More than a hundred years ago, in 1905, Henri Poincaré, a French theoretical physicist, first proposed the concept of gravitational waves. Later, in 1916, Albert Einstein predicted gravitational waves based on his general theory of relativity.
According to the theory, gravitation is not bodies attracting each other from a distance, as Newton proposed, but local deformation of the space-time continuum. The theory predicts that a system can lose energy by emitting gravitational waves in about the same way as a system of moving electrical charges emits electromagnetic waves.
However, for several decades, there was no evidence that gravitational waves exist.
Then, in 1974, Russell Hulse and Joseph Taylor Jr were searching for pulsars using the 300-m radio-telescope at the Arecibo observatory, Puerto Rico. They discovered a different type of pulsar – a highly magnetized rotating compact star emitting beams of electromagnetic radiation out of its magnetic poles. It was accompanied by an equally heavy companion. Hulse and Taylor were excited to see that the orbital period of the twin astronomical bodies was decreasing, losing energy through gravitational radiation, as predicted by Einstein’s general theory of relativity. That discovery won them a Nobel prize in 1993.
This discovery excited astrophysicists. The US and some European countries started exploring the possibility of detecting gravitational waves. That lead to a development of a large scientific instrument, the Laser Interferometer Gravitational-Wave Observatory or LIGO.
LIGO is an L-shaped vacuum tube with a beam splitter at the junction. Each arm is about a few kilometres long and has mirrors at the end. A laser beam that bounces off the mirror at one end splits into two parts at the junction. The beam returning from the other mirror interferes with the split part of the first reflected beam. The interference pattern that is formed changes when a gravitational wave passes through.
The changes in the pattern can help us detect gravitation waves. But, to locate the source, we need at least two such detectors. With more detectors, the accuracy improves.
LIGO started with two large observatories – one in Hanford, about 250 km southeast of Washington and the other in Livingston, Louisiana. Later, another large interferometer, the Virgo interferometer was set up near Pisa in Italy. Since 2007, Virgo and LIGO collaborate to detect gravitational waves. However, both failed to detect gravitational waves.
In 2015, LIGO was improved with advanced detectors. In 2016, the LIGO-Virgo Collaboration reported detecting gravitational waves for the first time. The team went on to record about 50 gravitational wave events. The LIGO Scientific Collaboration and the Virgo Collaboration are composed of several universities and research institutions and include many Indian scientists.
Recently, scientists on the LIGO-Virgo team observed two gravitational wave events. The scientists calculated that the source of the events was some 280 to 300 megaparsecs away from earth. Considering that a parsec is 3.26 light-years, it would take about a thousand years travelling at the speed of light to reach there.
The properties of gravitational waves indicated that the gravitational waves were emitted by a neutron star–black hole binary system. Neutron stars and black holes are formed after stars die. Stars lose mass and grow denser until they collapse in a supernova explosion. Depending on their size, some stars become black holes that capture anything around them, while others remain neutron stars, remnants of stars that are too small to become black holes.
LIGO Livingston and Virgo observed the first event, on 5 January 2020, at 9:54 pm, Indian time. The scientists who analysed the waves say that they were formed after an astronomical event in which a neutron star was swallowed up by a black hole. The mass of the neutron stars was estimated to be twice that of the sun, and the black hole is nine times the size of the sun.
The second event was detected by all three LIGO–Virgo detectors ten days later, on 15 January 2020 at 9:53 am, Indian time. In this event, a neutron star that was 50 per cent more massive than the sun was consumed by a black hole six times more massive than the sun.
The gravitational waves produced by these events took roughly ten million years to reach Earth.
The scientists say that the merging of these neutron star-black holes probably happened in the milky way itself because the masses of the neutron stars analyzed were similar to the masses of neutron stars distributed in the Milky Way.
These events are representative of the neutron star-black hole population in the universe, even when they have significantly unequal masses.
“We suspect neutron star-black hole mergers occur at least once a month within a billion-light year radius of Earth,” says Sheelu Abraham, Inter-University Centre for Astronomy and Astrophysics, Pune.
LIGO-India, the Indian Initiative in Gravitational-wave Observations, plans to create a gravitational wave detector in India. Meanwhile, there is a volunteer-distributed program, Einstein@Home, which provides searches for neutron stars in data from gravitational wave detectors and other large radio telescopes. About five hundred thousand volunteers from more than 200 countries are participating in this mega research effort. Volunteer scientists and enthusiasts can join Einstein@Home.
The Astrophysical Journal Letters, 915:L5 (24pp);
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