Gauge-invariant analysis of perturbations in Chaplygin gas unified models of dark matterand dark energy

Abstract

We exploit the gauge-invariant formalism to analyse the perturbative behaviourof two cosmological models based on the generalized Chaplygin gas describingboth dark matter and dark energy in the present universe. In the first model weconsider the generalized Chaplygin gas alone, while in the second one we add abaryon component to it. We extend our analysis also into the parameter rangeα>1, where the generalized Chaplygin gas sound velocity can be larger than that of light. In the firstmodel we find that the matter power spectrum is compatible with the observed one only forα<10−5, which makes the generalized Chaplygin gas practically indistinguishable fromΛCDM. In the second model we study the evolution of inhomogeneities of the baryon component. Thetheoretical power spectrum is in good agreement with the observed one for almost all values ofα.However, the growth of inhomogeneities seems to be particularly favoured either for sufficiently smallvalues of α or for . Thus, it appears that the viability of the generalized Chaplygin gas as acosmological model is stronger when its sound velocity is superluminal. Weshow that in this case the generalized Chaplygin gas equation of state canbe changed in an unobservable region in such a way that its equivalentk-essence microscopical model has no problems with causality.