Gauge-invariant definition of gravitational radiation in Robertson-Walker cosmologies

Gauge-invariant quantities are introduced which relate directly to the ideal fluid perturbations in Robertson-Walker backgrounds. It is shown how the vanishing of these gauge invariants is equivalent to the definition of purely radiative perturbations found in some early works. If one imposes a regularity condition, then the vanishing of just four gauge invariants is sufficient to define the condition of no perturbation of matter. The latter result is used to clarify the relationship to Hawking's definition of gravitational radiation