Vector modes in Type 3 New GR

TL;DR

This paper clarifies the nature of vector modes in Type 3 New General Relativity, emphasizing the importance of correctly analyzing equations of motion rather than substituting constraints into the Lagrangian.

Contribution

It corrects recent misconceptions by demonstrating the proper method to analyze vector modes in New GR, highlighting the pitfalls of substituting constraints into the Lagrangian.

Findings

Vector modes in Type 3 New GR are non-dynamical when correctly analyzed.

Substituting constraints into the Lagrangian can lead to incorrect conclusions about dynamical modes.

The proper analysis confirms previous results about degrees of freedom in New GR.

Abstract

Some time ago, we published the full count of degrees of freedom in the linearised weak gravity limit of arbitrary New GR models. We did it by considering the linear equations of motion and presented a thorough analysis with no ambiguity left. A bit later, we generalised it to linear cosmological perturbations and discussed the strong coupling issues that appear already at this level. Recently, there were claims that some dynamical modes had been missed in our work. However, the authors of the new claims did not look at the equations of motion and analysed the quadratic Lagrangian densities instead. In this paper, I take one of the most elementary cases, namely the vector modes in New GR of Type 3, and show what was their mistake that had led them to claiming that those were dynamical. The main message: Do not substitute constraint equations into a Lagrangian.