In June 2023, scientists unveiled the evidence that the Universe is reverberating with a background hum, or ‘ripples in the fabric of spacetime’ called gravitational waves. This cosmic sound is created by huge objects that keep moving around, colliding, and merging with each other. Scientists have been able to record the gravitational waves echoing through the Universe for the first time in the form of radio data. This breakthrough has been hailed as a major milestone that would open a new window into the Universe. This gravitational wave background is likened to hearing the hum of a large number of people, as if talking in a party, without being able to distinguish any individual voice.
Gravitational Waves
Gravitational waves are created over eons by astronomically dense objects such as supermassive black holes, orbiting around each other before they merge.
The possibility of the presence of gravitational waves in the Universe was predicted by Albert Einstein in his General Theory of Relativity more than a century ago. According to the theory, gravitational attraction results from ‘the bending of the fabric of spacetime by the equivalent of a heavy object’. The animation used to explain this theory shows that a curvature is created on a rubber sheet when a large ball is placed on it. Now, when you roll a smaller ball on this sheet, it first moves around the large ball along the curvature before falling into it. Einstein explained that similar to this animation, the Sun, the Earth, and other massive heavenly bodies form curvature around them, and this was the main reason that smaller objects get pulled towards these larger bodies. The movement of the Earth and other such bodies cause their curvature also to move; thus, creating a ripple-like effect or gravitational waves in the spacetime.
Einstein’s Theory of Relativity states that space and time do not exist as independent entities. It combines the three dimensions—height, width, and length—of space and one dimension of time into a single four-dimensional continuum, known as spacetime. The Theory of Relativity also states that if a body is moving in space, it would affect time also because space and time are interwoven.
After a decade, Einstein proposed that spacetime is not something that simply serves as a transparent, inert, static, or fixed background to all the events happening in the Universe. Rather, it is flexible and malleable, can interact with matter and get affected by it, and can also influence the events occurring in the Universe.
Laser Interferometer Gravitational Observatory
The presence of gravitational waves was first confirmed, in 2015, by the US and Italian observatories when they detected their creation by the collision of two black holes that took place around 1.3 billion years ago. These high-frequency waves were detected by conducting an experiment involving Laser Interferometer Gravitational Observatory (LIGO) detectors. All the subsequent detections made using LIGO were also of high-frequency waves. LIGO is a world-wide network of laboratories designed to detect the ripples (cosmic waves) created in spacetime by the movement of large celestial bodies.
LIGO-India—the Indian node in the international network of laboratories to explore the Universe—received the approval of the Indian government in February 2016 and will be set up in association with a group of Indian research institutions and the LIGO Laboratory, USA, along with its international partners. This ambitious project will be located in the Hingoli district of Maharashtra and is expected to conduct its scientific runs by 2030.
Detection of Low-Frequency Gravitational Wares
Gravitational waves come in a variety of wavelengths, frequencies, and intensities. As the gravitational wave detected in 2015 and all the subsequent detections of such waves involved the collision of relatively small black holes, they generated very feeble cosmic waves. The only waves that could be identified were those that were created right before the merger, when the energy released was at its peak. These, however, were brief gravitational wave flushes that barely lasted a few milliseconds.
The Universe is assumed to be reverberating with low-frequency gravitational waves, like the continuous hum of a background noise. In their attempt to discover cosmic waves of low frequency, scientists from all over the world accumulated under the banner of the International Pulsar Timing Array (InPTA) and used some of the finest radio telescopes to detect and study pulsars.
It was discovered, after 15 years of studying 25 pulsars, that some of the signals from neutron stars arrived a little early than others, with the differences varying by millionths of seconds. Scientists have suggested that such inconsistencies were produced by gravitational waves distorting spacetime. This variation in the time of receiving signals is an indicator of the presence of gravitational waves.
The low-frequency gravitational waves, as opposed to the ripples that had previously been observed, likely came from the collision of two enormous black holes, which are around millions of times bigger than the Sun. Such massive black holes are typically located at the centre of galaxies.
Pulsars are distant neutron stars characterised by their high rotational speed and the ability to emit short bursts of radiation, which are visible to the human eye from the Earth as dazzling flashes of light. As these bursts occur at incredibly regular intervals, pulsars are used by astronomers as ‘cosmic clocks’.
Importance of the Discovery
Scientists assert that gravitational waves are created when gigantic black holes, present at the centre of galaxies, collide and merge with each other. This recent discovery led scientists to believe that our Universe may have a background permeated with gravitational waves. Moreover, there are countless massive black holes that keep merging with each other. Gravitational waves are continuously generated long before these black holes merge, and also during the process of fusion, which might take millions of years to complete.
These findings would contribute to our growing understanding not only of the nature and origin of the Universe, but also of the formation and evolution of galaxies and blackholes. The discovery of low-frequency gravitational waves may one day also shed light on the mystery of the dark matter. It is estimated that dark matter and dark energy, which do not produce any light or electromagnetic waves, make up nearly 95 per cent of the Universe.
Up until now, we have learnt about distant objects by detecting electromagnetic waves emitted or reflected by such objects. It is believed that gravitational waves, like electromagnetic waves, are indicative of the event that produced them. In 2015, researchers were able to confirm their theory after they managed to trap gravitational waves produced by two black holes that combined into one.
Further insights, studies, and discoveries on gravitational waves may help us answer some of the most fundamental questions about spacetime, such as how and when galaxies have formed and changed over the course of time, and even shed light on the mysteries of the origin and evolution of life.
© Spectrum Books Pvt. Ltd.