Dark energy and dissipation

Most of the models leading to a current state of cosmic accelerated expansion fail to address the coincidence problem, i.e., that the dark energy density and the energy density of the matter fluid are of the same order precisely today. We show that a way to drive late acceleration and simultaneously solve the aforesaid problem is assuming the matter fluid dissipative [1]. 1 General setting The Friedmann equation plus the conservation equations for matter and dark energy in a Friedmann–Robertson–Walker universe dominated by these two components (non–interacting with one another), in terms of the density parameters, read 1 = Ωm +Ωφ +Ωk , (1) Ω̇ = (3γ − 2)H(Ω− 1)Ω , (2) Ω̇φ = [2 + (3γ − 2)Ω− 3γφ]ΩφH , (3) where Ω ≡ Ωm+Ωφ, and γ stands for the overall adiabatic index γ = (γmΩm+ γφΩφ)/Ω, with γm,φ ≡ 1 + (pm,φ/ρm,φ), and such that 1 ≤ γm ≤ 2 and 0 ≤ γφ < 1 (it should be noted that in general γm and γφ may vary with time). It is not our aim to propose a new dark energy model but rather to show that if one wishes to have late cosmic acceleration and simultaneosly solve the coincidence problem using general relativity, then the matter fluid component must be dissipative -in the sense explained belowirrespective of the potential of the scalar field [1].