Constraining Van der Waals quintessence with observations

Abstract

Abstract. We discuss an alternative approach to quintessence modifying the usual equation of state of the cosmological fluid in order to see if going further than the approximation of perfect fluid allows to better reproduce the available data. We consider a cosmological model comprising only two fluids, namely baryons (modelled as dust) and dark matter with a Van der Waals equation of state. First, the general features of the model are presented and then the evolution of the energy density, the Hubble parameter and the scale factor are determined showing that it is possible to obtain accelerated expansion choosing suitably the model parameters. We use the the data on the dimensionless coordinate distances to Type Ia supernovae and distant radio galaxies to see whether Van der Waals quintessence is viable to explain dark energy and to constrain its parameters. We then compare the model predictions with the estimated age of the universe and the position of the first three peaks of the anisotropy spectrum of the cosmic microwave background radiation. IntroductionIn the last few years an increasing bulk of data have been accumulated favouring the scenario of a spatially flat universe dominated by some form of dark energy. A first strong evidence came from the Hubble diagram of type Ia supernovae (hereafter SNeIa) that turned out to be best fitted by spatially flat accelerating cosmological models † Present address :

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

“…As it is well known, the perfect fluid EoS is only a crude approximation of a real fluid that is usually used in cosmology since it represents the simplest way to fit the available data. However, as first shown in the Van der Waals quintessence scenario [29,30] and further demonstrated here, abandoning the perfect fluid EoS still makes it possible to fit the available astrophysical data with the same accuracy so that the use of realistic EoS turns out to be motivated also a posteriori. In our opinion, the era of precision cosmology calls for precision theory so that time is come to abandon approximate description such as the perfect fluid one.…”

“…Motivated by these considerations, it is therefore worth exploring what are the consequences of abandoning the perfect fluid EoS. A first step in this direction has been performed by Capozziello et al [29] who have considered a model in which a single fluid with a Van der Waals EoS accounts for both dark matter and dark energy (see also [30]). From classical thermodynamics, we know that the Van der Waals EoS is best suited to describe the behaviour of real gas with a particular attention to the phase transitions phenomena.…”

Van der Waals quintessence stars

“…For this, it is important to understand the evolution of structure formation in the VDW scheme. It was shown that for large redshifts the VDW model is similar to the ΛCDM model [4], where the VDW dark matter plays the role of a cosmological constant, showing that structure formation could evolve in a similar manner as in the ΛCDM model, although these points need to be further investigated [4]. Now, it was also shown that due to a negative adiabatic sound speed, perturbations are unstable in the VDW quintessence scenario, and inevitably the role of anisotropic stresses and entropy production has to be taken into account.…”

“…The accelerated and decelerated periods depend on the parameters, α, β and γ of the equation of state, and in the limiting case α, β → 0, one recovers the dark energy equation of state, with γ = p/ρ < −1/3. It was also stressed that the perfect fluid equation of state p = γρ reflects an approximation of cosmic epochs describing stationary situations, where phase transitions are not considered [4]. Thus, one of the advantages of the VDW model is that it describes the transition from a matter field dominated era to a scalar field dominated epoch, without introducing scalar fields.…”

“…Furthermore, it is useful to explain the universe with a minimal number of ingredients, and the VDW gas actually treats dark matter and dark energy as a single fluid. The VDW quintessence scenario has also been confronted successfully with a wide variety of observational tests, by constraining the free parameters [4]. The success of this model has stimulated several different approaches.…”