Cosmological Principle

The cosmological principle is the working assumption in cosmology that, on sufficiently large scales, the universe is homogeneous and isotropic, meaning its large-scale structure looks essentially the same from every location and in every direction, with no special or privileged observing position. Albert Einstein applied general relativity to the universe as a whole under this simplifying assumption in 1917, and the principle was later named and articulated more explicitly by astronomer Edward Milne in the 1930s. The assumption underlies the Friedmann-Lemaitre-Robertson-Walker models that form the mathematical backbone of standard Big Bang cosmology, since a universe that is homogeneous and isotropic can be described by a single, relatively simple expansion history. Observational support includes the remarkable uniformity of the cosmic microwave background radiation across the sky and the large-scale distribution of galaxies, once averaged over sufficiently large volumes.