The cosmological principle

The cosmological principle states that the universe looks the same time at each point in space and in all directions for great distances the same, and also (spatial) homogeneity mentioned, the assumption that in every direction like look, is called (spatial) isotropy . A look at the starry sky with the naked eye immediately shows that the universe is not in the immediate vicinity of the earth is homogeneous and isotropic, because there are irregularly distributed star. On a larger scale are the stars of galaxies, although they are also very unevenly distributed and galaxies form. On an even larger scale, a honeycomb-like structure is seen, which consists of so-called filaments and voids. Following are current (2011) observations, it is no larger structures such as honeycomb, so that for very large distances, the assumption of homogeneity and isotropy a good approximation.

Applying the cosmological principle to the general relativity theory to simplify the Einstein field equations to the so-called Friedmann equations. The Friedmann equations therefore describe a homogeneous, isotropic universe. To solve the equations will be made from the present state of the universe and followed the development backwards in time. The exact solution depends in particular on the measured values ​​of the Hubble constant and density of various parameters that describe the mass and energy content of the universe. One then finds that the universe was once smaller (expansion of the universe), while it was hotter and denser. Formally, the solution leads to a time when the value of the scale factor vanishes, so the universe had no expansion and the temperature and density are infinite. This time is called "Big Bang", he is a formal singularity of the solution of the Friedmann equations. This, however, no statement about the physical reality of such an initial singularity is done because the equations of classical physics have only a limited scope and no longer applicable when quantum effects play a role, as is assumed in the very early, hot and dense universe. To describe the evolution of the universe at very early times, a theory of quantum gravity is required.