The model of a stationary state
Most of the cosmological models under consideration are based on the Copernican principle whereby, in a large scale, the Universe looks the same, regardless of the position of the observer. It implies that the cosmological parameters such as the pace of expansion of the Universe (Hubble’s constant), the chemical composition of matter, the average density of galaxies, the background radiation temperature and the mass distribution of stars turn out to be the same, regardless of location. This leads to the conclusion that in a sufficiently large scale (covering several clusters of galaxies) the Universe is isotropic (the same in all directions) and homogeneous (the same in all places). The symmetry of the Universe as expressed in the Copernican principle is very well confirmed by results of observations. Friedman's solutions of Einstein equations describing the expanding Universe also have such a symmetry.
An attempt was made to extend this principle to form a perfect Copernican principle. According to it, the Universe would look the same regardless not only of the position of an observer, but of the time (space age), during which the observations are conducted. In other words, the determined cosmological parameters were to be stable over time. The reason for such a test was only a commitment to the vision of the eternal, unchanging Universe.
Since the expansion of the Universe predicted by the general theory of relativity and confirmed by the discovery of receding galaxies by Hubble, denotes, e.g. a systematic decrease in the average density of matter in the Universe, in order to preserve its invariance it was assumed that there would be a continuous creation of matter. What is more, it would have to be the creation of hydrogen, deuterium, helium and lithium in proportion to their respective abundance in the Universe. This creation would not be detectable directly, since it would occur at a rate of one solar mass per year in a volume equivalent to the average volume occupied by a galaxy (or, in other words: one atom per one cubic meter per one billion years). This was the main postulate for the model of a stationary state proposed by Fred Hoyle in 1948 (1915-2001), Thomas Gold (1920-2004) and Hermann Bondi (1919-2005).
The stationary state model, struggling with many problems from its beginning, was overthrown by the observations of the cosmic microwave background radiation (along with subsequent observations of its temperature variation in the cosmic epochs) discovered in 1964, and by much evidence on the evolution of the brightness and chemical composition of entire populations of space objects (galaxies, quasars, large-scale structure).
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