Local Group Velocity Versus Gravity: Nonlinear Effects

Abstract

We use numerical simulations to study the relation between the velocity of the Local Group and its gravitational acceleration. This relation serves as a test for the kinematic origin of the CMB dipole and as a method for estimating beta. We calculate the misalignment angle between the two vectors and compare it to the observed value for the PSCz survey. The latter value is near the upper limit of the 95% confidence interval for the angle; therefore, the nonlinear effects are unlikely to be responsible for the whole observed misalignment. We also study the relation between the amplitudes of the LG velocity and gravity vectors. In an Omega=1 Universe, the smoothed gravity of the LG turns out to be a biased low estimator of the LG (unsmoothed) velocity. In an Omega=0.3 Universe, the estimator is biased high. The discussed biases are, however, only a few per cent, thus the linear theory works to good accuracy. The gravity-based estimator of the LG velocity has also a scatter that limits the precision of the estimate of beta in the LG velocity-gravity comparisons. The random error of beta due to nonlinear effects amounts to several per cent.