Nonlinear Effects in the Amplitude of Cosmological Density Fluctuations

The amplitude of cosmological density fluctuations, σ8, has been studied and estimated by analysing many cosmological observations. The values of the estimates vary considerably between the various probes. However, different estimators probe the value of σ8 in different cosmological scales and do not take into account the nonlinear evolution of the parameter at late times. We relate the linear and nonlinear values of σ8 with a variety of cosmological parameters and for a variety of models for the mapping between the linear and nonlinear regimes. We show that estimates of the amplitude of cosmological density fluctuations derived from cosmic flows are systematically higher than those inferred at early epochs from the CMB because of nonlinear evolution at later times. We discuss the past and future evolution of linear and nonlinear perturbations, derive corrections to the value of σ8 and compare amplitudes after accounting for these differences. Subject headings: cosmic flows, cosmological parameters from LSS, cosmological parameters from CMBR, galaxy dynamics