Unified and interacting models of the cosmological dark sector

Winfried Zimdahl

We argue that the accelerated expansion of the Universe can be understood either as a consequence of a cosmic viscosity [1] or as the result of an interaction in the dark sector [2, 3].
A viscous universe shares the same homogeneous and isotropic background dynamics as a generalized Chaplygin gas (GCG). But different from the latter it is characterized by a non-adiabatic perturbation dynamics. We demonstrate that matter power spectrum data favor viscous models over GCG models [1, 4].
Alternatively, the present accelerated expansion of the Universe is shown to result as a pure interaction phenomenon within the dark sector [2]. The possibility that a time varying interaction may give rise to a transient phase of acceleration is also discussed [3]. Such type of dynamics, apparently favored by a recent data analysis [5], cannot be obtained within the LCDM model.

[1] W.S. Hipólito-Ricaldi, H.E.S. Velten and W. Zimdahl, JCAP 0906 (2009) 016.
[2] W. Zimdahl and D. Pavón, Class. Quantum Grav. 24, 5641 (2007).
[3] J.C. Fabris, B. Fraga, N. Pinto-Neto and W. Zimdahl, JCAP 1004 (2010) 008.
[4] J.C. Fabris, H.E.S. Velten and W. Zimdahl, Phys. Rev. D 81, 087303 (2010).
[5] A. Shafieloo, V. Sahni and A.A. Starobinsky, Phys. Rev. D 80, 101301 (2009).