Gravitational Slip in the Parameterized Post-Newtonian Cosmology
A key signature of general relativity is that the two scalar potentials $\Phi$ and $\Psi$, when expressed in the longitudinal gauge, are equal in the absence of fluids with anisotropic stress. This is often expressed by stating that their ratio, the "gravitational slip", is equal to unity. However, the equality of $\Phi$ and $\Psi$ is typically broken in alternative theories of gravity. Observational constraints on the slip parameter are therefore of direct interest for testing Einstein's theory. In this paper we derive theory-independent expressions for the slip parameter on both large and small scales in Friedmann cosmologies, expressing it as a function of the post-Newtonian parameters. This is the final ingredient required for a complete parameterization of dust and dark energy-dominated cosmologies within the framework of Parameterized Post-Newtonian Cosmology (PPNC), which allows for the fully self-consistent modelling of cosmological observables without assuming any specific theory of gravity.