Introduction to Modified Gravity: From the Cosmic Speedup Problem to Quantum Gravity Phenomenology

TL;DR

This paper provides an overview of modified gravity theories, their historical development, and their relevance to cosmic acceleration, quantum gravity phenomenology, and singularity issues, emphasizing $f(R)$ and related theories.

Contribution

It offers a comprehensive introduction to various modified gravity models, highlighting their theoretical foundations and potential to address fundamental cosmological and quantum gravity problems.

Findings

$f(R)$ theories can be constrained by weak-field tests.

Palatini $f(R)$ and $f(R,Q)$ theories address quantum gravity phenomenology.

Modified gravity theories offer solutions to cosmic acceleration and singularities.

Abstract

These notes represent a summary of the introductory part of a course on modified gravity delivered at several Spanish Universities (Granada, Valencia, and Valladolid), at the University of Wisconsin-Milwaukee (WI, USA), and at the Karl-Franzens Universitaet (Graz, Austria) during the period 2008-2011. We begin with a discussion of the classical Newtonian framework and how special relativity boosted the interest on new theories of gravity. Then we focus on Nordstrom's scalar theories of gravity and their influence on Einstein's theory of general relativity. We comment on the meaning of the Einstein equivalence principle and its implications for the construction of alternative theories of gravity. We present the cosmic speedup problem and how $f(R)$ theories can be constrained attending to their weak-field behavior. We conclude by showing that Palatini f(R) and f(R,Q) theories can be used to address different aspects of quantum gravity phenomenology and singularity problems.