Parametrized Post-Friedmann Signatures of Acceleration in the CMB

TL;DR

This paper develops a generalized parametrized post-Friedmann framework to analyze how cosmic acceleration, including modified gravity, affects the CMB, providing explicit gauge expressions and constraints from WMAP data.

Contribution

It extends the covariant parametrized post-Friedmann approach to include matter, radiation, and curvature, enabling detailed CMB perturbation analysis in modified gravity models.

Findings

Modified gravity can reduce low multipole power in the CMB.

Explicit gauge expressions facilitate Einstein-Boltzmann code adaptation.

WMAP data constrains primary metric ratio parameters.

Abstract

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. Explicit expressions in the comoving, Newtonian and synchronous gauges facilitate the adaptation of Einstein-Boltzmann codes for solving CMB and matter perturbations in the linear regime. Using a comoving gauge code, we study the effect of metric evolution on the CMB through the integrated Sachs-Wolfe effect. Modified gravity can 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 WMAP on modified gravity models of the acceleration.