Essential idea: The modern field of cosmology uses advanced experimental and observational techniques to collect data with an unprecedented degree of precision and as a result very surprising and detailed conclusions about the structure of the universe have been reached.
Understandings: The cosmological principle; Fluctuations in the CMB; The cosmological origin of redshift; Critical density
Applications and skills: Describing the cosmological principle and its role in models of the universe; Describing and interpreting the observed anisotropies in the CMB; Deriving critical density from Newtonian gravitation
Guidance: Students must be familiar with the main results of COBE, WMAP and the Planck space observatory
Data Booklet:
The cosmological principle, at its heart, says that there is nothing special about our view of the universe, at least on large enough scales. Everyone, anywhere in the universe, should look at the night sky and see about the same distribution of stars, galaxies, nebulae, supernovae, etc. More generally, the laws of physics apply equally to all parts of the universe. The speed of light, the half-life of radioactive elements, the values of other physical constants and the relationships between them should not change. As astronomer William Keel puts it, the universe is not cheating physicists. Without the cosmological principle it is difficult to do physics as the explanations we generate with our knowledge of physics could be wrong.
Criticisms of the cosmological principle are that it is an assumption on all scales, and by some metrics is plainly untrue. The biggest was in which our view of universe seems to be special is that it includes us. We have so far uncovered no evidence for intelligent alien life. If it exists it must be rare, which makes our perspective special in that regard.
This is a difficult concept but just as a 2D sheet of paper can be deformed in 3D space, so our 3D universe could be deformed in a similar way. Einstein's theory of relativity tells us that matter distorts space on small to medium scales, but it is possible that the universe is curve on very large scales. Different possible configurations are described in terms of their curvature. This is best thought of as a description of what happens to parallel lines (such as parallel rays of light). In a flat universe they stay parallel to infinity. In a universe with overall positive curvature they will eventually converge and in a universe with negative curvature they will eventually diverge. A positively curved universe is also implied to be finite in terms of volume, while flat and negative curved universes are infinite.
Current evidence, derived from measurements of unevenness (anisotropy) in the Cosmic Microwave Background is that our universe is probably flat.
Oxford Physics: pages 675 - 679
Hamper HL (2014): pages 562 - 564
Page 226 - 228