Gravitational vacuum energy in our recently accelerating universe

TL;DR

This paper reviews cosmological observations and models explaining the universe's accelerated expansion, emphasizing the role of the cosmological constant and the challenges in distinguishing between dark energy and modified gravity.

Contribution

It clarifies the interpretation of vacuum energy in cosmology, discusses the minimal flat Lambda CDM model, and explores observational prospects to differentiate dark energy from dark gravity.

Findings

Flat Lambda CDM model fits current data well

Quantum vacuum effects do not directly measure vacuum energy

Future observations aim to distinguish dark energy from modified gravity

Abstract

We review current observations of the homogeneous cosmological expansion which, because they measure only kinematic variables, cannot determine the dynamics driving the recent accelerated expansion. The minimal fit to the data, the flat $\Lambda CDM$ model, consisting of cold dark matter and a cosmological constant, interprets $4\Lambda$ geometrically as a classical spacetime curvature constant of nature, avoiding any reference to quantum vacuum energy. (The observed Uehling and Casimir effects measure forces due to QED vacuum polarization, but not any quantum material vacuum energies.) An Extended Anthropic Principle, that Dark Energy and Dark Gravity be indistinguishable, selects out flat $\Lambda CDM$. Prospective cosmic shear and galaxy clustering observations of the growth of fluctuations are intended to test whether the 'dark energy' driving the recent cosmological acceleration is static or moderately dynamic. Even if dynamic, observational differences between an additional negative-pressure material component within general relativity (Dark Energy) and low-curvature modifications of general relativity (Dark Gravity) will be extremely small.