Large-scale instability in interacting dark energy and dark matter fluids

Understand

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.