Dark Energy Versus Modified Gravity

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.