  A detailed analysis of gravitational slip, a new post-general relativity
cosmological parameter characterizing the degree of departure of the laws of
gravitation from general relativity on cosmological scales, is presented. This
phenomenological approach assumes that cosmic acceleration is due to new
gravitational effects; the amount of spacetime curvature produced per unit mass
is changed in such a way that a universe containing only matter and radiation
begins to accelerate as if under the influence of a cosmological constant.
Changes in the law of gravitation are further manifest in the behavior of the
inhomogeneous gravitational field, as reflected in the cosmic microwave
background, weak lensing, and evolution of large-scale structure. The new
parameter, $\varpi_0$, is naively expected to be of order unity. However, a
multiparameter analysis, allowing for variation of all the standard
cosmological parameters, finds that $\varpi_0 = 0.09^{+0.74}_{-0.59} (2\sigma)$
where $\varpi_0=0$ corresponds to a $\Lambda$CDM universe under general
relativity. Future probes of the cosmic microwave background (Planck) and
large-scale structure (Euclid) may improve the limits by a factor of four.
