Cosmological perturbations in conformal gravity

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 conformal to 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 conformal to flat Robertson-Walker background, and find tensor fluctuations that grow far more rapidly than those that occur in the analogous standard case. 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.