  In a recent Letter [Phys. Rev. Lett. 114 091301 (2105)] the cause of the
acceleration of the present Universe has been identified with the shear
viscosity of an imperfect relativistic fluid even in the absence of any bulk
viscous contribution. The gist of this comment is that the shear viscosity, if
anything, can only lead to an accelerated expansion over sufficiently small
scales well inside the Hubble radius.
