Partially massless theory as a quantum gravity candidate

TL;DR

This paper explores the interactions of partially massless gravity with matter fields, proposing new interaction methods that preserve gauge symmetry and analyzing the resulting degrees of freedom and their dynamics.

Contribution

It introduces novel interaction vertices between PM gravity and matter fields that maintain gauge symmetry, and studies the resulting field equations and particle modes.

Findings

PM gravity has 4 unitary degrees of freedom.

Vector modes exhibit momentum-dependent dispersion relations.

Early universe vector modes are light-like, late-time modes slow down.

Abstract

We study partially massless gravity theory(PM gravity theory) and suggest an alternative way to add higher order interaction vertices to the theory. Rather than introducing self interaction vertices of the gravitational fields to the partially massless gravity action, we consider interactions with matter fields, since it is well known that addition of the self interaction terms necessarily breaks the $U(1)$ gauge symmetry that PM gravity theory enjoys. For the coupling with matter fields, we consider two different types of interaction vertices. The first one is given by an interaction Lagrangian density, $\mathcal L_{int}\sim h_{\mu\nu}T^{\mu\nu}$, where $h_{\mu\nu}$ is the PM gravity field and $T^{\mu\nu}$ is the stress-energy tensor of the matter fields. To retain the $U(1)$ gauge symmetry, the matter fields also transform accordingly and it turns out that the transform must be non-local in this case. The second type interaction is obtained by employing a gauge covariant derivative with the PM gravity field, where the PM gravity fields play a role of a gauge connection canceling the phase shift of the matter fields. We also study the actions and the equations of motion of the partially massless gravity fields. As expected, it shows 4 unitary degrees of freedom. 2 of them are traceless tensor modes and they are light like fields. The other 2 are transverse vector modes and their dispersion relation changes as background space time (de Sitter) evolutes. In the very early time, they are light like but in the very late time, their velocity become a half of speed of light. The vector mode dispersion relation shows momentum dependent behavior. In fact, the higher(lower) frequency modes show the faster(slower) velocity. We suggest their quantization, compute Hamiltonians to present their exited quanta and construct their free propagators.