Beyond the perfect fluid hypothesis for the dark energy equation of state

Abstract

Abandoning the perfect fluid hypothesis, we investigate here the possibility that the dark energy equation of state (EoS) w is a nonlinear function of the energy density ρ. To this aim, we consider four different EoS describing classical fluids near thermodynamical critical points and discuss the main features of cosmological models made out of dust matter and a dark energy term with the given EoS. Each model is tested against the data on the dimensionless coordinate distance to Type Ia Supernovae and radio galaxies, the shift and the acoustic peak parameters and the positions of the first three peaks in the anisotropy spectrum of the comic microwave background radation. We propose a possible interpretation of each model in the framework of scalar field quintessence determining the shape of the self interaction potential V (φ) that gives rise to each one of the considered thermodynamical EoS. As a general result, we demonstrate that replacing the perfect fluid EoS with more generar expressions gives both the possibility of successfully solving the problem of cosmic acceleration escaping the resort to phantom models.

Limits on decaying dark energy density models from the CMB temperature–redshift relation

“…If we take γ = 4/3, then we find m = 0.09 ± 0.10 and z T = 0.82 ± 0.10, while, if we leave both γ and m free to vary m = 0.20 [10,32]), although, on average, higher. In Fig.…”

“…Several models have been proposed to explain dark energy [7][8][9][10][11][12][13][14][15]. An alternative consists to consider a phenomenological decaying dark energy density with continuous creation of photons [16,18] or matter [15].…”

Baryon acoustic oscillations and dynamical dark energy

“…it has to be quadratic in angular and linear in radial distance measures. This A-parameter in combination with CMB and Sne Ia data was used to constrain the equation of state of dark energy [18,19,20,21], alternative dark energy models [23,24,25,26], as well as neutrinos and extra light particle masses [27,28].…”

Van der Waals quintessence stars

“…Several candidates have been proposed in the literature to explain this cosmic accelerated expansion, such as dark energy models, the generalized Chaplygin gas, modified gravity and scalar-tensor theories, tachyon scalar fields and specific braneworld models, such as the Dvali-Gabadadze-Porrati (DGP) model. However, it was pointed out that assuming that dark energy is governed by a perfect fluid equation of state may systematically induce wrong results and be misleading in inferring the nature of dark energy [1]. In this spirit, an alternative model was recently proposed without the presence of exotic fluids and modifications of the Friedmann equations, using a more complicated equation of state, namely, the van der Waals (VDW) equation of state [2,3], given by…”