Parametrized post-Friedmann signatures of acceleration in the CMB

We extend the covariant, parametrized post-Friedmann treatment of cosmic acceleration from modified gravity to an arbitrary admixture of matter, radiation, relativistic components, and spatial curvature. This generalization facilitates the adaptation of Einstein-Boltzmann codes for solving cosmic microwave background and matter perturbations in the linear regime. We use such a code to study the effect of metric evolution on the cosmic microwave background through the integrated Sachs-Wolfe effect. We discuss the ability of modified gravity to alter the low multipole spectrum, including lowering the power in the quadrupole. From a principal component description of the primary metric ratio parameter, we obtain general constraints from the Wilkinson Microwave Anisotropy Probe on modified gravity models of the acceleration.