7.9. Background and the Bumps on it and the Evolution of those Bumps
Equation 11 is our hot big bang description of the unperturbed Friedmann-Robertson-Walker universe. There are no bumps in it, no over-densities, no inhomogeneities, no anisotropies and no structure. The parameters in it are the background parameters. It describes the evolution of a perfectly homogeneous universe.
However, bumps are important. If there had been no bumps in the CMB thirteen billion years ago, no structure would exist today. The density bumps seen as the hot and cold spots in the CMB map have grown into gravitationally enhanced light-emitting over-densities known as galaxies (Fig. 7). Their gravitational growth depends on the cosmological parameters - much as tree growth depends on soil quality (see Efstathiou 1990 for the equations of evolution of the bumps). We measure the evolution of the bumps and from them we infer the background. Specifically, matching the power spectrum of the CMB (the C's which sample the z ~ 1000 universe) to the power spectrum of local galaxies (the P(k) which sample the z ~ 0 universe) we can constrain cosmological parameters. The limit on is an example.