Fr.: écoulement constant, ~ stationnaire
A flow in which the characterizing conditions, such as → streamlines or velocity at any given point, do not change with time.
Tacân, → flow; pâyâ "steady, constant," from pâyidan "to stand firm, to be constant, steady," from Mid.Pers. pattây-, pattutan "to last, endure, stay."
steady state theory
negare-ye hâlat-e pâyâ
Fr.: théorie de l'état stationnaire
A → cosmological model according to which the → Universe has no beginning and no end and maintains the same mean density, in spite of its observed expansion, by the continual creation of matter throughout all space. The theory was first put forward by Sir James Jeans in about 1920 and again in revised form in 1948 by Hermann Bondi and Thomas Gold. It was further developed by Sir Fred Hoyle to deal with problems that had arisen in connection with the alternative → Big Bang model. Observations since the 1950s have produced much evidence contradictory to the steady state theory and supportive of the Big Bang model. More specifically, the steady state theory attributed the → cosmic microwave background to → thermal radiation from → dust clouds, but this cannot account for a single → blackbody spectrum. Moreover, the steady state theory lacked a plausible mechanism for the creation of matter in space. See also → perfect cosmological principle.