Comments on gauge-invariance in cosmology
  We revisit the gauge issue in cosmological perturbation theory, and highlight its relation to the notion of covariance in general relativity. We also discuss the similarities and differences of the covariant approach in perturbation theory to the Bardeen or metric approach in a non-technical fashion. 
Karim A. Malik and David R. Matravers
astro-ph.CO gr-qc hep-ph