Cosmological perturbations in a family of deformations of general
  relativity
  We study linear cosmological perturbations in a previously introduced family
of deformations of general relativity characterized by the absence of new
degrees of freedom. The homogeneous and isotropic background in this class of
theories is unmodified and is described by the usual Friedmann equations. The
theory of cosmological perturbations is modified and the relevant deformation
parameter has the dimension of length. Gravitational perturbations of the
scalar type can be described by a certain relativistic potential related to the
matter perturbations just as in general relativity. A system of differential
equations describing the evolution of this potential and of the stress-energy
density perturbations is obtained. We find that the evolution of scalar
perturbations proceeds with a modified effective time-dependent speed of sound,
which, contrary to the case of general relativity, does not vanish even at the
matter-dominated stage. In a broad range of values of the length parameter
controlling the deformation, a specific transition from the regime of modified
gravity to the regime of general relativity in the evolution of scalar
perturbations takes place during the radiation domination. In this case, the
resulting power spectrum of perturbations in radiation and dark matter is
suppressed on the comoving spatial scales that enter the Hubble radius before
this transition. We estimate the bounds on the deformation parameter for which
this suppression does not lead to observable consequences. Evolution of scalar
perturbations at the inflationary stage is modified but very slightly and the
primordial spectrum generated during inflation is not noticeably different from
the one obtained in general relativity.
