No abstract

The generalized Chaplygin gas is usually defined as a barotropic perfect fluid with an equation of state p = −Aρ −α , where ρ and p are the proper energy density and pressure, respectively, and A and α are positive real parameters. It has been extensively studied in the literature as a quartessence prototype unifying dark matter and dark energy. Here, we consider an extended family of generalized Chaplygin gas models parameterized by three positive real parameters A, α and β, which, for two specific choices of β [β = 1 and β = (1 + α) /(2α)], is described by two different Lagrangians previously identified in the literature with the generalized Chaplygin gas. We show that, for β > 1/2, the linear stability conditions and the maximum value of the sound speed cs are regulated solely by β, with 0 ≤ cs ≤ 1 if β ≥ 1. We further demonstrate that in the nonrelativistic regime the standard equation of state p = −Aρ −α of the generalized Chaplygin gas is always recovered, while in the relativistic regime this is true only if β = (1 + α) /(2α). We present a regularization of the (α → 0, A → ∞) limit of the generalized Chaplygin gas, showing that it leads to a logarithmic Chaplygin gas model with an equation of state of the form p = A ln (ρ/ρ * ), where A is a real parameter and ρ * > 0 is an arbitrary energy density. We finally derive its Lagrangian formulation. * Electronic address: vasco.ferreira@astro.up.pt † Electronic address: pedro.avelino@astro.up.pt

We show that the observed rotation curves of spiral galaxies constrain the sound speed of the dark matter to be cs < 10 −4 c, where c is the speed of light in vacuum. Using the Modified Chaplygin Gas as a representative example of a class of unified dark energy models incorporating an effective dark matter component with a non-zero sound speed, we determine the most stringent constraint to date on the value of the constant contribution to the equation of state parameter in this class of models. Finally, we explain the reason why previous constraints using the Cosmic Microwave Background and Baryonic Acoustic Oscillations were not as competitive as the one presented in this paper and discuss the limitations of the recently proposed Extended Chaplygin Gas.