Cosmological Perturbations of Ultrarelativistic Plasmas

Scalar cosmological perturbations of a weakly self-interacting plasma mixed with a perfect radiation fluid are investigated. Effects of this plasma are considered through order $λ^{3/2}$ of perturbative thermal-field-theory in the radiation dominated universe. The breakdown of thermal perturbation theory at vastly subhorizon scales is circumvented by a Padé approximant solution. Compared to collisionless plasmas the phase speed and subhorizon damping of the plasma density perturbations are changed. An example for a self-interacting thermal field is provided by the neutrinos with effective 4-fermion interactions.

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