  In General Relativity, the constraint equation relating metric and density
perturbations is inherently nonlinear, leading to an effective non-Gaussianity
in the dark matter density field on large scales - even if the primordial
metric perturbation is Gaussian. Intrinsic non-Gaussianity in the large-scale
dark matter overdensity in GR is real and physical. However, the variance
smoothed on a local physical scale is not correlated with the large-scale
curvature perturbation, so that there is no relativistic signature in the
galaxy bias when using the simplest model of bias. It is an open question
whether the observable mass proxies such as luminosity or weak lensing
correspond directly to the physical mass in the simple halo bias model. If not,
there may be observables that encode this relativistic signature.
