General Relativity:

There is no absolute standard for position in the universe, nor is there an absolute standard for the velocity. The other manner of saying is that any reference frame is just as applicable as any other as long as acceleration does not occur. The ideas of "the center of the universe" and "at rest" are associative. When we measure position or velocity, we should do so with respect to something, like the Earth or the Sun or a space ship coasting via the void.