SKEWNESS OF COSMIC MICROWAVE BACKGROUND TEMPERATURE-FLUCTUATIONS DUE TO NONLINEAR GRAVITATIONAL-INSTABILITY

Skewness of temperature fluctuations of the cosmic microwave background produced by initially Gaussian adiabatic perturbations with the flat (Harrison-Zeldovich) spectrum, which arises as a result of nonlinear corrections to a gravitational potential at the matter-dominated stage, is calculated quantitatively. For the standard cold dark matter model, the effect appears to be smaller than expected previously and lies below the cosmic variance limit even for small angles. The sign of the skewness is opposite to that of the skewness of density perturbations.