves and develop gravitational-wave observations as an astronomical tool. LIGO’s original gravitational wave detectors (called Initial LIGO or iLIGO) were constructed as early as 1999. However, a need was felt to completely redesign LIGO’s instruments. The redesigning was done and the advanced version began to function in 2015.
Two large observatories were set up in the United States: at Livingston in Louisiana and Hanford in Washington. But with time, a need for more observatories was recognised. The European Variability of solar Irradiance and Gravity Oscillations (VIRGO) detector (located outside Pisa in Italy) came up in 2017. But the huge uncertainty in determining where in the sky the disturbance came from led to the idea that if observations can be made from another detector in a far-off position, it would help locate the source of the gravitational waves more accurately. So, a Japanese detector, KAGRA, or Kamioka gravitational-wave detector, located underground in the Kamioka mine in Kamioka-cho, Hida-city, Gifu-prefecture of Japan, was soon developed and joined the international network. In a collaboration with LIGO, a gravitational-wave detector is also being set up in India. In the meanwhile, the LIGO-India Scientific Collaboration (LISC) researchers continue to contribute in different areas of the LIGO-Virgo-KAGRA (LVK) collaboration efforts.
The initial LIGO observatories were funded by the National Science Foundation (NSF) of the US and were conceived and built by Caltech and MIT as well as operated by them.
How LIGO Works
Gravitational-wave detectors use high power lasers to carefully measure the time taken for light to travel between mirrors along two perpendicular arms. LIGO consists of a pair of huge interferometers, each having two arms which are 4 km long. (The KAGRA is a bit smaller than LIGO, as it has 3-km long arms, but with some newer technology, such as cryogenically cooled mirrors, its ability to detect gravitational waves is greatly improved.)
A lot of precision is needed as gravitational-wave detectors detect signals which are very faint (the gravitational wave): all external noise is weeded out as it may drown out the signal. The two LIGO detectors work as one unit to detect the signals. A single LIGO detector cannot confidently detect this disturbance on its own, as the signal is very weak. A random noise could give out a signal and it can easily mislead one into thinking that a genuine gravitational wave has been detected. When both the detectors detect the faint signal, there is less chance of the signal being a false one.
LIGO is not for seeing the incoming ripples in space-time, as in telescopes. This is because gravitational waves are not a part of the electromagnetic spectrum or light. They are not light waves but explained as a stretching of space-time due to immense gravity.
Gravitational waves are ‘ripples’ in space-time caused by some of the most violent and energetic processes in the universe. According to the general theory of relativity, massive accelerating objects (such as neutron stars or black holes orbiting each other) would disrupt space-time in such a way that ‘waves’ of distorted space would radiate from the source (like the movement of waves away from a stone thrown into a pond). When objects with such an immense gravity merge, the disturbance travels outwards from the place of merger, like the ripples on the surface of a water body. It is, therefore, that gravitational waves have been described as “ripples in the fabric of space time”. These ripples would travel at the speed of light through the universe, carrying with them information about their cataclysmic origins, as well as clues to the nature of gravity itself.
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 even remnants of gravitational radiation emanated when the universe was created. So, these objects or phenomena are seen as the sources for capturing and recording gravitational waves. The significance of studying the waves is because they are a largely unknown as well as fundamental phenomena. With many more detectors, it may be possible to map out the universe, using gravitational-wave astronomy. Accurate detection facilities in the future may help us detect and map signatures of gravitational waves bouncing off celestial objects.
Discoveries by LIGO
In the data collected from 2002 to 2010, no gravitational waves were detected but, with the Advanced LIGO Project in place, the improved detectors starting operations in 2015.
On September 14, 2015, the two LIGO detectors in the US registered a small disturbance which was recorded as a result of gravitational waves travelling outwards from a point 1.3 billion light years away from Earth. At the time, two massive black holes with masses 29 and 36 times that of the Sun are believed to have merged to give off gravitational-wave disturbances.
The detection led to the award of the 2017 Nobel Prize for Physics to Rainer Weiss, Kip Thorne, and Barry C. Barish “for decisive contributions to the LIGO detector and the observation of gravitational waves”.
The detection of gravitational waves was reported in 2016 by the LIGO Scientific Collaboration (LSC) and the Virgo Collaboration with the international participation of scientists from several universities and research institutions. Scientists involved in the project and analysis of the data for gravitational-wave astronomy are organised by the LSC.
Following the 2015 event, the two LIGO detectors in the USA detected another seven such binary black hole merger events.
By December 2018, LIGO had made more detections of gravitational waves, most of which were from binary black hole mergers. LIGO detected a collision of two neutron stars for the first time on August 17, 2017, which simultaneously produced optical signals detectable by conventional telescopes.
Several gravitational waves have since been detected. The newly released (October 2020) catalogue of gravitational waves consists of 90 events, 35 of which were discovered between November 2019 and March 2020 by astrophysicists from the LVK collaboration. Of the 35 events detected, 32 were most likely to be black hole mergers, and 3 were considered to be collisions between neutron stars and black holes (though some scientists think one of these could have been a merger of a heavy black hole and a light black hole) . The black holes are of different sizes, the most massive of them being around 90 times the mass of our Sun. Many of the black holes that formed from the mergers exceed 100 solar masses and are classed as intermediate-mass black holes. Indian researchers from the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, also contributed in various aspects of the cosmological analysis.
It is reported that the LIGO and Virgo instruments are being upgraded in preparation for the next observing run O4 (the fourth since the Advanced LIGO instrument came online in 2015) will begin in March 2023. Originally planned to begin in December 2022, unanticipated delays in finishing upgrades and commissioning the instruments caused this delay. The O4 which will also include the KAGRA detector.
With all of these new signals, it would be possible to better understand the black holes and neutron stars that exist in the greater universe. The many signals in the updated catalogue could also be used to further test Einstein’s theory of gravity or general relativity.
Gravitational waves have been established as a powerful tool to observe the sudden and violent merger events in the universe, which cannot otherwise be detected with even the most powerful telescopes. These black holes have been theorised by astrophysicists for a long time, but the recent observations by LIGO confirm that they exist.
The masses of black holes and neutron stars are of great importance in understanding the life of massive stars and their death in supernova explosions.
LIGO India
LIGO India is a planned advanced gravitational-wave observatory that will come up at Dudhala village in Hingoli district of Maharashtra. This project received in-principle approval from the government in 2016 and has been approved by the Government of India in April 2023 as a part of a worldwide network, including Virgo in Italy and KAGRA in Japan. The government would spend around US$ 320 million on this project, which is expected to complete by 2030. In November 2021, the Hingoli revenue department is reported to have handed over the land required for the project to the authorities concerned. LIGO-India is envisaged as a collaborative project between a consortium of Indian research institutions and the LIGO Laboratory in the USA, along with its international partners. The three institutes in India that will collaborate with the LIGO Laboratory in the US are: the Raja Ramanna Centre for Advanced Technology (RRCAT) in Indore, the Institute for Plasma Research (IPR) in Gandhinagar, and the Inter-University Centre for Astronomy and Astrophysics (IUCAA) in Pune. The first observations are expected by the end of this decade.
The LIGO-India project will be built by the Department of Atomic Energy (DAE) and the Department of Science and Technology (DST), Government of India, with a Memorandum of Understanding (MoU) with the National Science Foundation (NSF) of the USA, along with several national and international research and academic institutions. The LIGO-India facilities are to be funded by DAE and the DST, with DAE acting as the lead agency.
The LIGO detector at LIGO India will also have two arms each 4 km in length. However, the ultra-high precision large-scale apparatus is expected to show a unique ‘temperament’ determined by the local site characteristics.
Significance of LIGO India With LIGO India working along with the facilities in the US and Europe, scientists will be able to locate sources of gravitational waves over the entire sky. LIGO India will help pinpoint the source of gravitational waves much more accurately than current efforts allow; it will help in accurate calculations of sizes of black holes; and it will help to better understand the universe’s rate of expansion. There will be benefits to India’s astrophysics research, high end technological development, and human resource development in general. With most of the components being made in India, the technological expertise of Indian scientists and engineers is expected to improve.
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