The Physics of Cosmic Acceleration

TL;DR

This paper reviews the evidence for cosmic acceleration and surveys various theoretical explanations, including the cosmological constant, quintessence, and modified gravity theories, along with potential observational tests to distinguish them.

Contribution

It provides a comprehensive overview of the current theoretical landscape and discusses experimental approaches to differentiate among competing models of cosmic acceleration.

Findings

Evidence supports cosmic acceleration from multiple observations.

Various theories like dark energy and modified gravity are viable explanations.

Future tests could help discriminate between models.

Abstract

The discovery that the cosmic expansion is accelerating has been followed by an intense theoretical and experimental response in physics and astronomy. The discovery implies that our most basic notions about how gravity work are violated on cosmological distance scales. One simple fix is the introduction of a cosmological constant into the field equations for general relativity. However, the extremely small value of the cosmological constant, relative to theoretical expectations, has led theorists to explore a wide variety of alternative explanations that involve the introduction of an exotic negative-pressure fluid or a modification of general relativity. Here we briefly review the evidence for cosmic acceleration. We then survey some of the theoretical attempts to account for it, including the cosmological constant, quintessence and its variants, mass-varying neutrinos, and modifications of general relativity, such as scalar-tensor and $f(R)$ theories and braneworld scenarios. We discuss experimental and observational tests that may allow us to distinguish between some of the theoretical ideas that have been put forward.