Dark Energy as a Relic of the Vacuum-Energy Cancellation?

TL;DR

This paper explores a modified gravity model with an exponential Lagrangian, proposing that dark energy may originate from vacuum-energy cancellation, and analyzes its implications for cosmic acceleration and deSitter solutions.

Contribution

It introduces a new exponential Lagrangian for gravity, linking dark energy to vacuum-energy cancellation and providing a scalar-tensor framework analysis.

Findings

DeSitter solutions exist only with negative vacuum-energy ratio.

Negative cosmological constant can induce accelerated deSitter expansion.

The model predicts non-Einsteinian effects in quantum regimes.

Abstract

We analyze the dynamical implications of an exponential Lagrangian density for the gravitational field, as referred to an isotropic FRW Universe. Then, we discuss the features of the generalized deSitter phase, predicted by the new Friedmann equation. The existence of a consistent deSitter solution arises only if the ratio between the vacuum-energy density and that associated with the fundamental length of the theory acquires a tantalizing negative character. This choice allows us to explain the present universe dark energy as a relic of the vacuum-energy cancellation due to the cosmological constant intrinsically contained in our scheme. The corresponding scalar-tensor description of the model is addressed too, and the behavior of the scalar field is analyzed for both negative and positive values of the cosmological term. In the first case, the Friedmann equation is studied both in vacuum and in presence of external matter, while, in the second case, the quantum regime is approached in the framework of ''repulsive'' properties of the gravitational interaction, as described in recent issues in Loop Quantum Cosmology. In particular, in the vacuum case, we find a pure non-Einsteinian effect, according to which a negative cosmological constant provides an accelerating deSitter dynamics, in the region where the series expansion of the exponential term does not hold.