Vacuum Energy: If Not Now, Then When?

TL;DR

The paper argues that current cosmological data favor a low matter density universe with a substantial, nonzero vacuum energy, yielding a nearly flat geometry and an accelerating expansion. It discusses both a cosmological constant and dynamical vacuum energy (quintessence) as viable explanations, with observational hints from SN Ia, cluster abundances, and age dating supporting $\\Omega_m\\sim0.3-0.4$ and $\\Omega_X\\sim0.6-0.7$. The authors emphasize that inflation does not uniquely fix $\\Omega_m$ or $\\Omega_X$, and frame the cosmic coincidence problem within an anthropic context, suggesting that the observed ratio $\\Omega_\\Lambda/\\Omega_m$ is not surprising given structure formation constraints. They project a future dominated by vacuum energy, possibly driven by a rolling scalar field, and highlight the need to understand the nature of scalar fields and their implications for fundamental physics and cosmology.

Abstract

We review the cosmological evidence for a low matter density universe and a cosmological constant or dynamical vacuum energy and address the cosmolog$ coincidence problem: why is the matter density about one-half the vacuum energy {\em now}. This is reasonble, following the anthropic argument of Efstathiou and of Martel, Schapiro & Weinberg.