The strong CP problem, general covariance, and horizons

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 theta = 0, this might lead to a solution of the strong CP problem without extra observable dynamics.

Automated summary

The discussion of the strong CP problem in the context of quantum field theory with horizons highlights the constraints that general covariance places on the topological structure of the theory. This could potentially offer a solution to the strong CP problem without the need for additional observable dynamics.