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What’s Up With Gravity?

An artist's impression of gravitational waves.
An artist's impression of gravitational waves. 

Staff writer Anoushka Sinha discusses gravitational waves, a physical phenomenon involving invisible ripples in space-time. Research from King’s College London (KCL) in the field is examined.

Gravitational waves are caused by massive accelerating objects in space. Massive objects, such as stars, planets and galaxies, literally bend space and time, causing nearby objects to orbit, and fall into them. That’s why the Earth orbits the Sun, and why we (for the most part) remain fixated on the surface of the Earth.

Recent discoveries have landed gravitational waves at the centre of astrophysical research. But first, let’s turn back the clock a little.

Einstein’s theory of relativity predicted gravitational waves in the early 20th century, but they were not proven for another 60 years.

The work done on Einstein’s prediction of gravitational waves was furthered by professors in the King’s mathematics department in the 1950s and 1960s. Aiding in the initial designs of gravitational wave detectors, Professors Hermann Bondi and Felix Pirani worked together to mathematically prove the waves’ existence.

Eventually, the first gravitational waves were detected in 1974 by Russell Hulse and Joseph Taylor

Winners of the 1993 Nobel Prize in Physics, Hulse and Taylor tracked a binary pulsar they had discovered and found that the rate at which the stars were approaching each other at the same rate predicted if they were radiating gravitational waves.

Gravitational waves have become a popular field of research, and in the summer of 2023, a large team of scientists published a set of papers, suggesting that the waves are far more prevalent than previously thought, creating a ‘cosmic symphony’ throughout the Universe.

Gravitational waves are difficult to detect because they are all around us, all the time – hidden in plain sight, like air. One has to look at massive, violent objects in space (pulsars or blackholes) in order to see gravitational waves.

For two decades, LIGO (the Laser Interferometer Gravitational-wave Observatory) has detected high-frequency gravitational waves but, using six large radio telescopes, scientists are now able to detect low-frequency gravitational waves by tracking the radiation emitted by pulsars.

The James Webb Telescope, launched in 2021, is getting in on the action too; specialising in infrared, the telescope has been observing gravitational waves emitted by black holes.

The combined efforts have shown that gravitational waves are all around us, and understanding them leads us to a better understanding of the universe and the way it was created.

In December 2023, researchers at King’s, Professor Mairi Sakellariadou and Nikolaos Kouvatsos were involved in the development of a new method of mapping the universe using gravitational waves created by black holes and neutron star collisions. This would help physicists understand whether or not the Universe is isotropic (uniform in all directions).

Most notably, gravitational waves help us understand dark matter – an invisible matter which occupies a significant portion of the Universe. This method is known as gravitational lensing.

Since the discovery of the ‘cosmic symphony’ last year, researchers at LIGO and Virgo have been using gravitational waves to detect deep space collisions. The results have been phenomenal.

It was previously believed that there is a ‘mass gap’ between neutron stars (the densest stars) and black holes. However, the recent detection of a merger between a star about the size of our Sun, and a much larger object, indicates that the mass gap does not exist at all.

Black holes can be much less massive than previously thought, suggesting that previously proposed astrophysical models must be wrong; traditional models show that large stars collapse into either neutron stars or black holes, but scientists now believe that the transition must not be so smooth.

It is now hypothesised that during a supernova, some ejected mass and energy must fall back into the temporarily created neutron star, causing its mass to increase until it eventually turns into a black hole.

The possibility of using gravitational waves to learn about the universe – super massive objects in particular – has scientists giddy with excitement; perhaps the unknown will not feel quite as unknown anymore.

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