A theory which predicts that the speed of light is variable, not constant as suggested by Albert Einstein, may soon be tested and could change our understanding of the early universe if proven to be true.
Einstein observed that the speed of light remains the same in any situation, and this meant that space and time could be different in different situations.
The assumption that the speed of light is constant, and always has been, underpins many theories in physics, such as Einstein's theory of general relativity.
In particular, it plays a role in models of what happened in the very early universe, seconds after the Big Bang.
However, some researchers have suggested that the speed of light could have been much higher in this early universe.
Now, one of this theory's originators, Joao Magueijo from Imperial College London, working with Niayesh Afshordi at the Perimeter Institute in Canada, has made a prediction that could be used to test the theory's validity.
Structures in the universe, for example galaxies, all formed from fluctuations in the early universe - tiny differences in density from one region to another.
A record of these early fluctuations is imprinted on the cosmic microwave background - a map of the oldest light in the universe - in the form of a 'spectral index'.
Working with their theory that the fluctuations were influenced by a varying speed of light in the early universe, researchers used a model to put an exact figure on the spectral index.
Cosmologists are currently getting ever more precise readings of this figure, so that prediction could soon be tested - either confirming or ruling out the team's model of the early universe. The figure is 0.96478.
This is close to the current estimate of readings of the cosmic microwave background, which puts it around 0.968, with some margin of error.
"The theory, which we first proposed in the late-1990s, has now reached a maturity point - it has produced a testable prediction," said Magueijo.
"If observations in the near future do find this number to be accurate, it could lead to a modification of Einstein's theory of gravity," he said.
"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," Magueijo said.
"If true, it would mean that the laws of nature were not always the same as they are today," he said.
The research was published in the journal Physical Review D.
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