Despite the observations of clustering and the obvious importance of the existence of clumpiness to our existence as human beings, cosmologists usually rely on a simplifying assumption known as the Cosmological Principle. It states:
On the largest cosmic scales, the Universe is both homogeneous and isotropic.
Homogeneity means that there is no preferred location in the Universe. That is, no matter where you are in the Universe, if you look at the Universe, it will look the same.
Isotropy means that there is no preferred direction in the Universe. That is, from your current location, no matter which direction you look, the Universe will look the same.
Results from redshift surveys (e.g., SDSS or 2dF) of the distribution of relatively nearby galaxies seem to imply that the Universe isn't homogeneous and isotropic. In other words, the galaxies in one direction are not distributed in exactly the same way as the galaxies in another direction. However, the galaxies plotted in those diagrams only extend to a redshift of z < 0.2, which is equivalent to a distance of about 750 Mpc. When we study the most distant objects we can find at much larger distances from Earth, the structure appears to smooth out and become more homogeneous on the largest scales. For example, the all-sky map of the locations of objects detected by radio telescopes shown below reveals a much more uniform appearance. These objects are mostly expected to lie at higher redshifts than the ones in the pie slice diagram above, suggesting that when we consider the largest distance scales, the Universe appears to be homogeneous and isotropic. Thus, we currently find support for the Cosmological Principle in the distribution of galaxies in the Universe.
© 2009-2014 Richard Gelderman
