A Unified Treatment of Cosmological Perturbations from Super--Horizon to Small Scales

We study the evolution of cosmological perturbations, using a hybrid approximation scheme which upgrades the weak-field limit of Einstein’s field equations to account for post-Newtonian scalar and vector metric perturbations and for leading-order source terms of gravitational waves, while including also the first- and second-order perturbative approximations. Our equations, which are derived in the Poisson gauge, provide a unified description of matter inhomogeneities in a Universe filled with a pressureless and irrotational fluid and a cosmological constant, ranging from the linear to the highly nonlinear regime. The derived expressions for scalar, vector and tensor modes may have a wide range of cosmological applications, including secondary CMB anisotropy and polarization effects, cosmographic relations in a inhomogeneous Universe, gravitational lensing and the stochastic gravitational-wave backgrounds generated by nonlinear cosmic structures.