A quantum field theory in its algebraic description may admit many irregular
states. So far, selection criteria to distinguish physically reasonable states
have been restricted to free fields (Hadamard condition) or to flat spacetimes
(e.g. Buchholz-Wichmann nuclearity). We propose instead to use a modular
l^p-condition, which is an extension of a strengthened modular nuclearity
condition to generally covariant theories.
The modular nuclearity condition was previously introduced in Minkowski
space, where it played an important role in constructive two dimensional
algebraic QFT's. We show that our generally covariant extension of this
condition makes sense for a vast range of theories, and that it behaves well
under causal propagation and taking mixtures. In addition we show that our
modular l^p-condition holds for every quasi-free Hadamard state of a free
scalar quantum field (regardless of mass or scalar curvature coupling).
However, our condition is not equivalent to the Hadamard condition.Comment: 42 page