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If we consider the universe as a system, is there any net external force acting on it? I am thinking of whether or not the center of mass of the system moves. If there is no such a net external force then the center of mass of our universe must not move. Is it correct?

As a result, this center of mass can then be used as an absolute inertial frame of reference.

Qmechanic
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kiss my armpit
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    I can measure that for you... as soon as you show me where the doors to the outside of the universe are located. – CuriousOne Jan 03 '15 at 03:50
  • @CuriousOne, just as one can't tell if they are in an accelerating vehicle from the inside? – Kenshin Apr 02 '16 at 03:37
  • Probably a lot harder than that... vehicles always have a spontaneous disassembly mode... the universe, as far as we know, doesn't. :-) – CuriousOne Apr 02 '16 at 04:10

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The universe has no center. It also doesn't have a center of mass.

The notion of "a force exerted on the universe" would need the universe to be able to move somewhere, since that's what forces (classically) do - they move things. But the universe is all there is, including spacetime, and the notion of "spacetime moving somewhere" is ill-defined.

Rather tautologically, the universe is "all that there is" - if the were a notion of force on it, there would be a notion of something exerting it on it, and hence, there would be more than just the universe, which contradicts the meaning of universe in the first place.

No matter what kind of word games you are going to play, there isn't some (measurable, for the axiomatic, superfluous alternative, see Lorentz aether theory) absolute reference frame. Special and general relativity are self-consistent, well-tested physical theories.

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ACuriousMind
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  • These questions can be answered for the Observable Universe though right? I'm guessing the center would just be the Earth, and the center of mass could be calculated? – QCD_IS_GOOD Jan 03 '15 at 04:22
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    @JoshuaLin: Since the observable universe is indeed of finite size with earth as center, that is correct - but not particularly interesting, since it is a rather arbitrary subset of the universe. – ACuriousMind Jan 03 '15 at 13:59