Evrim Yazgin
Cosmos science journalist
A new test of Einstein’s “Theory of General Relativity” at scale has raised questions about this cornerstone of modern physics.
“General Relativity” provides a powerful predictive framework for one of the fundamental forces of nature — gravity. The theory is remarkably successful in describing gravity when talking about stars and planets. But different physical scales present challenges for Einstein’s theory.
In 1915, Einstein submitted the definitive version of his theory in “The Field Equations of Gravitation”. Since then, his theories have been put to the test many times.
Famously, astronomer Arthur Eddington provided the first observational proof of Einstein’s theory in 1919. Eddington showed that the curvature of space-time around our Sun would reveal the light from a star behind the Sun, just as Einstein’s theory predicted.
But “General Relativity” does not do a good job at small scale – where we start talking about quantum mechanics.
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Quantum theory counterintuitively tells us that the vacuum – that is, empty space – has an energy.
According to Einstein, the vacuum energy has “repulsive gravity” pushing empty space apart. Not only do we see this effect, but in 1998 it was shown that the universe is expanding.
Called “dark energy”, the force causing this expansion is many orders of magnitude smaller than the amount of vacuum energy predicted by quantum theory.
This “Cosmological Constant Problem” throws up questions like whether or not the vacuum energy exerts a gravitational force, why gravity is so weak, and, if not the vacuum energy, what is causing the accelerated expansion of the universe?
Not only do observations suggest there is some invisible “dark energy”, but also “dark matter”. In fact, cosmologists haven’t been able to account for around 95% of our universe. This has had physicists question whether Einstein’s theories may be incomplete.
To compound the problems, different methods of measuring the Hubble constant – the rate of cosmic expansion – give different answers. This additional problem is called the Hubble tension.
Now a new study, published in Nature Astronomy, suggests that Einstein’s theory has to be reassessed again – this time throwing up questions of how General Relativity performs on the largest of cosmic scales. The authors believe their approach could help answer some of the biggest questions about the universe.
For the first time, the researchers tackled three aspects of General Relativity on large scales: cosmic expansion, gravitational effects on light, and gravitational effects on matter.
Using a statistical computational simulation, the team modelled the gravity in the universe through its history. The parameters through time were estimated through analysis of the cosmic microwave background – the oldest visible data in the universe. The researchers also used observations of the shape and distribution of distant galaxies.
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Comparing their results with the standard cosmological theory based on Einstein’s predictions showed a mismatch. The disagreement, the authors note, is of low statistical significance. But it is there, suggesting gravity may work differently on large scales.
But the authors also say that solving the Hubble tension is not as simple as tweaking the theory of gravity. The full solution likely needs a new ingredient in the cosmological model. Such an ingredient would likely predate the fusion of electrons and protons to form hydrogen for the first time, just after the Big Bang.
The authors concede that there may be a much more human explanation – an error in the data.
But what the study shows is that observational data can be used to assess the validity of Einstein’s theory of gravity on large scales. Future applications of such statistical methods may yet resolve some of the biggest questions in the universe, challenging some of the most successful physical theories along the way.