The Cosmological Constant

TL;DR

The article surveys the cosmological constant as vacuum energy, its historical context, observational constraints, and theoretical issues. It explains how a nonzero Lambda modifies expansion, distances, and structure formation, and reviews SN Ia, CMB, and lensing constraints that together indicate a flat, Lambda-dominated universe with Omega_M around 0.3 and Omega_Lambda around 0.7. A significant portion discusses the cosmological constant problem and various proposed resolutions, including SUSY, string theory landscapes, anthropic reasoning, and dynamical dark energy. The work underscores that while Lambda provides a compelling fit to current data, its extremely small value remains a profound puzzle requiring new physics or deeper principles.

Abstract

This is a review of the physics and cosmology of the cosmological constant. Focusing on recent developments, I present a pedagogical overview of cosmology in the presence of a cosmological constant, observational constraints on its magnitude, and the physics of a small (and potentially nonzero) vacuum energy.

Figures (11)

• Figure 1: Dynamics for $\Omega = \Omega_{\rm M} + \Omega_\Lambda$. The arrows indicate the direction of evolution of the parameters in an expanding universe.
• Figure 2: Expansion histories for different values of $\Omega_{\rm M}$ and $\Omega_\Lambda$. From top to bottom, the curves describe $(\Omega_{\rm M}, \Omega_\Lambda) = (0.3, 0.7)$, $(0.3, 0.0)$, $(1.0, 0.0)$, and $(4.0, 0.0)$.
• Figure 3: Hubble diagram (distance modulus vs. redshift) from the High-Z Supernova Team riess1. The lines represent predictions from the cosmological models with the specified parameters. The lower plot indicates the difference between observed distance modulus and that predicted in an open-universe model.
• Figure 4: Hubble diagram from the Supernova Cosmology Project perlmutter3. The bottom plot shows the number of standard deviations of each point from the best-fit curve.
• Figure 5: Constraints in the $\Omega_{\rm M}$-$\Omega_\Lambda$ plane from the High-Z Supernova Team riess1.