3.6.1  Steady-State Universe

Before the prevailing Big Bang model, the Steady-State theory was popular in the 1920s, and it remained so for some time even after Hubble”s discovery. In this theory, the density of matter in the expanding Universe remains unchanged due to a continuous creation of matter, thus adhering to the perfect cosmological principle, which asserts that the observable Universe is basically the same at any time as well as at any place. This theory was developed further in 1948 by Hermann Bondi (1919-2005), Thomas Gold (1920-2004), and Fred Hoyle (1915-2001). Before that, in 1931, Einstein considered a Steady-State model of the expanding Universe, but he quickly abandoned the idea.

In the 1950s and 60s, many observations began to support the idea that the Universe was in fact changing, and bright radio sources, such as quasars and radio galaxies, were found only at large distances, indicating that early stages were denser and hotter than closer galaxies. In 1961, statistical tests based on radio-source surveys had ruled out the Steady-State model in the minds of most cosmologists, although some proponents insisted that the radio data were suspect. Furthermore, after the discovery of the cosmic microwave background radiation in 1964, which was predicted by the Big Bang theory, the Steady-State theory was refuted again. Hawking described this discovery as “the final nail in the coffin of the Steady-State theory”.

The Steady-State theory tried to explain microwave background radiation as the result of light from ancient stars that has been scattered by galactic dust. However, the cosmic microwave background level is very even in all directions, making it difficult to explain how it could have been generated by numerous point sources and the microwave background radiation shows no evidence of characteristics such as polarization that are normally associated with scattering. Furthermore, its spectrum is so close to that of an ideal black body that it could hardly be formed by the superposition of contributions from a multitude of dust clumps at different temperatures as well as at different redshifts.

In 1972, Steven Weinberg wrote: “The Steady-State model does not appear to agree with the observedversusrelation or with source counts ... In a sense, this disagreement is a credit to the model; alone among all cosmologies, the Steady-State model makes such definite predictions that it can be disproved even with the limited observational evidence at our disposal. The Steady-State model is so attractive that many of its adherents still retain hope that the evidence against it will eventually disappear as observations improve. However, if the cosmic microwave radiation ... is really black-body radiation, it will be difficult to doubt that the Universe has evolved from a hotter denser early stage.” Weinberg (1972)

Since this discovery, the Big Bang theory has been considered to provide the best explanation of the origin of the Universe. In most astrophysical publications, the Big Bang is implicitly accepted and is used as the basis of more complete theories.

In 1993, a variant of this theory, the Quasi-Steady-State cosmology, was proposed by Hoyle and others to explain additional features unaccounted for in the initial proposal. The theory suggests pockets of creation occurring over time within the Universe, sometimes referred to as minibangs, mini-creation events, or little bangs. Other modifications were also made after the observation of an accelerating Universe in 1998.