Cosmic variance
= Cosmic variance
{wiki}
Cosmic variance is the sampling uncertainty caused by observing only one realization of a cosmological random field. For a Gaussian isotropic full-sky temperature field and an ideal noiseless estimate, $\operatorname{Var}(\widehat C_\ell)=2C_\ell^2/(2\ell+1)$ because there are only $2\ell+1$ independent real angular degrees of freedom.