[Paper Review] Modified Gravity and Cosmology: An Update by the CANTATA Network

Emmanuel N. Saridakis, Ruth Lazkoz+51 more|arXiv (Cornell University)|May 20, 2021

Cosmology and Gravitation TheoriesPhysics and Astronomy73 references167 citations

TL;DR

A comprehensive review of recent developments in modified gravity and cosmology, outlining theoretical consistency checks, background and perturbation predictions, and confrontation with observational data. It summarizes viable theories and future directions.

ABSTRACT

General Relativity and the $Λ$CDM framework are currently the standard lore and constitute the concordance paradigm. Nevertheless, long-standing open theoretical issues, as well as possible new observational ones arising from the explosive development of cosmology the last two decades, offer the motivation and lead a large amount of research to be devoted in constructing various extensions and modifications. All extended theories and scenarios are first examined under the light of theoretical consistency, and then are applied to various geometrical backgrounds, such as the cosmological and the spherical symmetric ones. Their predictions at both the background and perturbation levels, and concerning cosmology at early, intermediate and late times, are then confronted with the huge amount of observational data that astrophysics and cosmology are able to offer recently. Theories, scenarios and models that successfully and efficiently pass the above steps are classified as viable and are candidates for the description of Nature. This work is a Review of the recent developments in the fields of gravity and cosmology, presenting the state of the art, high-lighting the open problems, and outlining the directions of future research. Its realization was performed in the framework of the COST European Action ``Cosmology and Astrophysics Network for Theoretical Advances and Training Actions''.

Motivation & Objective

• Motivate the exploration of gravity and cosmology beyond General Relativity and Lambda-CDM.
• Summarize theoretical consistency criteria for extended theories of gravity.
• Review predictions on background evolution and perturbations across cosmic history.
• Classify viable models that pass theoretical and observational viability tests.
• Outline open problems and future research directions within the COST network framework.

Proposed method

• Survey and synthesize theoretical frameworks of modified gravity and cosmology.
• Assess theories against consistency criteria and geometrical backgrounds (cosmological and spherical symmetry).
• Compare predictions at background and perturbation levels with observational data across early, intermediate, and late times.
• Identify viable models that pass theoretical and observational tests.
• Highlight open problems and future research agendas within the CANTATA network.

Experimental results

Research questions

• RQ1 What are the theoretical consistency requirements for modified gravity theories?
• RQ2 How do extended gravity theories modify background cosmology and cosmological perturbations?
• RQ3 Which models remain viable when confronted with current observational data across different epochs?
• RQ4 What are the major open problems and directions for future research in gravity and cosmology?

Key findings

• The work provides a state-of-the-art review of gravity and cosmology developments.
• It emphasizes the process of testing theories for theoretical consistency, background behavior, and perturbations.
• The review identifies theories and scenarios that pass these tests as viable candidates for describing Nature.
• The paper highlights open problems and outlines future research directions within the COST European Action framework.
• The update aggregates a wide range of models and approaches proposed in the literature up to 2023.