Unphysical properties in a class of interacting dark energy models. (arXiv:1909.10336v2 [gr-qc] UPDATED)
<a href="http://arxiv.org/find/gr-qc/1/au:+Marttens_R/0/1/0/all/0/1">R. von Marttens</a>, <a href="http://arxiv.org/find/gr-qc/1/au:+Borges_H/0/1/0/all/0/1">H. A. Borges</a>, <a href="http://arxiv.org/find/gr-qc/1/au:+Carneiro_S/0/1/0/all/0/1">S. Carneiro</a>, <a href="http://arxiv.org/find/gr-qc/1/au:+Alcaniz_J/0/1/0/all/0/1">J. S. Alcaniz</a>, <a href="http://arxiv.org/find/gr-qc/1/au:+Zimdahl_W/0/1/0/all/0/1">W. Zimdahl</a>

Models with non-gravitational interactions between the dark matter and dark
energy components are an alternative to the standard cosmological scenario.
These models are characterized by an interaction term, and a frequently used
parameterization is $Q = 3xi H rho_{x}$, where $H$ is the Hubble parameter
and $rho_{x}$ is the dark energy density. Although current observations
support such a model for negative values of the interaction parameter $xi$, we
show here that this interval of values of $xi$ leads the model to predict a
violation of the Weak Energy Condition (WEC) for the dark matter density,
regardless of the value of the equation-of-state parameter of the dark energy
component. This violation is accompanied by unphysical instabilities of matter
perturbations.

Models with non-gravitational interactions between the dark matter and dark
energy components are an alternative to the standard cosmological scenario.
These models are characterized by an interaction term, and a frequently used
parameterization is $Q = 3xi H rho_{x}$, where $H$ is the Hubble parameter
and $rho_{x}$ is the dark energy density. Although current observations
support such a model for negative values of the interaction parameter $xi$, we
show here that this interval of values of $xi$ leads the model to predict a
violation of the Weak Energy Condition (WEC) for the dark matter density,
regardless of the value of the equation-of-state parameter of the dark energy
component. This violation is accompanied by unphysical instabilities of matter
perturbations.

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