Lowering the strong coupling mode of modified teleparallel gravity theories

TL;DR

This paper explores how to mitigate the strong coupling problem in modified teleparallel gravity by adjusting the emergence of new degrees of freedom to lower perturbative orders, using the effective field theory approach.

Contribution

It introduces a modification to $f(T)$ gravity that shifts the appearance of new degrees of freedom to cubic order, providing a novel way to address strong coupling issues.

Findings

Scalar perturbations are non-dynamical up to cubic order in conformally equivalent $f(T)$.

A simple modification lowers the emergence of new DoFs to cubic order.

Proposes a new classification scheme based on perturbative order of new DoFs.

Abstract

We investigate the strong coupling problem in modified teleparallel gravity theories using the effective field theory (EFT) approach, demonstrating that it is possible to shift the emergence of new degrees of freedom (DoFs) to lower orders in perturbation theory. We first focus on the case of $f(T)$ gravity, and we show that in its conformally equivalent form the scalar perturbations are non-dynamical up to the cubic action. We then propose a simple modification of the theory, which lowers the appearance of new DoFs to cubic order, compared to the quartic order in standard $f(T)$ gravity. Our work opens a new avenue to address the issue of strong coupling in modified teleparallel gravity, and suggests a new classification scheme of these theories based on the perturbative order at which new DoFs appear.