Cosmological perturbation theory

Summary

Cosmological perturbation theory studies small departures from a homogeneous and isotropic universe. It writes the metric and the matter content as a Friedmann–Lemaître–Robertson–Walker background plus small perturbations. It then solves the Einstein field equations to first order in the perturbations. It is the basic tool to explain how galaxies and the anisotropies of the cosmic microwave background grew from tiny initial fluctuations. A main difficulty is the choice of coordinates, called the gauge, which changes the perturbations without changing the physics.