Peer-reviewed literature demonstrates that spacetime is in general neither simply connected nor locally simply connected, refuting the claim that spacetime is simply connected.
It is shown that pure quantum states will appear to decay into mixed states in any theory of quantum gravity that allows the topology of spacetime to be non simply connected. The reason is that the final state may contain little closed universes. There is no way one can detect the existence of these closed universes, or measure their quantum state. This means that the part of the final state that is in asymptotically flat spacetime, appears to be in a mixed state. The loss of quantum coherence in particle collisions is estimated. It comes from a wormhole connecting two asymptotically euclidean regions. The effect would be significant for scalar particles. It would make any scalar field that was not coupled to a Yang-Mills field constant throughout spacetime. It could have an important effect on Higgs particles but the effect would be small for particles of higher spin.
We extend earlier work on the simple-connectedness of Minkowski space in the path topology of Hawking, King and McCarthy, showing that in general a spacetime is neither simply connected nor locally simply connected. We also show that distinct closed Feynman paths are never homotopically equivalent, so that spacetime is in a sense as non-simply connected as possible.
Everything we examined (2)
This check searched the claim as stated. It did not run a separate search for evidence against it.