The Relaxed Universe: towards solving the cosmological constant problem dynamically from an effective action functional of gravity

TL;DR

The paper proposes a modified gravity model that dynamically relaxes a large cosmological constant, reproduces the universe's expansion history, and avoids fine-tuning, offering a preliminary solution to the cosmological constant problem.

Contribution

It introduces an F(R,G) gravity model that dynamically relaxes the cosmological constant without fine-tuning and aligns with observed cosmic expansion.

Findings

Large CC is relaxed dynamically in the model.

The model reproduces the universe's expansion history.

It alleviates the coincidence problem.

Abstract

We present an unconventional approach for addressing the old cosmological constant (CC) problem in a class of F(R,G) models of modified gravity. For a CC of arbitrary size and sign the corresponding cosmological evolution follows an expansion history which strikingly resembles that of our real universe. The effects of the large CC are relaxed dynamically and there is no fine-tuning at any stage. In addition, the relaxation mechanism alleviates the coincidence problem. The upshot is that a large cosmological constant and the observed cosmic expansion history coexist peacefully in the Relaxed Universe. This model universe can be thought of as an interesting preliminary solution of the cosmological constant problem, in the sense that it provides a successful dynamical mechanism able to completely avoid the fine-tuning problem (the toughest aspect of the CC problem). However, since the Relaxed Universe is formulated within the context of modified gravity, it may still suffer of some of the known issues associated with these theories, and therefore it can be viewed only as a toy-model proposal towards a final solution of the CC problem.