  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, as in QCD, it means
that different topological sectors of the theory can only sum incoherently,
because the degrees of freedom beyond the horizon must be traced over for
general covariance to apply. This might lead to a solution of the so-called
strong CP problem without extra observable dynamics.
