Dynamics of cosmological perturbations in thermal λφ⁴ theory

Using a recent thermal-field-theory approach to cosmological perturbations, the exact solutions that were found for collisionless ultrarelativistic matter are generalized to include the effects from weak self-interactions in a λφ⁴ model through order λ^3/2. This includes the effects of a resummation of thermal masses and associated nonlocal gravitational vertices, thus going far beyond classical kinetic theory. Explicit solutions for all the scalar, vector, and tensor modes are obtained for a radiation-dominated Einstein–de Sitter model containing a weakly interacting scalar plasma with or without the admixture of an independent component of perfect radiation fluid. © 1996 The American Physical Society.