We formulate an effective field theory (EFT) of coupled dark energy (DE) E
dark matter (DM) interacting through energy and momentum transfers. In the DE
sector, we exploit the EFT of vector-tensor theories with the presence of a
preferred time direction on the cosmological background. This prescription
allows one to accommodate shift-symmetric and non-shift-symmetric scalar-tensor
theories by taking a particular weak coupling limit, with and without
consistency conditions respectively. We deal with the DM sector as a
non-relativistic perfect fluid, which can be described by a system of three
scalar fields. By choosing a unitary gauge in which the perturbations in the DE
and DM sectors are eaten by the metric, we incorporate the leading-order
operators that characterize the energy and momentum transfers besides those
present in the conventional EFT of vector-tensor and scalar-tensor theories and
the non-relativistic perfect fluid. We express the second-order action of
scalar perturbations in real space in terms of time- and scale-dependent
dimensionless EFT parameters and derive the linear perturbation equations of
motion by taking into account additional matter (baryons, radiation). In the
small-scale limit, we obtain conditions for the absence of both ghosts and
Laplacian instabilities and discuss how they are affected by the DE-DM
interazioni. We also compute the effective DM gravitational coupling $G_{\rm
eff}$ by using a quasi-static approximation for perturbations deep inside the
DE sound horizon and show that the existence of momentum and energy transfers
allow a possibility to realize $G_{\rm eff}$ smaller than in the uncoupled case
at low redshift.

Questo articolo esplora i giri e le loro implicazioni.

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