Onset of the nonlinear regime in unified dark matter models

Abstract

We discuss the onset of the non-linear regime in the context of unified dark matter models involving a generalised Chaplygin gas. We show that the transition from dark matter-like to dark energy-like behaviour will never be smooth. In some regions of space the transition will never take place while in others it may happen sooner or later than naively expected. As a result the linear theory used in previous studies may break down late in the matter dominated era even on large cosmological scales. We study the importance of this effect showing that its magnitude depends on the exact form of the equation of state in the low density regime. We expect that our results will be relevant for other unified dark matter scenarios particularly those where the quartessence candidate is a perfect fluid.

Letter: Generalized Chaplygin Gas with α = 0 and the ΛC D M Cosmological Model

“…Some authors (see, for example, reference [6]) claim that the comparison with observation indicates that α is peaked around zero. At same time, it is generally argued in the literature that the case α = 0 is equivalent to a ΛCDM model [7,8,6]. The aim of the present letter is to show that, if this is true for the background solutions, it is not true for the linearized equations.…”

Purely KinetickEssence as Unified Dark Matter

“…It has been argued that this behavior is problematic for the model, since the large sound speed at late times leads to either blow-up of the perturbations (for negative α) or excessive damped oscillations (for positive α), with a resulting limit of α < 10 −5 [20]. (However, nonlinear clustering may significantly alter the evolution of density perturbations in this model [21]; see also the discussion in Refs. [22,23]).…”

Nonlinear Chaplygin gas cosmologies

“…The effect of small scale nonlinearities has been recognized as having a strong potential impact on the large scale evolution of the Universe, in particular in the context of UDE scenarios [52][53][54][55][56][57][58]. However, it has been argued that, in the case of the Chaplygin gas, nonlinear effects would be too small to significantly affect the above linear results [53] (see also [52]).…”