Variable Chaplygin Gas

We consider a variable Chaplygin gas model, which is characterized by an exotic equation of state, P = −A(a)/ρ, where A(a) is a positive function of the scale factor. By assuming a power-law function A(a) we find that the variable Chaplygin gas evolves as from dust to quiessence. We show that the model is consistent with current type Ia supernova data for a broad range of parameters. ∗e-mail address: [email protected] Recent observations of type Ia supernovae (SNe Ia) suggest that the expansion of the universe is accelerating and that two-thirds of the total energy density exists in a dark energy component with negative pressure [1]. In addition, measurements of the cosmic microwave background (CMB) [2] and the galaxy power spectrum [3] also indicate the existence of the dark energy. The simplest candidate for the dark energy is a cosmological constant Λ, which has pressure PΛ = −ρΛ. Specifically, a reliable model should explain why the present amount of the dark energy is so small compared with the fundamental scale (fine-tuning problem) and why it is comparable with the critical density today (coincidence problem). The cosmological constant suffers from both these problems. One possible approach to constructing a viable model for dark energy is to associate it with a slowly evolving and spatially homogeneous scalar field φ, called “quintessence” [4, 5]. Such a model for a broad class of potentials can give the energy density converging to its present value for a wide set of initial conditions in the past and possess tracker behavior. Recently, the Chaplygin gas model was proposed as an alternative to both the cosmological constant and to quintessece in explaining the accelerating universe [6]. The Chaplygin gas is characterized by an exotic equation of state P = −A/ρ, where A is a positive constant. An attractive feature of the model is that it can naturally explain both dark energy and dark matter. The reason is that the Chaplygin gas behaves as dust-like matter at early stage and as a cosmological constant at later stage. Some possible motivations for this model from the field theory point of view are discussed in Refs. [7]. The Chaplygin gas appears as an effective fluid associated with d-branes [8] and can also be derived from the Born-Infeld action [9]. An interesting range of models was found to be consistent with SN Ia data [10], CMB peak locations [11] and other observational data [12]. The Chaplygin gas model has been extensively studied in the literature [13]. In this Letter we consider a variable Chaplygin gas model and analyze its cosmological consequences. We then constrain the parameters associated with the variable Chaplygin gas using the SN Ia data. Let us now consider a variable Chaplygin gas characterized by the equation of state