Cosmological Dark Energy: Prospects for a Dynamical Theory

TL;DR

This paper explores a dynamical approach to dark energy involving vacuum energy screening via quantum effects, proposing a model where dark energy naturally adjusts to the horizon scale without fine tuning.

Contribution

It introduces a new mechanism based on the quantum trace anomaly that allows the cosmological constant to relax dynamically to zero on large scales.

Findings

Trace anomaly leads to additional gravity terms consistent with the Equivalence Principle.

Scalar degrees of freedom from the anomaly can relax Lambda to zero.

Potential signatures in CMB fluctuations are discussed.

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.