Hypnosifl wrote:PsiSquared wrote:Let's take a simple example. Suppose the frame-of-reference in which ansible signals move, is tied to the motion of the center-of-mass of the universe.
Current models of cosmology generally use the assumption that the universe doesn't have any center of mass, see Where is the centre of the universe? from the Usenet Physics FAQ.
I have a degree of trouble with this. Not with the expansion or relative frames of reference. That's all fine. But assuming if the universe is non-infinite, there would have to be a center of mass, yes? Sure, this is just an arbitrary categorization, and a different, smaller categorization would have a different center of mass(I presume the CoM for the solar system would be within, or very close to the sun, for instance), but it should still have one, yes?
Of course, we likely cannot get the information to know exactly where that is...but even the observable universe, as a smaller subset of the entire universe, should have a center of mass.