On the viability of $f(Q)$ gravity models

TL;DR

This paper investigates the conditions under which energy conservation holds in $f(Q)$ gravity models, revealing that most non-linear models violate conservation unless $Q$ is constant, highlighting fundamental issues in the theory.

Contribution

It demonstrates that energy conservation in $f(Q)$ gravity is model-dependent and generally violated in non-linear cases, except for linear models, with implications for the theory's consistency.

Findings

Energy conservation is equivalent to the affine connection's field equation in $f(Q)$ gravity.

Non-linear $f(Q)$ models generally do not satisfy energy conservation unless $Q$ is constant.

Linear $f(Q)= ext{constant} imes Q + ext{constant}$ models preserve energy conservation.

Abstract

In general relativity, the contracted Bianchi identity makes the field equation compatible with the energy conservation, likewise in $f(R)$ theories of gravity. We show that this classical phenomenon is not guaranteed in the symmetric teleparallel theory, and rather generally $f(Q)$ model specific. We further prove that the energy conservation criterion is equivalent to the affine connection's field equation of $f(Q)$ theory, and except the $f(Q)=\alpha Q+\beta$ model, the non-linear $f(Q)$ models do not satisfy the energy conservation or, equivalently the second field equation in every spacetime geometry; unless $Q$ itself is a constant. So the problem is deep-rooted in the theory, several physically motivated examples are provided in the support.