  We describe how causality and energy-momentum conservation constrain the
superhorizon behaviour of perturbation variables in a general FRW spacetime.
The effect of intrinsic curvature upon the horizon scale is discussed and
`white noise' results for the power spectra of generic `causally-seeded'
superhorizon perturbations are obtained. We present a detailed derivation of
the superhorizon energy density power spectrum for curved universes, and
provide elegant mathematical arguments for a curved universe analogue of the
Bessel function addition theorem. This yields physical insight into both the
familiar $k^4$ result for flat space and the $K \not= 0$ case: curvature
effects are seen to be evident from as early as half the curvature scale.
  We consider the implications of these results for the physically and
numerically significant issue of setting consistent initial conditions.
