How Come Cosmic Inflation Doesn’t Break The Speed Of Light?
“In an inflationary Universe, any two particles, beyond a tiny fraction of a second, will see the other one recede from them at speeds appearing to be faster-than-light. But the reason for this isn’t because the particles themselves are moving, but rather because the space between them is expanding. Once the particles are no longer at the same location in both space and time, they can start to experience the general relativistic effects of an expanding Universe, which — during inflation — quickly dominates the special relativistic effects of their individual motions. It’s only when we forget about general relativity and the expansion of space, and instead attribute the entirety of a distant particle’s motion to special relativity, that we trick ourselves into believing it travels faster-than-light. The Universe itself, however, is not static. Realizing that is easy. Understanding how that works is the hard part.”
It’s true that nothing can move faster than the cosmic speed limit, the speed of light, and that no two particles can move faster than light relative to one another. So how, then, do you explain the fact that during inflation, two particles that begin a subatomic distance away from one another are, after just a tiny fraction of a second, are then billions of light years apart? The answer is because special relativity only applies, strictly, to particles that occupy the same location as one another in both space and time. If they’re separated, then the Universe is under no obligation to be static, and space is free to expand and/or contract. You cannot figure your apparent motion with special relativity alone, but need to factor in the effects of general relativity as well. And that’s where things get really weird.