Cosmological Principle

If one wants to use General Relativity to make a model of the Universe, one has to make an assumption about how matter in the universe is distributed. The simplest one you can make is that the Universe appears roughly the same everywhere and in every direction. That is, the matter in the universe is homogeneous and isotropic when averaged.

This is called the Cosmological Principle.

Astronomers make certain assumptions when they study the universe as a whole. These assumptions may be difficult to prove or verify in practice, but they form an essential starting point for cosmology. The first is the idea that the laws of physics can be applied across the universe. It is a very bold assumption, because our laws of physics are only determined precisely in laboratories on Earth and they may not apply exactly over all time and space. Hubble had to assume that Cepheid variables always worked the same way in order to demonstrate that many of the nebulae were distant galaxies. Astronomers are quite confident that physics is not wildly different elsewhere in the universe. We see the same types of stars and galaxies everywhere we look. We see spectral lines from the same elements billions of light years away that we do in nearby stars. These observations lend support to the

Viewed up close, a beach consists of grains of sand and shells and pebbles of many different sizes. From afar, all we see is a beach. The universe is isotropic if it looks the same in all directions. In other words, no observation can be made that will identify an edge or a center. The concept of isotropy is supported by the fact that galaxies do not bunch up in any direction in the sky and by the fact that we observe the same Hubble relation in different directions in the sky. Large telescopes have been used to count faint and distant galaxies in different direction and the numbers are always statistically the