Observational tests of non-adiabatic Chaplygin gas

Abstract

In a previous paper [1] it was shown that any dark sector model can be mapped into a nonadiabatic fluid formed by two interacting components, one with zero pressure and the other with equation-of-state parameter ω = −1. It was also shown that the latter does not cluster and, hence, the former is identified as the observed clustering matter. This guarantees that the dark matter power spectrum does not suffer from oscillations or instabilities. It applies in particular to the generalised Chaplygin gas, which was shown to be equivalent to interacting models at both background and perturbation levels. In the present paper we test the non-adiabatic Chaplygin gas against the Hubble diagram of type Ia supernovae, the position of the first acoustic peak in the anisotropy spectrum of the cosmic microwave background and the linear power spectrum of large scale structures. We consider two different samples of SNe Ia, namely the Constitution and SDSS compilations, both calibrated with the MLCS2k2 fitter, and for the power spectrum we use the 2dFGRS catalogue. The model parameters to be adjusted are the present Hubble parameter, the present matter density and the Chaplygin gas parameter α. The joint analysis best fit gives α ≈ −0.5, which corresponds to a constant-rate energy flux from dark energy to dark matter, with the dark energy density decaying linearly with the Hubble parameter. The ΛCDM model, equivalent to α = 0, stands outside the 3σ confidence interval. This result is still valid if we use, as supernovae samples, the SDSS and Union2.1 compilations calibrated with the SALT2 fitter.

On the concordance of cosmological data in the case of the generalized Chaplygin gas

“…Using supernovae data calibrated with different fitters in a joint analysis, even smaller values of α are found in [30], such as α ≃ −0.5 or α ≃ −0.36, which shows the sensitivity of the chosen calibration method of SNe Ia data. Using additional data sets, a joint analysis carried out in [23] finds the 1σ estimation α = 0.004 +0.013 −0.010 .…”

“…This generated again a large interest in the Chaplygin cosmologies and its variants, see e. g. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]. Most papers assume a flat universe when the allowed parameter space is estimated according to various cosmological data.…”

Friedmann-Lemaitre cosmologies via roulettes and other analytic methods

“…of a generalized Chaplyin fluid model [36,37,38]. Inserting (5.25) into the law of energy conservation, (4.3), we obtain 26) where C > 0 is an integration constant.…”

Evidence for cosmological particle creation?

“…Note that for α = −1/2 the ansatz (2) reduces to the linear case Λ = 2ΓH whereas for α = 0 the standard model with constant Λ and no matter creation is readily recovered. The background solution corresponding to the ansatz (2) is given by [11] In the asymptotic future the cosmic fluid (3) tends to a cosmological constant whereas for early times it behaves as pressureless matter, i.e.,From this expression, it is clear that for the same density at high redshifts, negative values of α imply a higher present density as compared to the standard model, owing to the late-time matter creation process. Before testing the hypothesis of cosmological matter creation against large-scale structure observations, it is necessary to verify whether the interacting vacuum clusters.…”

“…Note that for α = −1/2 the ansatz (2) reduces to the linear case Λ = 2ΓH whereas for α = 0 the standard model with constant Λ and no matter creation is readily recovered. The background solution corresponding to the ansatz (2) is given by [11] …”