Dark Energy Versus Modified Gravity

Abstract

Understanding the reason for the observed accelerated expansion of the Universe represents one of the fundamental open questions in physics. In cosmology, a classification has emerged among physical models for this acceleration, distinguishing between dark energy and modified gravity. In this review, we provide a brief overview of models in both categories as well as their phenomenology and characteristic observable signatures in cosmology. We also introduce a rigorous distinction between dark energy and modified gravity based on the strong and weak equivalence principles. Expected final online publication date for the Annual Review of Nuclear and Particle Science Volume 66 is October 19, 2016. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.