Gauge in Cosmology (2023/11/14)
Why is there gauge freedom when analyzing cosmological perturbations? I think this lies in the fact that our background universe is homogeneous and isotropic, and density perturbations are perturbatively small. An observer in FLRW effectively experiences no gravity in the background level and thus has no preferred inertial frame background universe. The observer thus has the freedom to choose an inertial frame, as long as it doesn’t violate the perturbation limit, to describe his observable universe, and this introduces the gauge freedom.