The Cosmological Principle

The cosmological principle states that, when viewed on a large enough scale, the universe is homogeneous and isotropic, and the laws of physics are universal.Homogeneous means that matter is distributed uniformly throughout the universe. Any large region of space (hundreds of megaparsecs across) contains roughly the same number of galaxies, and the same density of matter, as any other large region.Isotropic means that the universe looks the same in every direction. An observer sees the same large-scale distribution of galaxies whichever way they look.Universal laws: the same laws of physics, such as gravitation and electromagnetism, apply everywhere in the universe.On small scales the universe is clearly not uniform. Matter is clumped into stars, galaxies and clusters of galaxies, with huge empty voids between them. The principle only applies on the very largest scales.An important consequence is that the universe has no centre and no edge. If it had a centre, it would look different from different places, so it would not be homogeneous. Our position in the universe is not special: observers in any other galaxy would see the same overall picture that we see.The cosmological principle is supported by observations such as the even distribution of galaxies in large surveys and the very uniform cosmic microwave background. It underpins the models of the expanding universe, such as Hubble's law and the Big Bang theory.