Nonlinear Cosmological Power Spectra in Real and Redshift–Space

We present an expression for the nonlinear evolution of the cosmological power spectrum based on following Lagrangian trajectories. This is simplified using the Zel’dovich approximation to trace particle displacements, assuming Gaussian initial conditions. The model is found to exhibit the transfer of power from large to small scales expected in self-gravitating fields. Some exact solutions are found for power–law initial spectra. We have extended this analysis into redshift–space and found a solution for the nonlinear, anisotropic redshift–space power spectrum in the limit of plane–parallel redshift distortions. The quadrupole–to–monopole ratio is calculated for the case of power–law initial spectra. We find that the shape of this ratio depends on the shape of the initial spectrum, but when scaled to linear theory depends only weakly on the redshift–space distortion parameter, β. The point of zero–crossing of the quadrupole, k0, is found to obey a simple scaling relation and we calculate this scale in the Zel’dovich approximation. This model is found to be in good agreement with a series of N -body simulations on scales down to the zero-crossing of the quadrupole, although the wavenumber at zero-crossing is underestimated. These results are applied to the quadrupole–monopole ratio found in the merged QDOT plus 1.2 Jy IRAS redshift survey. Using a likelihood technique we have estimated that the distortion parameter is constrained to be β > 0.5 at the 95% level. The local primordial spectral slope is not well constrained, but the likelihood analysis suggests n ≈ −2 in the translinear regime. The zero–crossing scale of the quadrupole is k0 = 0.5 ± 0.1hMpc −1 and from this we infer the amplitude of clustering is σ8 = 0.7± 0.05. We suggest that the success of this model is due to nonlinear redshift–space effects arising from infall onto caustics and is not dominated by virialised cluster cores. The latter should start to dominate on scales below the zero–crossing of the quadrupole, where our model breaks down.