Transcript: Astronomers make a distinction between the physical universe, all that there is, and the observable universe, all that we can see. The distinction comes about because early in the cosmic expansion any two points in space were separating at faster than the velocity of light. This sounds like a violation of relativity. Special relativity however, which says that the velocity of light is the maximum speed for any signal applies, only to local reference frames. General relativity is the theory that governs the cosmic expansion, and general relativity places no speed limit on the expansion of the big bang. The time when points were moving apart faster than the velocity of light was a time about five billion years ago when the universe was forty percent of its current size or redshift of 1.25. Back much further, three hundred thousand years after the big bang at the time of the microwave background radiation, points in space were moving apart at forty times the velocity of light. Thus there are regions of physical space that we have never seen in the history of the universe, implying that the physical universe is potentially much larger than the observable universe.