Cosmological Dark Energy: Prospects for a Dynamical Theory

TL;DR

The paper addresses the cosmological constant problem by proposing a dynamical relaxation of $\Lambda$ driven by infrared fluctuations tied to the quantum trace anomaly and the conformal factor of the metric. Using an infrared-dominant conformal-sector effective field theory, it derives a non-local anomaly action $S_{anom}$ that can be localized with auxiliary scalar fields, predicting an infrared fixed point in which the dimensionless cosmological term $\lambda$ flows to zero with increasing volume and suggesting horizon-scale signatures in the CMB via conformal invariance. It further argues that conformal fluctuations induce deviations from the Harrison–Zel'dovich spectrum and may generate non-Gaussianities, providing observable tests. The authors propose a horizon-boundary dark energy scenario, akin to a gravastar, where the interior de Sitter region dynamically adjusts to boundary conditions at the cosmological horizon, offering a natural mechanism to explain the observed $\Omega_{\Lambda}\approx 0.74$ without fine-tuning, while highlighting the need for a fully dynamical cosmological model and confrontation with data.

Abstract

We present an approach to the problem of vacuum energy in cosmology, based on dynamical screening of Lambda on the horizon scale. We review first the physical basis of vacuum energy as a phenomenon connected with macroscopic boundary conditions, and the origin of the idea of its screening by particle creation and vacuum polarization effects. We discuss next the relevance of the quantum trace anomaly to this issue. The trace anomaly implies additional terms in the low energy effective theory of gravity, which amounts to a non-trivial modification of the classical Einstein theory, fully consistent with the Equivalence Principle. We show that the new dynamical degrees of freedom the anomaly contains provide a natural mechanism for relaxing Lambda to zero on cosmological scales. We consider possible signatures of the restoration of conformal invariance predicted by the fluctuations of these new scalar degrees of freedom on the spectrum and statistics of the CMB, in light of the latest bounds from WMAP. Finally we assess the prospects for a new cosmological model in which the dark energy adjusts itself dynamically to the cosmological horizon boundary, and therefore remains naturally of order H^2 at all times without fine tuning.