The cosmological constant and the relaxed universe

TL;DR

This paper explores the unavoidable role of the cosmological constant in dark energy models, discussing theoretical challenges and recent approaches to dynamically neutralize its large predicted value without fine-tuning.

Contribution

It reviews recent methods for neutralizing a large cosmological constant dynamically, including effective dark energy components and modified gravity approaches.

Findings

Dynamical relaxation of the cosmological constant is feasible.

Modified gravity can make the universe insensitive to vacuum energy.

The old cosmological constant problem remains a significant theoretical challenge.

Abstract

We study the role of the cosmological constant (CC) as a component of dark energy (DE). It is argued that the cosmological term is in general unavoidable and it should not be ignored even when dynamical DE sources are considered. From the theoretical point of view quantum zero-point energy and phase transitions suggest a CC of large magnitude in contrast to its tiny observed value. Simply relieving this disaccord with a counterterm requires extreme fine-tuning which is referred to as the old CC problem. To avoid it, we discuss some recent approaches for neutralising a large CC dynamically without adding a fine-tuned counterterm. This can be realised by an effective DE component which relaxes the cosmic expansion by counteracting the effect of the large CC. Alternatively, a CC filter is constructed by modifying gravity to make it insensitive to vacuum energy.