The steady state theory is a cosmological model of the universe in which the universe is expanding, yet it is spatially identical to how it always has been and always will be. This is due to a constant state of matter creation in the universe, which allows the universe to expand yet create sufficient matter to ensure that at all times and in all directions the universe is the same. The sense of universal homogenization is often referred to as the perfect cosmological principal and is a key factor of the theory. The steady state theory, and similar variations, is often seen as the most likely alternative to the big bang theory and cosmological model.
During the 20th century, certain discoveries in the physical sciences led to the development of new cosmological theories such as the steady state theory. Prior to this time, most theories typically indicated that the universe was the same in every direction, and always had been the same and always would be the same. There was no sense of universal expansion, however, and so the size of the universe was thought to be a constant.
The steady state theory, however, stemmed from two major sources: Albert Einstein’s theory of general relativity, and observations by astronomer Edwin Hubble that indicated that the universe was expanding. These discoveries made the previous vision of a static universe scientifically unsound, and so new theories were needed to explain how the universe came to be what had been observed. The steady state theory and the big bang theory were among the two leading models of the universe, and in a certain way they are fairly opposing models.
According to the steady state theory, the universe is expanding but the distribution of matter throughout the universe is equal and constant. This sense of a homogenous universe is, perhaps, aesthetically appealing and logically ideal in a satisfying way. In order to account for equal distribution of matter in a universe that is expanding, however, new matter must be created to make up for the expansion of the universe.
The steady state theory typically attributes this creation to a “C-field” that not only creates new matter but also causes the expansion of the universe. One of the major flaws with this theory, one which lends strength to big bang theories, is that in a homogenous universe there should be an equal distribution of objects like quasars, which would be evidence of an old, steady universe. Such objects have only been observed at great distances, however, which support the idea that these older bodies in the universe have developed over time and distance, and not through steady and spontaneous creation. Though many physicists tend to view the big bang theory as the most likely model of the universe, there is still some defense of the steady state theory and similar concepts such as the quasi-steady state model.