One of the oddest things about our universe is that it has some fundamental constants. These constants—such as the speed of light in a vacuum—have numerical values that don’t change no matter what conditions you test them under.

The speed of light in a vacuum had been measured by many physicists in the 19th century and found to always have the same value (299,792,458 meters per second, though it took until the 1970s to reach that level of precision). But physicists had to wait until Albert Einstein developed his theory of relativity to understand why this was the case.

As you probably know from your high-school physics, light can travel both as electromagnetic waves and as particles, or photons. Einstein hypothesized that, if something could travel faster than the speed of light, it would break fundamental physical laws by being able to observe, relatively speaking, a stationary electromagnetic wave. Thus, for his theory of relativity to work, he hypothesized that the speed of light must remain constant.

In the century since Einstein developed his theory, every conclusion he made has been proven to be correct. But what if his assumption that the speed of light in a vacuum remains constant was faulty from the outset? That’s a question that João Magueijo of Imperial College London raised in 1998.

Magueijo proposed that to solve one of the biggest physics problems, called the “horizon problem,” we might have to challenge the idea that the speed of light is constant. The problem states that the universe reached a uniform temperature long before energy-carrying photons traveling at constant speed could have had the time to reach all corners of the expanding universe.

The most accepted explanation for the horizon problem is something called inflation. It suggests that, after the Big Bang, the temperature evened out before the universe went through a rapid phase of expansion. But the inflation theory doesn’t sit well with many physicists, mainly because nobody can explain why inflation started and why it stopped.

Magueijo’s search for an alternative explanation is what made him question the speed of light. While it’s tempting to ditch a cornerstone of physics to solve other problems, it’s not wise to do so without setting out a testable hypothesis. That is why, until now, Magueijo’s hypothesis remained on the fringes of physics.

All that is about to change because of a study that is to be published on Nov. 28 in the journal Physical Review. Along with Niayesh Afshordi of Perimeter Institute, Magueijo has proposed a testable hypothesis. They propose that, at the very beginning of the universe, light and gravity traveled at different speeds. If photons moved faster than gravity, it would have given them enough time to travel to all parts of the universe and thus help reach temperature equilibrium.

Other physicists can test the hypothesis by measuring something called cosmic microwave background (CMB) radiation, which is a fossilized impression of the early universe that we can measure even today. The hypothesis suggests that the CMB should reflect the change in the speed of light and the speed of gravity as the temperature of the universe changed.

The value they predict, called the spectral index, which describes the density ripples in the universe, is 0.96478. The most recent figure for the spectral index, based on a satellite measuring the CMB, is 0.968, which is quite close. If future measurements of the CMB show a mismatch, it would mean the speed of light remains constant, and that Magueijo’s and Afshordi’s theory can be thrown away.

What if measurements of the spectral index don’t match Magueijo’s and Afshordi’s prediction? “That would be great—I won’t have to think about these theories again,” Magueijo told the New Scientist.

Meanwhile, our accepted theories of physics would still explain most things in the universe, but not quite everything.