If there is one thing humans and any intelligent aliens living in a galaxy far, far away would agree upon, it is this — the speed of light in vacuum is constant, and has remained the same throughout our universe’s existence.
Or has it?
In a new study, to be published in the upcoming edition of the journal Physical Review, physicists from the Perimeter Institute in Canada argue that in the early universe, the speed of light may not have been what it is now. In fact, the researchers say, when the universe was still in its infancy, light may have outpaced gravity — which Einstein’s theory of general relativity tells us currently propagates at the speed of light.
In order to understand the significance of this yet-to-be-tested hypothesis, one needs to travel back in time a little — roughly 13.8 billion years back, when our universe burst into existence.
Our current model of the universe’s formation tells us that it underwent a brief period of exponential expansion trillionths of trillionths of a second after the Big Bang — a phenomenon called “cosmic inflation.” During this period, space-time burgeoned from being smaller than a proton to a fabric that stretched across light-years.
Here’s where we run into a problem. We do not have any proof that this inflation took place, and a potential discovery of primordial gravitational waves — something that was hailed as a “smoking gun” of cosmic inflation — was later debunked.
However, if scientists discard this theory, we would have nothing left to explain what is known as the horizon problem, which deals with the fact that the universe has a uniform temperature. If, as we currently believe, the speed of light is constant and nothing can travel faster than photons, how did these heat-carrying particles have enough time to reach all the corners of the universe?
The cosmic inflation theory attempts to solve this problem by suggesting that in the first few seconds of the universe’s existence, regions of space-time that are now separated by billions of light-years were in contact, allowing the temperature to even out. This was then followed by an extremely brief period of exponential expansion, during which this uniformity spread throughout the cosmos.
On the other hand, if the speed of light and gravity did in fact differ in the early universe, as the new study suggests, it could solve this vexing problem without invoking the inflation theory.
“If photons moved faster than gravity just after the big bang, that would have let them get far enough for the universe to reach an equilibrium temperature much more quickly,” the researchers to New Scientist.
Of course, it’s also entirely possible that back then, gravity may have travelled slower than it does now. Either way, it’s the relative speed of light and gravity that matters, insofar as the horizon problem is concerned.
And, the researchers claim that unlike the cosmic inflation theory — which, as things stand now, is difficult to rule out — this one can actually be tested.
If the speed of light and gravity did once vary, it would have left distinctive signatures on the cosmic microwave background — the residual radiation from the Big Bang. Specifically, the researchers say that the value of the spectral index, which describes the initial ripples in the density of the cosmos, should be 0.96478 if their hypothesis is right.
Currently, the best estimate we have of this quantity — calculated based on observations made using the Planck satellite — is 0.968, which is extremely close to the researchers’ estimate. Further observations by the Planck satellite would now be needed to see if the spectral index matches the estimate.