Modified gravity approaches to the cosmological constant problem

TL;DR

This paper reviews various modified gravity theories as potential solutions to the cosmological constant problem, discussing their mechanisms, challenges, and observational status in a comprehensive manner.

Contribution

It provides a modern, concise overview of different modified gravity approaches and their potential to address the cosmological constant problem, including recent theoretical developments.

Findings

Modified gravity can potentially alleviate cosmological constant issues

Screening mechanisms are crucial for compatibility with observations

Many models remain consistent with current astrophysical data

Abstract

The cosmological constant and its phenomenology remain among the greatest puzzles in theoretical physics. We review how modifications of Einstein's general relativity could alleviate the different problems associated with it that result from the interplay of classical gravity and quantum field theory. We introduce a modern and concise language to describe the problems associated with its phenomenology, and inspect no-go theorems and their loopholes to motivate the approaches discussed here. Constrained gravity approaches exploit minimal departures from general relativity; massive gravity introduces mass to the graviton; Horndeski theories lead to the breaking of translational invariance of the vacuum; and models with extra dimensions change the symmetries of the vacuum. We also review screening mechanisms that have to be present in some of these theories if they aim to recover the success of general relativity on small scales as well. Finally, we summarise the statuses of these models in their attempt to solve the different cosmological constant problems while being able to account for current astrophysical and cosmological observations.