The steady-state theory, a cosmology theory proposed by Thomas Gold, Herman Bondi, and Fred Hoyle in 1948, is based on the
idea that the universe is infinite in space and time. According to the theory, Hubble's constant does not change with time. The density of galaxies in the universe also remains unchanged, and the expansion of the universe is ascribed to a repulsive force. New matter supposedly is being
created continuously, so that the density of galaxies remains the same despite their mutual recession. Within an average volume equivalent to the volume occupied by a galaxy, a new one is created once every 12 billion years. This corresponds to a
creation rate, roughly, of one hydrogen atom per cubic kilometer per year. Such a rate is too small to be observed directly.The steady-state theory was a milestone in the development of modern cosmology, but it has been discarded for the big bang theory by most cosmologists because it cannot reasonably account for the 2.7 K microwave background radiation discovered in 1965. It is still supported by Hoyle and others, however, in a modified form called the quasi-steady-state theory. This theory proposes an infinite universe maintained not by the continuous creation of matter but by sporadic bursts of creationÑsuch as the unexplained bursts of gamma rays observed in gamma-ray astronomy.