Anyone who took Physics 101 has had this fact drilled into their head: The speed of light is a constant, traveling at 186,000 miles per second. In fact, it’s the cornerstone of much of modern physics, especially Einstein’s theory of special relativity, reports Joseph Dussault at the Christian Science Monitor.

But that may not be the case. In a new paper published the journal Physical Review D, João Magueijo, a theoretical physicist at Imperial College London, and Niayesh Afshordi, of the University of Waterloo in Canada, explore the idea that the speed of light was different in the past—zipping by much faster in the universe's infancy than it does today.

Ian Sample at The Guardian explains:

Magueijo and Afshordi came up with their theory to explain why the cosmos looks much the same over vast distances. To be so uniform, light rays must have reached every corner of the cosmos, otherwise some regions would be cooler and more dense than others. But even moving at 1bn km/h, light was not traveling fast enough to spread so far and even out the universe’s temperature differences.

To explain this discrepancy, physicists developed the inflation theory, which suggests that the early universe was much smaller, allowing temperatures to even out. Then overtime it has expended to reach its current size. But this idea is often criticized because it requires the creation of a set of conditions that would only exist in the universe's infancy—something that cannot be easily tested.

Magueijo and Afshordi's concept, however, has been gaining traction. “The theory, which we first proposed in the late-1990s, has now reached a maturity point—it has produced a testable prediction,” he says in a press release. “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.”

The duo tested this idea against the universe’s Cosmic Microwave Background (CMB), which is the radiation created shortly after the Big Bang that fills the universe. Based on the researchers model, the CMB acts as a sort of timeline for universe fluctuations, recording how the speed of gravity and the speed of light change with variations in temperature, reports Michael Brooks at New Scientist.

“In our theory, if you go back to the early universe, there’s a temperature when everything becomes faster. The speed of light goes to infinity and propagates much faster than gravity,” Afshordi tells Sample. “It’s a phase transition in the same way that water turns into steam.”

Brooks explains:

This fixes a value called the spectral index, which describes the initial density ripples in the universe, at 0.96478—a value that can be checked against future measurements. The latest figure, reported by the CMB-mapping Planck satellite in 2015, place the spectral index at about 0.968, which is tantalisingly close.

Even if the numbers don’t match, the researchers say they will be happy. “That would be great—I won’t have to think about these theories again,” Magueijo tells Brooks. “This whole class of theories in which the speed of light varies with respect to the speed of gravity will be ruled out.”

The concept has not gone without criticism, Sample reports. David Marsh at the Centre for Theoretical Cosmology at Cambridge University says the idea has too many theoretical issues that have not been worked out, while inflation seems to make more and more sense. “The predictions of inflation developed by Stephen Hawking and others more than 30 years ago have been tested by cosmological observations and faced those tests remarkably well,” he tells Sample. “Many scientists regard inflation as a simple and elegant explanation of the origin of galaxies in the universe.”