Dynamically avoiding fine-tuning the cosmological constant: the "Relaxed Universe"

TL;DR

This paper presents a class of gravitational models that can dynamically relax a large initial cosmological constant to its small observed value without fine-tuning, using purely gravitational effects and potentially linking to early universe inflation.

Contribution

It introduces a new class of action functionals involving scalar curvature and Gauss-Bonnet invariant that achieve dynamical CC relaxation without scalar fields, connecting to standard cosmology.

Findings

Successfully mimics standard cosmological epochs

Provides a natural solution to the cosmological constant problem

Suggests a link between CC relaxation and primordial inflation

Abstract

We demonstrate that there exists a large class of action functionals of the scalar curvature and of the Gauss-Bonnet invariant which are able to relax dynamically a large cosmological constant (CC), whatever it be its starting value in the early universe. Hence, it is possible to understand, without fine-tuning, the very small current value of the CC as compared to its theoretically expected large value in quantum field theory and string theory. In our framework, this relaxation appears as a pure gravitational effect, where no ad hoc scalar fields are needed. The action involves a positive power of a characteristic mass parameter, M, whose value can be, interestingly enough, of the order of a typical particle physics mass of the Standard Model of the strong and electroweak interactions or extensions thereof, including the neutrino mass. The model universe emerging from this scenario (the "Relaxed Universe") falls within the class of the so-called LXCDM models of the cosmic evolution. Therefore, there is a "cosmon" entity X (represented by an effective object, not a field), which in this case is generated by the effective functional and is responsible for the dynamical adjustment of the cosmological constant. This model universe successfully mimics the essential past epochs of the standard (or "concordance") cosmological model (LCDM). Furthermore, it provides interesting clues to the coincidence problem and it may even connect naturally with primordial inflation.