Steady-State Theory

The steady-state model is based on a set of four assumptions collectively known as the perfect cosmological principle. The first assumption is that physical laws are universal. Any science experiment, if performed under identical conditions, will have the same result anywhere in the universe because physical laws are the same everywhere in the universe. Second, on a sufficiently large scale the universe is homogeneous. We know there is large scale structure in the universe, such as clusters of galaxies; so, we assume that the universe is homogenous only on scales large enough for even the largest structures to average out. Third, we assume that the universe is isotropic, meaning that there is no preferred direction in the universe. Fourth, we assume that over sufficiently long times the universe looks essentially the same at all times.

Collectively, the first three of the above assumptions are the cosmological principle (not to be confused with the perfect cosmological principle). In a nutshell, the cosmological principle states that the universe looks essentially the same at any location in the universe. This principle remains a largely untested assumption because we cannot travel to every location in the universe to perform experiments to test the assumption. Adding the fourth assumption, that the universe does not change on the large scale with time, gives us the perfect cosmological principle. Essentially, the universe looks the same at all times as well as at all locations within the universe. The perfect cosmological principle forms the philosophical foundation for the steady-state theory. With the addition of the fourth assumption, the universe does not evolve in the steady-state theory, so observational evidence that the universe evolves would be evidence against the steady-state theory.