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We point out, the scalar sector of gravitational perturbations may be excitedby an isolated astrophysical system immersed in a universe whose acceleratedexpansion is not due to the cosmological constant, but due to extra fielddegrees of freedom. This is true even if the source of gravitational radiationdid not couple directly to these additional fields. We illustrate this byconsidering a universe driven by a single canonical scalar field. By workingwithin the gauge-invariant formalism, we solve for the electric components ofthe linearised Weyl tensor to demonstrate that both the gravitational masslessspin-2 (transverse-traceless) tensor and the (Bardeen) scalar modes aregenerated by a generic astrophysical source. For concreteness, the Dark Energyscalar field is either released from rest, or allowed to asymptote to theminimum in a certain class of potentials; and we compute the traceless tidalforces induced by gravitational radiation from a hypothetical compact binarysystem residing in such a universe. Though their magnitudes are very smallcompared to the tensors', spin zero gravitational waves in such a canonicalscalar driven universe are directly sensitive to both the Dark Energy equationof state and the eccentricity of the binary's orbit.

Scalar Gravitational Waves Can Be Generated Even Without Direct Coupling Between Dark Energy and Ordinary Matter