The Cosmological Principle

On a large enough scale, the universe is both isotropic and homogeneous.

ISOTROPY: There is no preferred direction in space. (All directions are alike.)

HOMOGENEITY: One randomly-chosen large volume of the universe will have the same physical properties as another randomly-chosen large volume of the universe. (All places are alike.)

2-dimensional Examples of Isotropy and Homogeneity:

Isotropic Forest

Anisotropic Forest ("Trails" are preferred directions)

Which 2-d universes are homogeneous and isotropic?

Q: How Large is "Large Enough"??

A: 1.5 billion light years (or more!)

The universe is a big place. Distances are measured in light years (1 ly = 9.47x1012 km = 63,200 AU) or, even more commonly, in Megaparsecs (1 Mpc = 3.26x10 6 ly = 3.09x1019 km).

Is the cosmological principle nothing more than a convenient assumption without which we would be unable to make progress in studying the entire universe? NO!

Unsettling Consequence of the Cosmological Principle:

On to Defining the Universe or back to start.