Phase transitions in perturbed stiff fluid Friedmann-Robertson-Walker cosmological models

The authors consider one-soliton perturbations of a flat Friedmann-Robertson-Walker (FRW) cosmological model, with an ideal fluid with pressure equal to energy density (stiff fluid), in the case where the 'pole trajectory' parameter is negative, thereby introducing singularities along certain null hypersurfaces. Starting with a metric that approaches asymptotically the FRW background, they show that it is possible to construct extensions through these hypersurfaces such that the energy-momentum tensor T_mu nu is finite and satisfies the energy conditions. One of the extensions is C^infinity and displays a smooth transformation where the stiff fluid becomes 'tachyonic' (and vice versa), similar to one already discussed by Tabensky and Taub (1973). The other extension is only C¹, providing a sort of 'shock front', with continuity in T_mu nu , that has an associated 'null dust to stiff fluid phase transformation' of the form described by Chandrasekhar and Xanthopoulos (1986).