Covariant and gauge-invariant approach to cosmological density fluctuations.

摘要: It has been known for a long time that the gauge problem plagues study of density perturbations in cosmology. The quantity {delta}{mu}/{mu} (the fractional variation along world line) usually determined perturbation calculations is completely dependent on chosen. Even fully covariant approach Hawking (1966) not immune. Bardeen's major paper (1980) determines set gauge-invariant quantities are related to but those themselves. We give simple alternative representation fluctuations. This both and invariant; thus it sidesteps usual problems. basic used represent inhomogeneities {ital comoving} fractional} gradient} of} the} energy} orthogonal} to} fluid} flow}. Our description does make assumption this gradient small. Exact (fully nonlinear) propagation equations derived quantity. They then linearized appropriate case an almost-Robertson-Walker universe. Their solutions obtained which can be compared with standard theory; we recover growing decaying modes. Thus resultmore » standard, its derivation transparent. interpretation Bardeen variables terms our formalism.« less