The strong CP problem is solved by use of quantum gravity. It is argued that the coherent-state parameters αᵢ describing the effect of wormholes (baby universes) adjust themselves so as to make the strong interactions CP invariant. There remains though a small CP breaking so that {θ}{} is not exactly zero. The main prediction is that we do not need the axion proposed by Peccei and Quinn.
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Nielsen et al. (1989) studied this question.
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