Gravity models with nonlinear symmetry realization

TL;DR

This paper examines three models with nonlinear conformal symmetry realizations, highlighting their limitations in describing inflation and the presence of ghost degrees of freedom, which may be mitigated by additional symmetries.

Contribution

It introduces and analyzes three nonlinear conformal symmetry models, revealing their equivalence and issues with ghost degrees of freedom and potential solutions.

Findings

The simplest model describes only decelerating universe expansion.

The models are equivalent through variable reparametrization.

Ghost degrees of freedom are present but can be removed with extra symmetries.

Abstract

Three models with nonlinear realizations of conformal symmetry are discussed. The simplest model can only describe a universe expanding with a deceleration and does not include inflation. The other models are equivalent up to a variables reparametrization. All these models contain ghost degrees of freedom which may be excluded with an additional symmetry of the target space.