Why doesn’t the expanding Universe break the speed of light? | by Ethan Siegel | Starts With A Bang! | Jul, 2024

Just 13.8 billion years after the hot Big Bang, we can see objects up to 46.1 billion light-years away. No, this doesn’t violate relativity.

If there’s one rule that most people know about the Universe, it’s that there’s an ultimate speed limit that nothing can exceed: the speed of light in a vacuum. If you’re a massive particle, not only can’t you exceed that speed, but you’ll never reach it; you can only approach the speed of light. If you’re massless, you have no choice; you can only move at one speed through spacetime: the speed of light if you’re in a vacuum, or some slower speed if you’re in a medium. The faster your motion through space, the slower your motion through time, and vice versa as well. There’s no way around these facts, as they’re the fundamental principle on which relativity is based.

And yet, when we look out at distant objects in the Universe, they seem to defy our common-sense approach to logic. Through a series of precise observations, we’re confident that the Universe is precisely 13.8 billion years old. The most distant galaxy we’ve seen so far is presently 32 billion light-years away, the most distant light we see corresponds to a point presently 46.1 billion light-years away, and galaxies beyond…