Canonical gauge-invariant variables for scalar perturbations in   synchronous coordinates
  Under an appropriate change of the perturbation variable Lifshitz-Khalatnikov propagation equations for the scalar perturbation reduce to d'Alembert equation. The change of variables is based on the Darboux transform. 
Zdzislaw A. Golda, Andrzej Woszczyna and Karolina Zawada