Non-adiabatic Chaplygin gas

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We consider the perturbation dynamics for the cosmic baryon fluid and determine the corresponding power spectrum for a Λ(t)CDM model in which a cosmological term decays into dark matter linearly with the Hubble rate. The model is tested by a joint analysis of data from supernovae of type Ia (SNIa) (Constitution and Union 2.1), baryonic acoustic oscillation (BAO), the position of the first peak of the anisotropy spectrum of the cosmic microwave background (CMB) and large-scale-structure (LSS) data (SDSS DR7). While the homogeneous and isotropic background dynamics is only marginally influenced by the baryons, there are modifications on the perturbative level if a separately conserved baryon fluid is included. Considering the present baryon fraction as a free parameter, we reproduce the observed abundance of the order of 5% independently of the dark-matter abundance which is of the order of 32% for this model. Generally, the concordance between background and perturbation dynamics is improved if baryons are explicitly taken into account. *

Section: Observational Analysiscontrasting

confidence: 63%

“…In this paper we extend a previously established decaying vacuum model [6][7][8][9][10] by including a separately conserved baryon fluid with a four-velocity that differs from the four-velocity of the DM component. The basic ingredient of this model is a DE component with an energy density proportional to the Hubble rate.…”

Section: Introductionmentioning

confidence: 99%

Baryonic matter perturbations in decaying vacuum cosmology

Marttens¹,

Hipólito-Ricaldi²,

Zimdahl³

2014

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“…3 in Ref. [5] that the gravitational potential for α = −1/2 presents approximately the same strength and evolution as in the ΛCDM model. This guarantees, by the way, that the CMB power spectrum will also have the correct normalisation.…”

Section: Resultsmentioning

confidence: 70%

Observational tests of non-adiabatic Chaplygin gas

Carneiro

Pigozzo

2014

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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.

“…This means, on the other hand, that there is no pressure term in the perturbed equations, which guarantees the absence of instabilities in the matter power spectrum. For a detailed analysis of the perturbed equations, see [15] (for the particular case α = −1/2 see also [16], where conserved baryons are explicitly included).…”

mentioning

confidence: 99%

Evidence for cosmological particle creation?

Pigozzo

Carneiro

Alcaniz

et al. 2016

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A joint analysis of the linear matter power spectrum, distance measurements from type Ia supernovae and the position of the first peak in the anisotropy spectrum of the cosmic microwave background indicates a cosmological, late-time dark matter creation at 95% confidence level.In spite of its general agreement with current observations, the standard model based on conserved cold dark matter plus a cosmological constant (ΛCDM) presents some theoretical issues that motivate the study of more general models, either in the realm of modified gravity theories or in the context of General Relativity, by introducing extensions of the dark sector. Among those issues we can refer, for example, to the so-called "coincidence problem", which is alleviated in models with interactions in the dark sector, models that have been deserving attention in the literature for a long time (see, for instance, [1] and references therein). Apart theoretical motivations, there are also observational reasons to include interactions in the dark sector, as the appearance of observational tensions when the standard model is tested against data of the large-scale clustering of galaxies (LSS), on one hand, and distance measurements of type Ia supernovae (SNe Ia), on the other. Some analyses of the linear mass power spectrum provide a value of Ω m0 ∼ 0.2 − 0.3 [2,3] for the present relative matter density, whereas from SNe Ia data one obtains Ω m0 ∼ 0.3 − 0.4 [4,5] (see also [6] for an analysis of CMB data). With more recent galaxy surveys the value obtained for the matter density in the ΛCDM case presents a better agreement with that derived from distance measurements [7,8]. Nevertheless, a tension still persists, particularly when LSS tests are compared to CMB results [9].This tension between different observational tests, if not related to systematic errors, may be interpreted as a signature of late-time dark matter creation. The idea is that, while the power spectrum depends strongly upon the matter density at the time of matter-radiation equality, which determines the spectrum turn-over and profile, distance measurements depend strongly upon the present value of the matter density. Therefore, if matter is created during the matter-dominated phase but the fitting model assumes matter conservation, the present matter density derived from the power spectrum will appear lower than that obtained from SNe Ia observations. In order to verify this possibility, let us take, for simplicity, a substract formed only by pressureless dark matter with energy density ρ and a vacuum term with energy density Λ and equation of state p Λ = −Λ. The balance equation for the total energy assumes the forṁwhere H =ȧ/a is the expansion rate and the second equality defines the rate of matter creation Γ. Clearly, a matter creation process is concomitant with a time decay of the vacuum term. Taking the derivative of the spatially flat Friedmann equation ρ + Λ = 3H 2 and substituting into (1) we findΛ = 2ΓḢ. In the particular case of a constant creation rate we obtain, ap...