Einstein was incorrect about the speed of light being a fixed constant in our universe, a new theory suggests. A team of physicists are backing an idea that the speed of light is not constant and have made a prediction that can be tested.

The speed of light is exactly 299,792,458 metres per second, and is a value that is thought to never change. The constancy of the speed of light is a powerful assumption in many fundamental areas of physics, including cosmology, quantum mechanics, and electromagnetism.

This month, a paper published in Physical Review D [Arxiv] challenges this view. Professor João Magueijo and Professor Niayesh Afshordi, who are both theoretical physicists at Imperial College in the UK and the Perimeter Institute of Theoretical Physics in Canada respectively, claim that the speed of light could have been faster during the early universe.

Magueijo and Afshordi have figured a way to test their theory by looking at the universe’s tiny density fluctuations observable on a map of the cosmic microwave background (CMB).

It is widely believed that the CMB is the energy left over from the early moments after the Big Bang. Although there are small pockets where the energy is varies, on a wide scale the CMB is mostly uniform possibly due to inflation of our universe.

The theory that the speed of light varies is in direct conflict with inflation, the idea that the universe rapidly ballooned in size. Inflation explains why the CMB is pretty stable. Different parts of the universe were influenced by each other and all expanded at the same rate, keeping the CMB nearly constant.

But in order for that to be true, the speed of light must have been faster, the physicists claim. The current value means that not all the regions of the universe would have been able to influence each other as there would not have been enough time for light to reach the edge of the universe, something known as the horizon problem.

The theory was first proposed in the 1990s. Magueijo said the theory had now “reached a maturity point”, where it has now “produced a testable prediction”.

“The idea that the speed of light could be variable was radical when first proposed, but with a numerical prediction, it becomes something physicists can actually test. If true, it would mean that the laws of nature were not always the same as they are today,” said Magueijo. ®