cosmology - the branch of astrophysics that studies the origin and evolution and structure of the universebig bang theory, big-bang theory - (cosmology) the theory that the universe originated 20 billion years ago from the cataclysmic explosion of a small mass of matter at extremely high density and temperature nebular hypothesis - (cosmology) the theory that the solar system evolved from a hot gaseous nebula planetesimal hypothesis - (cosmology) the theory that the solar system was formed by the gravitational accumulation of planetesimals astrophysics - the branch of astronomy concerned with the physical and chemical properties of celestial bodies big bang - (cosmology) the cosmic explosion that is hypothesized to have marked the origin of the universe inflation - (cosmology) a brief exponential expansion of the universe (faster than the speed of light) postulated to have occurred shortly after the big bang closed universe - (cosmology) a universe in which there is sufficient matter to halt the expansion that began with the big bang and to cause the universe to collapse again; the visible matter is only 10 percent of the matter required for closure but there may be large amounts of dark matter Hubble constant, Hubble's constant - (cosmology) the ratio of the speed of recession of a galaxy (due to the expansion of the universe) to its distance from the observer; the reciprocal of the Hubble constant is the age of the universe ylem - (cosmology) the original matter that (according to the big bang theory) existed before the formation of the chemical elements dark matter - (cosmology) a hypothetical form of matter that is believed to make up 90 percent of the matter in the universe; it is invisible (does not absorb or emit light) and does not collide with atomic particles but exerts gravitational force |