Cosmic Acceleration, Dark Energy and Fundamental Physics

TL;DR

This paper reviews the evidence for cosmic acceleration caused by dark energy, discusses various theoretical explanations including quantum vacuum energy and modifications to gravity, and highlights upcoming tests to understand its nature.

Contribution

It provides a comprehensive overview of dark energy theories, observational evidence, and future experiments to distinguish between different models of cosmic acceleration.

Findings

Universe's expansion is accelerating due to dark energy

Quantum vacuum energy estimates are vastly larger than observed

Upcoming experiments will test dark energy's nature and gravity theories

Abstract

A web of interlocking observations has established that the expansion of the Universe is speeding up and not slowing, revealing the presence of some form of repulsive gravity. Within the context of general relativity the cause of cosmic acceleration is a highly elastic (p\sim -rho), very smooth form of energy called ``dark energy'' accounting for about 75% of the Universe. The ``simplest'' explanation for dark energy is the zero-point energy density associated with the quantum vacuum; however, all estimates for its value are many orders-of-magnitude too large. Other ideas for dark energy include a very light scalar field or a tangled network of topological defects. An alternate explanation invokes gravitational physics beyond general relativity. Observations and experiments underway and more precise cosmological measurements and laboratory experiments planned for the next decade will test whether or not dark energy is the quantum energy of the vacuum or something more exotic, and whether or not general relativity can self consistently explain cosmic acceleration. Dark energy is the most conspicuous example of physics beyond the standard model and perhaps the most profound mystery in all of science.