Winds from protogalactic starbursts and quasars can drive shocks that heat, ionize, and enrich the intergalactic medium. The Sedov-Taylor solution for pointlike explosions adequately describes these blast waves early in their development, but as the time since the explosion (t — t_1) approaches the age of the universe (t), cosmological effects begin to alter the blast wave's structure and growth rate. This paper presents an analytical solution for adiabatic blast waves in an expanding universe, valid when the IGM is homogeneous and contains only a small fraction of the total mass density (Ω_(IGM < < Ω_0). In a flat universe, the solution applies until the age of the universe approaches t_1Ω_(IGM)~(-3/2), at which time the self-gravity of the matter associated with the shock compresses the shocked IGM into a thin shell. When Ω_(IGM) approx < 0.03, blast waves starting after z ～ 7 and containing more than 10~(57) ergs remain adiabatic to relatively low z, so this solution applies over a wide range of the parameter space relevant to galaxy formation. Using this analytical solution, we examine the role protogalactic explosions might play in determining the state of intergalactic gas at z ～ 2-4. Since much of the initial energy in galaxy-scale blast waves is lost through cosmological effects, photoionization. by a protogalaxy is much more efficient than shock ionization. Shocking the entire IGM by z ～ 4, when it appears to be substantially ionized, is most easily done with small explosions (approx < 10~(56) ergs) having a high comoving number density (approx >1 Mpc~(-3)). Larger scale explosions could also fill the entire IGM, but if they did so, they would raise the mean metallicity of the IGM well above the levels observed in Lyα clouds. Since the metal abundances of Lyα clouds are.-small, the metals in these clouds were probably produced in small-scale bursts of star formation, rather than in large-scale explosions. The H I column densities of protogalactic blast waves are much smaller than those of typical Lyα clouds, but interactions between shocks and preexisting Lyα clouds can potentially amplify the neutral column densities of preexisting clouds by a large factor.
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