Relaxing a large cosmological constant

TL;DR

This paper proposes an extension to General Relativity with invariant terms that dynamically relax the cosmological constant, leading to a late-time de Sitter universe that resembles LCDM but with testable differences.

Contribution

It introduces a novel class of invariant terms in the field equations that automatically reduce the cosmological constant's value over cosmic time.

Findings

Models relax the initial vacuum energy to a small positive value.

The universe naturally evolves into an eternal de Sitter phase.

Predicted imprints distinguish these models from standard LCDM.

Abstract

The cosmological constant (CC) problem is the biggest enigma of theoretical physics ever. In recent times, it has been rephrased as the dark energy problem in order to encompass a wider spectrum of possibilities. It is, in any case, a polyhedric puzzle with many faces, including the cosmic coincidence problem, i.e. why the density of matter is presently so close to the CC density. However, the oldest, toughest and most intriguing face of this polyhedron is the big CC problem, namely why the measured value of the CC at present is so small as compared to any typical density scale existing in high energy physics, especially taking into account the many phase transitions that our Universe has undergone since the early times, including inflation. In this letter, we propose to extend the field equations of General Relativity by including a class of invariant terms that automatically relax the value of the CC irrespective of the initial size of the vacuum energy in the early epochs. We show that, at late times, the Universe enters an eternal de Sitter stage mimicking a tiny positive cosmological constant. Thus, these models could solve the big CC problem and have also a bearing on the cosmic coincidence problem. Remarkably, they mimic the LCDM model to a large extent, but they still leave some characteristic imprints that should be testable in the next generation of experiments.