Any unified dark matter cosmology can be decomposed into dark matter interacting with vacuum energy, without introducing any additional degrees of freedom. We present observational constraints on an interacting vacuum plus dark energy corresponding to a generalised Chaplygin gas cosmology. We consider two distinct models for the interaction leading to either a barotropic equation of state or dark matter that follows geodesics, corresponding to a rest-frame sound speed equal to the adiabatic sound speed or zero sound speed, respectively. For the barotropic model, the most stringent constraint on α comes from the combination of CMB+SNIa+LSS(m) gives α < 1.44 × 10 −5 at the 95% confidence level, which indicates that the barotropic model must be extremely close to the ΛCDM cosmology. For the case where the dark matter follows geodesics, perturbations have zero sound speed, and CMB+SNIa+gISW then gives the much weaker constraint −0.1522 < α < 0.2556 at the 95% confidence level.

Abstract. In this paper, we revisit generalized Chaplygin gas (GCG) model as a unified dark matter and dark energy model. The energy density of GCG model is given as ρGCG/ρGCG0, where α and Bs are two model parameters which will be constrained by type Ia supernova as standard candles, baryon acoustic oscillation as standard rulers and the seventh year full WMAP data points. In this paper, we will not separate GCG into dark matter and dark energy parts any more as adopted in the literatures. By using Markov Chain Monte Carlo method, we find the result: α = 0.00126 +0.000970+0.00268 −0.00126−0.00126 and Bs = 0.775

No abstract

In this Letter, a modified Chaplygin gas (MCG) model of unifying dark energy and dark matter with the exotic equation of state pMCG = BρMCG− A ρ α M CG is constrained from recently observed data:the 182 Gold SNe Ia, the 3-year WMAP and the SDSS baryon acoustic peak. It is shown that the best fit value of the three parameters (B,Bs,α) in MCG model are (-0.085,0.822,1.724). Furthermore, we find the best fit w(z) crosses -1 in the past and the present best fit value w(0) = −1.114 < −1, and the 1σ confidence level of w(0) is −0.946 ≤ w(0) ≤ −1.282. Finally, we find that the MCG model has the smallest χ 2 min value in all eight given models. According to the Alaike Information Criterion (AIC) of model selection, we conclude that recent observational data support the MCG model as well as other popular models.

Kinematical models are constrained by the latest observational data from geometry-distance measurements, which include 557 type Ia supernovae (SNIa) Union2 data and 15 observational Hubble data. Considering two parameterized deceleration parameter, the values of current deceleration parameter q0, jerk parameter j0 and transition redshift zT , are obtained. Furthermore, we show the departures for two parameterized kinematical models from ΛCDM model according to the evolutions of jerk parameter j(z). Also, it is shown that the constraint on jerk parameter j(z) is weak by the current geometrical observed data.

We present improved constraints on an interacting vacuum model using updated astronomical observations including the first data release from Planck. We consider a model with one dimensionless parameter, α, describing the interaction between dark matter and vacuum energy (with fixed equation of state w = −1). The background dynamics correspond to a generalised Chaplygin gas cosmology, but the perturbations have a zero sound speed. The tension between the value of the Hubble constant, H0, determined by Planck data plus WMAP polarisation (Planck+WP) and that determined by the Hubble Space Telescope (HST) can be alleviated by energy transfer from dark matter to vacuum (α > 0). A positive α increases the allowed values of H0 due to parameter degeneracy within the model using only CMB data. Combining with additional datasets of including supernova type Ia (SN Ia) and baryon acoustic oscillation (BAO), we can significantly tighten the bounds on α. Redshift-space distortions (RSD), which constrain the linear growth of structure, provide the tightest constraints on vacuum interaction when combined with Planck+WP, and prefer energy transfer from vacuum to dark matter (α < 0) which suppresses the growth of structure. Using the combined datasets of Planck+WP+Union2.1+BAO+RSD, we obtain the constraint on α to be −0.083 < α < −0.006 (95% C.L.), allowing low H0 consistent with the measurement from 6dF Galaxy survey. This interacting vacuum model can alleviate the tension between RSD and Planck+WP in the ΛCDM model for α < 0, or between HST measurements of H0 and Planck+WP for α > 0, but not both at the same time. PACS numbers: 98.80. -k, 98.80.Es

We investigate observational constraints on the generalized Chaplygin gas (GCG) model as the unification of dark matter and dark energy from the latest observational data: the Union SNe Ia data, the observational Hubble data, the SDSS baryon acoustic peak and the fiveyear WMAP shift parameter. The result is obtained that the best-fit values of the GCG model parameters with their confidence level are A s = 0.73 (2σ ). Furthermore, in this model, we can see that the evolution of equation of state (EOS) for dark energy is similar to quiessence, and its current best-fit value is w 0de = −0.96 with the 1σ confidence level −0.91 ≥ w 0de ≥ −1.00.

As is known above 90% of the energy content in Universe is made of unknown dark component. Usually this dark fluid is separated into two parts: dark matter and dark energy. However, it may be a mixture of these two energy components, or just one exotic unknown fluid. This property is dubbed as dark degeneracy. With this motivation, in this paper, a unified dark fluid having constant adiabatic sound speed c 2 s = α, which is in the range [0, 1], is studied. At first, via the energy conservation equation, its energy density, ρ d /ρ d0 = (1 − Bs) + Bsa −3(1+α) where Bs is related to integration constant from energy conservation equation as another model parameter, is presented. Then by using Markov Chain Monte Carlo method with currently available cosmic observational data sets which include type Ia supernova Union 2, baryon acoustic oscillation and WMAP 7-year data of cosmic background radiation, we show that small values of α are favored in this unified dark fluid model. Furthermore, we show that smaller values of α < 10 −5 are required to match matter (baryon) power spectrum from SDSS DR7. *