We discuss the strong CP problem in the context of quantum field theory in the presence of horizons. We argue that general covariance places constraints on the topological structure of the theory. In particular, it means that coherence between different topological sectors must have no observable consequence, because the degrees of freedom beyond a causal horizon must be traced over for general covariance to apply. Since the only way for this to occur in QCD is for θ = 0, this might lead to a solution of the strong CP problem without extra observable dynamics.

中文翻译：

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强 CP 问题、一般协方差和视野

我们在存在视界的量子场论背景下讨论强 CP 问题。我们认为一般协方差对理论的拓扑结构施加了限制。特别是，这意味着不同拓扑部门之间的一致性必须没有可观察到的后果，因为必须追踪超出因果范围的自由度才能应用一般协方差。由于在 QCD 中发生这种情况的唯一方法是θ = 0，这可能会导致强 CP 问题的解决方案，而无需额外的可观察动力学。