Imprints of interacting dark energy on cosmological perturbations
  We investigate the characteristic modifications in the evolving cosmological
perturbations when dark energy interacts with dust-like matter, causing the
latter's background energy density fall off with time faster than usual.
Focusing in particular to the late-time cosmic evolution, we show that such an
interaction (of a specific form, arising naturally in a scalar-tensor
formulation, or a wide range of modified gravity equivalents thereof), can have
a rather significant effect on the perturbative spectrum, than on the
background configuration which is not expected to get distorted much from
$\L$CDM. Specifically, the matter density contrast, which is by and large
scale-invariant in the deep sub-horizon limit, not only gets dragged as the
interaction affects the background Hubble expansion rate, but also receives a
contribution from the perturbation in the (scalar field induced) dark energy,
which oscillates about a non-zero mean value. As such, the standard
parametrization ansatz for the the matter density growth factor becomes
inadequate. So we modify it suitably, and also find a numerical fit of the
growth index in terms of the background parameters, in order to alleviate the
problems that arise otherwise. Such a fit enables direct estimations of the
background parameters, as well as the growth parameter and the reduced Hubble
parameter, which we duly carry out using a redshift space distortion (RSD)
subsample and its combination with the observational Hubble data. On the whole,
the parametric estimates show consistency with the general observational
constraints on the background level cosmology, as well as the constraints on
scalar-tensor gravity from astrophysical observations, apart from having
significance in the domain of cosmological perturbations.
