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Universal rotation [Q&A]

Vinay

This question came from a group — why is it that everything we know of seems to rotate, is it because the Universe as a whole is rotating? Can we infer anything about the Universe’s rotation from the fact that everything seems to rotate?

Not really. Suppose you begin with a Universe that is filled with matter that has zero angular momentum. As this matter collapses into smaller agglomerations, such as galaxies, stars, etc., it does so imperfectly and randomly. That is, the material that goes towards making any given clump comes from arbitrary directions, and on average the angular momenta of all these accumulations will not cancel out. So the final products will end up having non-zero residual angular momenta.

This argument doesn’t work in reverse. We will get the same kind of uncanceled angular momenta in smaller clumps regardless of whether the Universe started out in a state of zero or non-zero angular momentum, so this kind of argument cannot be used to infer anything about the original state of the Universe.

That said, if the Universe were rotating as a whole, we should be able to see its effects, for example in anisotropies in the cosmic microwave background, and no such signature has been detected. Furthermore, if the entire Universe is rotating around a specific axis, that axis could be considered a preferred direction that breaks the isotropic symmetry of physical laws, and consequently break the conservation of angular momentum. This last doesn’t seem to be happening, so it is fair to assume that the Universe has a net zero angular momentum.