  Nonlocal models of cosmology might derive from graviton loop corrections to
the effective field equations from the epoch of primordial inflation. Although
the Schwinger-Keldysh formalism would automatically produce causal and
conserved effective field equations, the models so far proposed have been
purely phenomenological. Two techniques have been employed to generate causal
and conserved field equations: either varying an invariant nonlocal effective
action and then enforcing causality by the ad hoc replacement of any advanced
Green's function with its retarded counterpart, or else introducing causal
nonlocality into a general ansatz for the field equations and then enforcing
conservation. We point out here that the two techniques access very different
classes of models, and that neither one of them may represent what would
actually arise from fundamental theory.
