The Cosmological Constant and Dark Energy

TL;DR

The paper scrutinizes whether a cosmological constant or a slowly evolving dark-energy component explains cosmic acceleration within the Friedmann-Lemaitre framework. It surveys a broad set of observational probes—supernovae, CMB anisotropies, large-scale structure, galaxy clusters, and lensing—to constrain the matter density parameter $\Omega_{M0}$ and the dark-energy density $\Omega_{\Lambda0}$, finding consistent evidence for low $\Omega_{M0}$ (roughly $0.15-0.4$) and substantial dark energy (roughly $\Omega_{\Lambda0}\sim0.6-0.7$), though with systematic uncertainties. The discussion spans both a true cosmological constant scenario and dynamical dark-energy models, notably scalar-field quintessence with inverse-power-law potentials and the XCDM parametrization, highlighting the potential for future observations to reveal evolution in the dark-energy density. The work emphasizes cross-checks between independent cosmological tests, the role of inflation and CDM in shaping structure, and the ongoing challenge of connecting dark energy to fundamental physics, including possible high-energy theory embeddings. Overall, the paper argues that while LambdaCDM remains broadly consistent with current data, detecting dynamical dark energy or deviations in gravity would have profound implications for cosmology and fundamental physics.

Abstract

Physics invites the idea that space contains energy whose gravitational effect approximates that of Einstein's cosmological constant, Lambda; nowadays the concept is termed dark energy or quintessence. Physics also suggests the dark energy could be dynamical, allowing the arguably appealing picture that the dark energy density is evolving to its natural value, zero, and is small now because the expanding universe is old. This alleviates the classical problem of the curious energy scale of order a millielectronvolt associated with a constant Lambda. Dark energy may have been detected by recent advances in the cosmological tests. The tests establish a good scientific case for the context, in the relativistic Friedmann-Lemaitre model, including the gravitational inverse square law applied to the scales of cosmology. We have well-checked evidence that the mean mass density is not much more than one quarter of the critical Einstein-de Sitter value. The case for detection of dark energy is serious but not yet as convincing; we await more checks that may come out of work in progress. Planned observations might be capable of detecting evolution of the dark energy density; a positive result would be a considerable stimulus to attempts to understand the microphysics of dark energy. This review presents the basic physics and astronomy of the subject, reviews the history of ideas, assesses the state of the observational evidence, and comments on recent developments in the search for a fundamental theory.