The formation of shock waves in the recombination era of the Universe

The formation and evolution of shock waves in a two-component fluid during the recombination era of the universe is studied. The fluid consists of photons and ionized hydrogen gas with interaction due to Thomson scattering. The analysis uses the expansion procedure in powers of the coordinates near the traveling wave front. Incorporating discontinuities in the first derivatives of the physical quantities, an analytical solution is obtained which answers the questions as to when the discontinuities appear in the physical quantities themselves. Of the two possible waves (a radiation-dominated and a matter-dominated wave) only the matter-dominated wave shows the shock phenomena. However, the shock is induced by the radiation due to Thomson scattering. Therefore, the time for the shock formation depends on the initial amplitude of the wave (as in usual shock phenomena) and on the collision frequency of the photons.