Cosmological Perturbations in Conformal Gravity

TL;DR

This paper explores cosmological perturbations within conformal gravity, revealing how conformal invariance influences gauge choices and leads to unique fluctuation behaviors, including rapid growth of tensor modes and energyless fluctuations.

Contribution

It provides the first detailed analysis of first-order metric fluctuations in conformal gravity across various cosmological backgrounds, highlighting differences from standard gravity.

Findings

Trace of metric fluctuations decouples from gravitational equations.

Tensor fluctuations grow faster than in standard cosmology.

Conformal fluctuations around de Sitter are energyless.

Abstract

We present the first steps needed for an analysis of the perturbations that occur in the cosmology associated with the conformal gravity theory. We discuss the implications of conformal invariance for perturbative coordinate gauge choices, and show that in the conformal theory the trace of the metric fluctuation kinematically decouples from the first-order gravitational fluctuation equations. We determine the equations that describe first-order metric fluctuations around the illustrative conformally flat de Sitter background. Via a conformal transformation we show that such fluctuations can be constructed from fluctuations around a flat background, even though the fluctuations themselves are associated with a perturbative geometry that is not itself conformal to flat. We extend the analysis to fluctuations around other cosmologically relevant backgrounds, such as the conformally-flat Robertson-Walker background, and find tensor fluctuations that grow far more rapidly than those that occur in the analogous standard case. We comment on some recent work by 't Hooft and by Maldacena on deriving conformal gravity from Einstein gravity or vice versa. We show that while the standard gravity tensor fluctuations around a de Sitter background are also fluctuation solutions in the conformal theory, in the conformal case they do not carry energy.