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Incosmology, thesteady-state model orsteady-state theory was an alternative to theBig Bang theory. In the steady-state model, the density of matter in theexpanding universe remains unchanged due to a continuous creation of matter, thus adhering to theperfect cosmological principle, a principle that says that theobservable universe is always the same at any time and any place. Astatic universe, where space is not expanding, also obeys the perfect cosmological principle, but it cannot explain astronomical observations consistent with expansion of space.
From the 1940s to the 1960s, the astrophysical community was divided between supporters of the Big Bang theory and supporters of the steady-state theory. The steady-state model is now rejected by mostcosmologists,astrophysicists, andastronomers.[1]The observational evidence points to a hot Big Bang cosmology with a finiteage of the universe, which the steady-state model does not predict.[2]
Cosmological expansion was originally seen through observations byEdwin Hubble. Theoretical calculations also showed that thestatic universe, as modeled byAlbert Einstein (1917), was unstable. The modern Big Bang theory, first advanced by FatherGeorges Lemaître, is one in which the universe has a finite age and has evolved over time through cooling, expansion, and the formation of structures through gravitational collapse.
On the other hand, the steady-state model says while the universe is expanding, it nevertheless does not change its appearance over time (theperfect cosmological principle). E.g., the universe has no beginning and no end. This required that matter be continually created in order to keep the universe's density from decreasing. Influential papers on the topic of a steady-state cosmology were published byHermann Bondi,Thomas Gold, andFred Hoyle in 1948.[3][4] Similar models had been proposed earlier byWilliam Duncan MacMillan, among others.[5]
It is now known that Albert Einstein considered a steady-state model of the expanding universe, as indicated in a 1931 manuscript, many years before Hoyle, Bondi and Gold. However, Einstein abandoned the idea.[6]
Problems with the steady-state model began to emerge in the 1950s and 60s – observations supported the idea that the universe was in fact changing. Bright radio sources (quasars andradio galaxies) were found only at large distances (therefore could have existed only in the distant past due to the effects of thespeed of light on astronomy), not in closer galaxies. Whereas the Big Bang theory predicted as much, the steady-state model predicted that such objects would be found throughout the universe, including close to our own galaxy. By 1961, statistical tests based on radio-source surveys[7] provided strong evidence against the steady-state model. Some proponents likeHalton Arp insist that the radio data were suspect.[1]: 384
Gold and Hoyle (1959)[8] considered that matter that is newly created exists in a region that is denser than the average density of the universe. This matter then may radiate and cool faster than the surrounding regions, resulting in a pressure gradient. This gradient would push matter into an over-dense region and result in a thermal instability and emit a large amount of plasma. However, Gould and Burbidge (1963)[9]realized that the thermalbremsstrahlung radiation emitted by such a plasma would exceed the amount of observedX-rays. Therefore, in the steady-state cosmological model, thermal instability does not appear to be important in the formation of galaxy-sized masses.[10]
In 1964 thecosmic microwave background radiation was discovered as predicted by the Big Bang theory. The steady-state model attempted to explain the 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 be generated by numerous point sources, and the microwave background radiation does not show the polarization characteristic of scattering. Furthermore, its spectrum is so close to that of an idealblack 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 differentredshifts.Steven Weinberg wrote in 1972:
"The steady state model does not appear to agree with the observeddL versusz relation or withsource 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 itcan 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."[11]
Since this discovery, the Big Bang theory has been considered to provide the best explanation of the origin of the universe. In mostastrophysical publications, the Big Bang is implicitly accepted and is used as the basis of more complete theories.[12]: 388
Quasi-steady-state cosmology (QSS) was proposed in 1993 by Fred Hoyle,Geoffrey Burbidge, andJayant V. Narlikar as a new incarnation of the steady-state ideas meant to explain additional features unaccounted for in the initial proposal. The model suggests pockets of creation occurring over time within the universe, sometimes referred to asminibangs,mini-creation events, orlittle bangs.[13] After the observation of anaccelerating universe, further modifications of the model were made.[14] The Planck particle is a hypotheticalblack hole whoseSchwarzschild radius is approximately the same as itsCompton wavelength; the evaporation of such a particle has been evoked as the source of light elements in an expanding steady-state universe.[15]
Astrophysicist andcosmologistNed Wright has pointed out flaws in the model.[16] These first comments were soon rebutted by the proponents.[17] Wright and other mainstream cosmologists reviewing QSS have pointed out new flaws and discrepancies with observations left unexplained by proponents.[18]
[T]he Steady State theorists' ideas are largely discredited today...
the Chicago astronomer William MacMillan not only assumed that stars and galaxies were distributed uniformly throughout infinite space, he also denied 'that the universe as a whole has ever been or ever will be essentially different from what it is today.'
