A new large-scale test of Einstein’s “general relativity theory” has raised questions about this cornerstone of modern physics.
“General relativity” provides a powerful predictive framework for one of nature’s fundamental forces: 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 presented the definitive version of his theory in “The Field Equations of Gravitation”. Since then, his theories have been tested many times.
Famously, astronomer Arthur Eddington provided the first observational proof of Einstein’s theory in 1919. Eddington showed that the curvature of spacetime around our Sun would reveal light from a star behind the Sun , just as Einstein’s theory predicted.
But “general relativity” doesn’t do a good job on the small scale – where we start talking about quantum mechanics.
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Quantum theory counterintuitively tells us that a vacuum – that is, empty space – has energy.
According to Einstein, vacuum energy has a “repulsive gravity” that pulls empty space apart. Not only do we see this effect, but in 1998 the universe was shown to be expanding.
Called “dark energy,” the force behind this expansion is several orders of magnitude less than the amount of vacuum energy predicted by quantum theory.
This “cosmological constant problem” raises questions such as whether or not vacuum energy exerts a gravitational force, why gravity is so weak, and if it is not vacuum energy, what is it? what causes the accelerated expansion of the universe?
Not only do observations suggest that there is invisible ‘dark energy’, but also ‘dark matter’. In fact, cosmologists have not been able to account for about 95% of our universe. This caused physicists to wonder if Einstein’s theories might be incomplete.
To compound the problems, different methods of measuring Hubble’s constant – the rate of cosmic expansion – give different answers. This additional problem is called the Hubble tension.
Now a new study, published in natural astronomy, suggests that Einstein’s theory needs to be re-evaluated – this time raising questions about how general relativity works 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, researchers addressed three aspects of large-scale general relativity: cosmic expansion, gravitational effects on light, and gravitational effects on matter.
Using a statistical computer simulation, the team modeled gravity in the universe through its history. Parameters over 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 standard cosmological theory based on Einstein’s predictions showed a discrepancy. The disagreement, the authors note, is of low statistical significance. But it’s there, suggesting gravity may work differently on a large scale.
But the authors also say solving the Hubble strain isn’t as simple as tweaking the theory of gravity. The complete solution probably requires 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 admit 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 a large scale. Future applications of these statistical methods could still solve some of the biggest questions in the universe, challenging some of the most successful physical theories along the way.
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