Dark Energy and the Accelerating Universe

TL;DR

This paper reviews the observational evidence for the universe's accelerating expansion, explores dark energy theories, and discusses future observational strategies to understand this cosmic mystery.

Contribution

It provides a comprehensive overview of current evidence, theoretical models, and future probes related to dark energy and cosmic acceleration.

Findings

Cosmic acceleration is well-supported by multiple observations.

Dark energy constitutes about 76% of the universe's energy content.

Various theories, including vacuum energy and modified gravity, aim to explain acceleration.

Abstract

The discovery ten years ago that the expansion of the Universe is accelerating put in place the last major building block of the present cosmological model, in which the Universe is composed of 4% baryons, 20% dark matter, and 76% dark energy. At the same time, it posed one of the most profound mysteries in all of science, with deep connections to both astrophysics and particle physics. Cosmic acceleration could arise from the repulsive gravity of dark energy -- for example, the quantum energy of the vacuum -- or it may signal that General Relativity breaks down on cosmological scales and must be replaced. We review the present observational evidence for cosmic acceleration and what it has revealed about dark energy, discuss the various theoretical ideas that have been proposed to explain acceleration, and describe the key observational probes that will shed light on this enigma in the coming years.