On the large-scale instability in interacting dark energy and dark matter fluids

Details On the large-scale instability in interacting dark energy and dark matter fluids On the large-scale instability in interacting dark energy and dark matter fluids

2009-01-21

arxiv.org/abs/0901.3272v2

Phys.Rev.D79:043526,2009

Astronomy and Astrophysics

Astrophysics

Cosmology and Extragalactic Astrophysics

Updated to match version published in February edition of Phys.Rev.D

Scientific paper

10.1103/PhysRevD.79.043526

Recently, Valiviita et al. (2008) have reported a large-scale early-time instability in coupled dark energy and dark matter models. We take the same form of energy-momentum exchange and specialise to the case when the interaction rate is proportional to Hubble's parameter and the dark energy density only. Provided the coupling is made small enough for a given equation of state parameter, we show that the instability can be avoided. Expressions are derived for non-adiabatic modes on super-horizon scales in both the radiation and matter dominated regimes. We also examine the growth of dark matter perturbations in the sub-horizon limit. There we find that the coupling has almost no effect upon the growth of structure before dark energy begins to dominate. Once the universe begins to accelerate, the relative dark matter density fluctuations not only cease to grow as in uncoupled models, but actually decay as the universe continues to expand.

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