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 lambda(3/2) of perturbative therm al-field theory in the radiation dominated universe. The breakdown of thermal perturbation theory at vastly subhorizon scales is circumvente d by a Pade approximant solution. Compared to collisionless plasmas th e phase speed and subhorizon damping of the plasma density perturbatio ns are changed. An exampled for a self-interacting thermal field is pr ovided by the neutrinos with effective 4-fermion interactions.