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The Question

(Submitted August 18, 1997)

Hawking wrote about a singularity (physical as well as mathematical term), and the possibility that space begun its life from one of these. If space is expanding at speed comparable to the speed of light and if every galaxy is (approximately) considered to be homogeneous (speed of all material bodies within one galaxy is the same), would it mean that every galaxy has its own 'TIME' regarding to some reference time in spot of singularity? If so, possibility of parallel worlds would be reality.

The Answer

Your question touches on one of the fundamental concepts of relativity. Observers moving relative to each other have their own 'time' in the sense that they may not agree whether two events happen at the same time. So there is no way to set up a single time system for the whole universe. Since the universe is expanding the relative speed of galaxies increases with distance. This means that there may be galaxies far enough away from us that the distance to them is increasing faster than the speed of light. This might seem to conflict with Special Relativity but it happens because space itself is expanding. We are completely cut off from these regions of spacetime. There is no way to communicate with them because that would require us to send information faster than the speed of light. Maybe this is what you mean by parallel worlds, but they are not really parallel worlds. They are just regions of our universe that we can never reach or communicate with.

Damian Audley and Tess Jaffe
for the Ask an Astrophysicist team.

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