Contemplating the fate of modified gravity

TL;DR

This paper reviews various models of modified gravity motivated by cosmological puzzles, highlighting the lack of experimental evidence and the challenge of identifying the simplest effective modifications to general relativity.

Contribution

It provides a comprehensive overview of existing approaches to modified gravity and discusses the current state of research in the field.

Findings

Limited experimental evidence for non-GR gravity regimes

Diverse theories with no unifying guiding principle

Focus on simplest modifications to address specific problems

Abstract

Possible models of modified gravity are being extensively studied now, with most phenomenological motivations coming from puzzles and tensions in cosmology due to a natural desire to better fit the known and newly coming data. At the same time, available experimental evidence is limited for testing gravity as a force beyond the regimes in which the theory of general relativity has proven to be successful. This situation leads researchers to look for ``the simplest modification'' to general relativity in a certain class of models, which is enough to solve one or more problems. As a result, we are lost amid a variety of theories with no deeper guiding principle. We give a general review of existing approaches and discuss the current state of the art.