Physical cosmology
= Physical cosmology
{wiki}
<Physical cosmology> connects the universe's history to the measurable behaviour of its contents. The <Friedmann equations> describe cosmic expansion; <atomic physics>, <radiation pressure> and <gravity> determine what that expanding universe looks like. A particularly revealing example is the route from <cosmological recombination> through the <photon-baryon fluid> to the <Cosmic microwave background acoustic peaks>. The universe supplies the <initial conditions> and the laboratory; the prediction must still pass through a model of the telescope before it can be compared with observations.
The practical reward is that one sky contains several different tests. The <Cosmic microwave background power spectrum> carries information about the <sound horizon>, the <cosmological baryon density> and cosmic geometry. These are related quantities with different observational signatures. <Angular sound-horizon degeneracy> explains why measuring a characteristic angle alone cannot count the universe's <baryons>.