The Cosmological Principle
On a large enough scale, the universe is both
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:
- surface of the "cue" ball used for playing pool (billiards)
- infinite forest of identical trees
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!
- Maps of the locations of galaxies in the universe
- Cosmic Microwave Background Radiation (CMBR)
Unsettling Consequence of the Cosmological Principle:
- There can be NO center to the universe! (e.g.,
there is no "center" to the surface
of an unmarked sphere)
On to Defining the Universe or back to