2 research outputs found
Large-scale instability in interacting dark energy and dark matter fluids
If dark energy interacts with dark matter, this gives a new approach to the
coincidence problem. But interacting dark energy models can suffer from
pathologies. We consider the case where the dark energy is modelled as a fluid
with constant equation of state parameter w. Non-interacting constant-w models
are well behaved in the background and in the perturbed universe. But the
combination of constant w and a simple interaction with dark matter leads to an
instability in the dark sector perturbations at early times: the curvature
perturbation blows up on super-Hubble scales. Our results underline how
important it is to carefully analyze the relativistic perturbations when
considering models of coupled dark energy. The instability that we find has
been missed in some previous work where the perturbations were not consistently
treated. The unstable mode dominates even if adiabatic initial conditions are
used. The instability also arises regardless of how weak the coupling is. This
non-adiabatic instability is different from previously discovered adiabatic
instabilities on small scales in the strong-coupling regime.Comment: 15 pages, 5 figures. New reference; published versio
Large-scale instability in interacting dark energy and dark matter fluids
If dark energy interacts with dark matter, this gives a new approach to the
coincidence problem. But interacting dark energy models can suffer from
pathologies. We consider the case where the dark energy is modelled as a fluid
with constant equation of state parameter w. Non-interacting constant-w models
are well behaved in the background and in the perturbed universe. But the
combination of constant w and a simple interaction with dark matter leads to an
instability in the dark sector perturbations at early times: the curvature
perturbation blows up on super-Hubble scales. Our results underline how
important it is to carefully analyze the relativistic perturbations when
considering models of coupled dark energy. The instability that we find has
been missed in some previous work where the perturbations were not consistently
treated. The unstable mode dominates even if adiabatic initial conditions are
used. The instability also arises regardless of how weak the coupling is. This
non-adiabatic instability is different from previously discovered adiabatic
instabilities on small scales in the strong-coupling regime.Comment: 15 pages, 5 figures. New reference; published versio