How can the universe expand faster than the speed of light?
We deduce that the universe is expanding from the observation that galaxies are receding from us, and receding faster the farther away they are from us. One can imagine it like a balloon (in 2D) or cake (in 3D), with an average distance between galaxies that increases with time (right now astronomers think that the expansion is actually accelerating, actually, but that’s a story for a future lecture!) There is also evidence that there was a period of time (called “inflation”) in the history of the universe during which the expansion happened so fast that parts of the universe moved away from each other faster than the speed of the light (again, more on this later). This does not violate the principle that nothing can move faster than the speed of light, because it was space itself that was expanding; the constraint on objects to never move faster than the speed of light actually applies only to local motions of objects relative to each other.
