First order phase transitions are well-motivated and extensively studied
    sources of gravitational waves (GWs) from the early Universe. The vacuum energy
    released during such transitions is assumed to be transferred primarily either
    to the expanding walls of bubbles of true vacuum, whose collisions source GWs,
    or to the surrounding plasma, producing sound waves and turbulence, which act
    as GW sources. In this Letter, we study an alternative possibility that has so
    far not been considered: the released energy gets transferred primarily to
    feebly interacting particles that do not admit a fluid description but simply
    free-stream individually. We develop the formalism to study the production of
    GWs from such configurations, and demonstrate that such GW signals have
    qualitatively distinct characteristics compared to conventional sources and are
    potentially observable with near-future GW detectors.

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