Does the expansion of space slow the speed of light?

Hannah - So, does the speed of light change? Dr. Andrew Pontzen from University College London provides the answer.

Andrew - So far as anyone can tell, the speed of light in our universe is absolutely fixed. It does not change.

Hannah - The speed of light travelling in the near vacuum of space clocks in at just shy of 300 million meters per second or 186,000 miles per second. So, if space is expanding, doesn't that mean that objects in space are also expanding including atoms in our meter ruler that we use to measure distance by? Well, space expansion isn't actually uniform. Our Milky Way and the galaxy next to us, Andromeda are moving closer to each other but in general, galaxies are getting further apart as space expands. In this case, does this non-uniform expansion affect the meter rule somehow and therefore, affects speed of light? Well, back to cosmologist Andrew Pontzen.

Andrew - As you can imagine, the fact that the universe is expanding and distances change over time, does make this a bit more complicated. You might think it even makes what we call speed of light ambiguous because distances mean something different at different times. But actually, it doesn't. You can still define what we mean by the speed of light pretty well because it has an effect for instance on the structure of atoms themselves. This is through something called the fine structure constant which is just a number that the speed of light appears in. So, if the speed of light were different, the structure of the atoms would also be different. Because of that, by looking through telescopes, we can actually tell that the structure of atoms hasn't changed over time and that's what makes us pretty confident that the speed of light must be fixed.

Hannah - Okidok, rest assured, our speed of light is not decreasing and time is not speeding up either, or is it?

Andrew - There is a possible complication which is something called string theory which is a possible theory of everything, sort of a better version of physics. It does allow for this fine structure constant to change. So, it is possible that in our universe, the fine structure constant and so the speed of light is effectively changing over very long timescales. But whenever anybody has looked for any evidence of that, they have found no evidence. So, as far as we can tell, the speed of light is fixed.