Dynamic model of spherical perturbations in the Friedmann universe. II.   retarding solutions for the ultrarelativistic equation of state
  Exact linear retarding spherically symmetric solutions of Einstein equations linearized around Friedmann background for the ultrarelativistic equation of state are obtained and investigated. Uniqueness of the solutions in the $C^{1} $ class is proved. 
Yu.G. Ignatyev, N. Elmakhi
gr-qc math-ph math.MP