Cosmological Tests of Gravity

TL;DR

This review discusses modified gravity theories as alternatives to dark energy, focusing on their theoretical frameworks, screening mechanisms, and the current and future experimental tests across various cosmic scales.

Contribution

It provides a comprehensive classification of modified gravity models, details their theoretical features, and summarizes the status and prospects of experimental tests on multiple scales.

Findings

High precision tests on laboratory and solar system scales confirm general relativity.

Astrophysical tests on galaxy and large-scale structures are in early stages but promising.

Future observations will improve constraints on modified gravity models.

Abstract

Modifications of general relativity provide an alternative explanation to dark energy for the observed acceleration of the universe. We review recent developments in modified gravity theories, focusing on higher dimensional approaches and chameleon/f(R) theories. We classify these models in terms of the screening mechanisms that enable such theories to approach general relativity on small scales (and thus satisfy solar system constraints). We describe general features of the modified Friedman equation in such theories. The second half of this review describes experimental tests of gravity in light of the new theoretical approaches. We summarize the high precision tests of gravity on laboratory and solar system scales. We describe in some detail tests on astrophysical scales ranging from ~kpc (galaxy scales) to ~Gpc (large-scale structure). These tests rely on the growth and inter-relationship of perturbations in the metric potentials, density and velocity fields which can be measured using gravitational lensing, galaxy cluster abundances, galaxy clustering and the Integrated Sachs-Wolfe effect. A robust way to interpret observations is by constraining effective parameters, such as the ratio of the two metric potentials. Currently tests of gravity on astrophysical scales are in the early stages --- we summarize these tests and discuss the interesting prospects for new tests in the coming decade.