in cosmology, a view that the universe is always expanding but maintaining a constant average density, matter being continuously created to form new stars and galaxies at the same rate that old ones become unobservable as a consequence of their increasing distance and velocity of recession. A steady-state universe has no beginning or end in time; and from any point within it the view on the grand scale—i.e., the average density and arrangement of galaxies—is the same. Galaxies of all possible ages are intermingled.

The theory was first put forward by Sir James Jeans (Jeans, Sir James) in about 1920 and again in revised form in 1948 by Hermann Bondi (Bondi, Sir Hermann) and Thomas Gold (Gold, Thomas). It was further developed by Sir Fred Hoyle (Hoyle, Sir Fred) to deal with problems that had arisen in connection with the alternative big-bang hypothesis. Observations since the 1950s have produced much evidence contradictory to the steady-state picture and supportive of the big-bang model (q.v.).