Collective Properties of Gravitational Perturbations in Thermal Environments on a Dynamical Background

A thermal plasma dresses soft gauge fields, turning them into collective modes with a plasmon gap and Landau damping. In this paper, we ask whether a thermal radiation plasma does the same to gravitational perturbations. For gravity, the thermal soft scale coincides with the Hubble rate. Any such gap therefore lies at the horizon scale, where the expansion of the background cannot be neglected. We model radiation by a conformal scalar and compute its linear response to metric perturbations directly on a flat FLRW background. We test this response against the diffeomorphism Ward identities. The adiabatic dispersion relation yields a gapped collective mode with ω p squared = 8 H 2/5, where H is the conformal Hubble rate. The imaginary part of its frequency, however, is dominated by the background expansion and does not represent Landau damping. In the radiation background, the tensor modes obey a real-time integro-differential equation that reproduces Weinberg's damping of gravitational waves. The initial-time boundary of the retarded response keeps superhorizon modes frozen until horizon reentry despite the gap. The superhorizon growth found in earlier analyses arises from discarding the memory of the retarded response and from a local-equilibrium state prescription that violates the Ward identities. More generally, the Ward identities forbid any local state response beyond the collisionless kernel.