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Scientists on Thursday (June 28) said they have found evidence to suggest that the universe is replete with low-frequency gravitational waves – ripples in the fabric of space-time that are created by huge objects moving around, colliding, and merging with each other, and predicted by Albert Einstein’s General Theory of Relativity more than 100 years ago.
Gravitational waves were first detected in 2015 using an experiment, involving Laser Interferometer Gravitational Observatory (LIGO) detectors. But those waves were of high frequency, believed to be produced by the merger of two relatively small black holes that took place about 1.3 billion years ago. All the subsequent detections after that were also of high-frequency waves. This, however, has changed now.
In a bid to discover low-frequency gravitational waves, scientists used an entirely different technology compared to the one used eight years ago, as per different studies published on Thursday that were carried out by radio astronomers representing five different international teams including Indian Pulsar Timing Array (InPTA).
The researchers used six large radio telescopes around the world, including the one in Pune, to study objects called pulsars — distant rapidly-rotating neutron stars that emit pulses of radiation, observed from the Earth as bright flashes of light. These bursts take place at extremely precise intervals, and therefore scientists use pulsars as ‘cosmic clocks’.
After examining 25 pulsars over a period of 15 years, it was noted that some of the signals from these neutron stars arrived a little early while a few others were late, the discrepancies ranging in millionths of seconds. Scientists have proposed that the observed inconsistencies were due to deformities caused in spacetime by gravitational waves.
“These irregularities showed consistent effects of the presence of gravitational waves,” said Bhal Chandra Joshi, senior NCRA scientist and the man behind InPTA.
Moreover, unlike the previously detected ripples, these low-frequency gravitational waves probably emerged from a colliding pair of very large, ‘monster’, black holes, millions of times bigger than our Sun, as per the researchers. Such large black holes are usually found at the centre of galaxies.
Scientists have been looking for low-frequency gravitational waves for decades. They believed that such ripples are perpetually rolling through space like background noise. Pairs of supermassive black holes, sitting at the centre of galaxies, merge across the universe, generating gravitational waves. The latest breakthrough provides enough data to suggest that there is a gravitational wave background, which exists in our universe.
“Like you have a whole spectrum of electromagnetic waves, from microwaves to radio waves, you can have a wide range of gravitational waves of different wavelengths, frequencies and energies. The gravitational wave that was detected in 2015, and all subsequent detections after that, involved mergers of black holes that were relatively small in size. The gravitational waves produced by them are relatively feeble. Only the waves produced just ahead of the merger, when the energy released was maximum, could be detected. But these are like flashes of gravitational waves, lasting for maybe a few milliseconds,” said Ashoka University vice-chancellor Somak Raychaudhury, a former director of Pune-based Inter-University Centre for Astronomy and Astrophysics (IUCAA).
“There are much more massive black holes that are constantly merging, black holes that are millions or billions of times larger than our Sun usually at the centre of the galaxies. They can produce detectable gravitational waves from times much before their merger. In fact, the merger process can take millions of years, providing a steady supply of gravitational waves. And there are many such events happening all the time. So, there is a sort of gravitational wave background that exists all the time,” he said.
The discovery will help scientists expand their knowledge about the nature and evolution of the universe. It will also help them gain more information about the environment around massive black holes.
In his theory of gravitation, Isaac Newton postulated that the force that makes an apple fall to Earth is also the one that keeps the moon in its orbit around the Earth. Essentially, every celestial body exerts an attractive force on every other. This force, he proposed, was proportional to the masses of the two bodies and inversely proportional to the square of the distance between them. So, the greater the distance between the bodies, the lower the gravitational force between them. This resulted in his famous equation of gravitation, which was written as:
F = G × m1 × m2/r^2
Here, F is the strength of the force of gravitation, m1 and m2 are the masses of the two bodies, r is the distance between them and G is a universal constant that Newton introduced and whose value was calculated to be 6.674 × 10^(-21) Newton sq m/sq kg.
Newton’s gravitational law described the motion of heavenly bodies with amazing accuracy and withstood the test of time for about three centuries. In fact, this law is enough to guide most of the activities of modern space programmes. But the law had some deficiencies.
Though mathematically accurate, it doesn’t say why two bodies are attracted to each other — a problem that Newton himself acknowledged during his lifetime. The second deficiency came to the fore after Einstein, in 1905, published his Special Theory of Relativity, establishing that nothing could travel faster than the speed of light. But the gravitational force exerted on the two bodies seemed to be propagating instantaneously, over any large distance, without any delay at all. Time does not even figure in Newton’s gravitational equation.
These two deficiencies were addressed by Einstein in his General Theory of Relativity, published a decade later. He had already shown, with Special Relativity, that space and time were not independent entities but had to be woven together as spacetime. With General Relativity, which was essentially a new theory of gravitation, Einstein took a huge leap of thought.
While formulating his theory, Einstein proposed that gravitational attraction was a result of the bending of the fabric of spacetime by the equivalent of a heavy object. This is very often elucidated by an animation in which a large ball is placed on a rubber sheet, creating a curvature in the sheet. When a smaller ball is rolled on the rubber sheet, it moves around the large ball along the curvature for a while before falling into it.
Einstein said the Sun, the Earth and all other bodies formed similar curvatures around them, and this was the reason for smaller objects getting pulled towards them. But since the Earth, sun and everything else are also moving, the curvature around them moves too. This creates ripples in spacetime, just like a moving boat in water creates ripples. It is these ripples that Einstein called gravitational waves.
Pretty much everything that we know about faraway objects, or those too small to be seen, has come through the detection of the electromagnetic waves either emitted or reflected by them.
These electromagnetic waves, of which visible light is also a part, very often carry information that is characteristic of the objects they are emitted by. Over the years, we have built different kinds of detectors to trap these waves and read the information they are carrying.
But as almost 95 per cent of the universe is known to consist of dark matter and dark energy, which don’t emit any light or any other electromagnetic waves, most of the cosmos remains ‘invisible’ to astrophysicists and astronomers. This changed when scientists for the first time detected gravitational waves eight years ago.
Like electromagnetic waves, gravitational waves, too, are thought to be characteristic of the event that generated them. This is how scientists confirmed the existence of black holes, which can’t be ‘seen’ – researchers trapped gravitational waves produced by two black holes that coalesced into one and were able to verify their hypothesis on September 14, 2015.
Gravitational waves essentially give us the ability to ‘see’ what remains ‘invisible’ to us in the universe. They can tell us about our origin and answer fundamental questions about outer space, such as how different galaxies have emerged and evolved over the entire course of the universe.
In his Special Theory of Relativity, Einstein proposed that space and time don’t exist as independent entities, combining the three dimensions (height, width and depth) of space and one dimension of time into a single four-dimensional continuum, known as spacetime.
At first, the idea might seem strange but we have always perceived life in four dimensions. For instance, if you want to meet a friend, you need to specify the time and place for your meeting. Because if you just tell them a place, then you could be waiting for them for quite a while, thinking you haven’t already missed them. And if you just tell them a time, then you could be waiting for each other in entirely different regions of the world. Therefore, it isn’t possible for space and time to exist independently.
Einstein further added that if a body is moving in space, it would affect time also because, as we saw earlier, space and time are interwoven.
A decade later, in his General Theory of Relativity, the scientist proposed that spacetime was not a mere transparent, inert, static or fixed background to all the events in the universe. Instead, spacetime was flexible and malleable, interacted with matter, was influenced by it, and in turn, influenced the events that take place there. It was like a soft fabric that responds to and gets deformed by a heavy object placed on it.
