Why is massive gravity a big deal?

Patrycja Ubysz reports on one possible theory on the constant expansion of the universe

Patrycja Ubysz
10th February 2020
Gravity, as trivial as it is in our day to day lives, is still not fully understood.

"What is there to understand?" you might ask. Pushing your favourite mug over the edge of a table will inevitably result in it being shattered on the floor, simple as that. There has been debate regarding the nature of gravity for years and scientists seem not to have reached a consensus just yet.

Einstein’s famous theory of relativity assumes that every matter will gravitationally attract all the other matter, regardless of how far apart they are. For example, a galaxy could attract another galaxy millions light years away, therefore gravity would contribute to shrinking of the Universe.

Einstein believed the Universe does not change its size. He included a cosmological constant in his theory, acting as a “reverse gravity” to balance out the attractive forces and allow the Universe to remain static. He did not expect the “reverse gravity”, named later a dark energy, to play a much greater role. When accelerating expansion of the Universe was proven, he removed the constant from his theories.

The concept of dark energy as something not only causing but also accelerating expansion, as well as the nature of gravity itself, still puzzles cosmologists. Multiple theories arose but none of them entirely adhere to the known laws of physics and serve as an explanation the majority of scientists would agree with.

Some theories, including Einstein’s, assume existence of a massless graviton mediating gravity which moves at the speed of light. Other theories such as Quantization, which describes an interaction with gravitons, is causing severe theoretical issues when it is applied to gravity.

Claudia de Rham, a physicist at Imperial College London, revisited seemingly a simple yet controversial alteration to Einstein’s theory in her research. What if gravitons possess mass? Research on gravitational waves, for which the Nobel Prize in 2017 was awarded, suggests that if gravitons existed and adhered to de Rham’s theory, their mass could not exceed 10-58 kg.

What implications does this tiny mass have on universe expansion? Unlike Einstein’s massless gravitons, the massive particles would lose their attractive abilities with very large distances. Galaxies from opposite ends could not attract each other anymore and the expansion would be undisturbed. They could only move slightly slower than light and so could be detected on Earth by observing the delay between the light signal and gravity wave from distant objects.

Although many physicists are sceptical towards the de Rham’s theory, it has its enthusiasts. Detection of a massive graviton would truly revolutionise our knowledge of the Universe and put an end to the discussion of the nature of gravity.

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