An outline of radiatively-driven cosmology
  A Big Bang universe consisting, before recombination, of H, D, 3He, 4He, 6Li, and 7Li ions, electrons, photons, and massless neutrinos, at closure density, with a galaxy-size perturbation spectrum but no large-scale structure, will evolve into the universe as we now observe it. Evolution during the first billion years is controlled by radiation. Globular clusters are formed by radiatively-driven implosions, galaxies are formed by radiatively triggered gravitational collapse of systems of globular clusters, and voids are formed by radiatively-driven expansion. After this period the strong radiation sources are exhausted and the universe has expanded to the point where further evolution is determined by gravity and universal expansion. 
Robert L. Kurucz (Smithsonian Astrophysical Observatory)