  We study linear perturbations of the Friedmann-Lemaitre-Robertson-Walker
(FLRW) cosmological model in the Regge-Wheeler formalism which is a standard
framework to study perturbations of spherically-symmetric black holes. In
particular, we show that the general solution of linear perturbation equations
can be given in terms of two copies of a master scalar satisfying scalar wave
equation on the FLRW background (with a Regge-Wheeler/Zerilli type potential)
thus representing two gravitational degrees of freedom, and one scalar
satisfying a transport type equation representing (conformal) matter
perturbation. We expect the Regge-Wheeler formalism to be easily extended to
include nonlinear perturbations, akin to to the recent work [Phys. Rev. D 96,
124026 (2017)].
