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Abstract: We demonstrate that creation of dark-matter particles at a constant rate implies the existence of a cosmological term that decays linearly with the Hubble rate. We discuss the cosmological model that arises in this context and test it against observations of the first acoustic peak in the cosmic microwave background (CMB) anisotropy spectrum, the Hubble diagram for supernovas of type Ia (SNIa), the distance scale of baryonic acoustic oscillations (BAO) and the distribution of large scale structures (LSS). We show that a good concordance is obtained, albeit with a higher value of the present matter abundance than in the ΛCDM model. We also comment on general features of the CMB anisotropy spectrum and on the cosmic coincidence problem. [Copyright &y& Elsevier]

Abstract: We investigate observational constraints on dark energy models from lookback time (LT) estimates of 32 old passive galaxies distributed over the redshift interval . To build up our LT sample we combine the age measurements for these 32 objects with estimates of the total age of the Universe, as obtained from current CMB data. We show that LT data may provide bounds on the cosmological parameters with accuracy competitive with type Ia Supernova methods. In order to break possible degeneracies between models parameters, we also discuss the bounds when our lookback time versus redshift sample is combined with the recent measurement of the baryonic acoustic oscillation peak and the derived age of the Universe from current CMB measurements. [Copyright &y& Elsevier]

Abstract: A new class of accelerating cosmological models driven by a one-parameter version of the general Chaplygin-type equation of state is proposed. The simplified version is naturally obtained from causality considerations with basis on the adiabatic sound speed plus the observed accelerating stage of the universe. We show that very stringent constraints on the unique free parameter describing the simplified Chaplygin model can be obtained from a joint analysis involving the latest SNe type Ia data and the recent Sloan Digital Sky Survey measurement of baryon acoustic oscillations (BAO). In our analysis we have considered separately the SNe type Ia gold sample measured by [A.G. Riess et al., Astrophys. J. 607 (2004) 665] and the supernova legacy survey (SNLS) from [P. Astier et al., Astron. Astrophys. 447 (2006) 31]. At 95.4% (c.l.), we find for BAO+ gold sample, and while BAO+SNLS analysis provides and . [Copyright &y& Elsevier]

Abstract: A class of modified gravity, known as -gravity, has presently been applied to Cosmology as a realistic alternative to dark energy. In this Letter we use the most recent Type-Ia Supernova (SNe Ia) data, the so-called Union sample of 307 SNe Ia, to place bounds on a theory of the form within the Palatini approach. Given the complementarity of SNe Ia data with other cosmological observables, a joint analysis with measurements of baryon acoustic oscillation peak and estimates of the CMB shift parameter is also performed. We show that, for the allowed intervals of n, , and β, models based on gravity in the Palatini approach can produce the sequence of radiation-dominated, matter-dominated, and accelerating periods without need of dark energy. [Copyright &y& Elsevier]

Abstract: Determining the mechanism behind the current cosmic acceleration constitutes a major question nowadays in theoretical physics. If the dark energy route is taken, this problem may potentially bring to light new insights not only in cosmology but also in high energy physics theories. Following this approach, we explore in this Letter some cosmological consequences of a new time-dependent parameterization for the dark energy equation of state (EoS), which is a well behaved function of the redshift z over the entire cosmological evolution, i.e., . This parameterization allows us to divide the parametric plane in defined regions associated to distinct classes of dark energy models that can be confirmed or excluded from a confrontation with current observational data. By assuming a flat universe, a statistical analysis involving the most recent observations from type Ia supernovae, baryon acoustic oscillation peak, Cosmic Microwave Background shift parameter and Hubble evolution is performed to check the observational viability of the EoS parameterization here proposed. [Copyright &y& Elsevier]

Abstract: One of the most important questions nowadays in physics concerns the nature of the so-called dark energy. It is also a consensus among cosmologists that such a question will not be answered on the basis only of observational data. However, it is possible to diminish the range of possibilities for this dark component by comparing different dark energy scenarios and finding which models can be ruled out by current observations. In this paper, by assuming three distinct parametrizations for the low-redshift evolution of the dark energy equation of state (EOS), we consider the possibility of discriminating between evolving dark energy and ΛCDM models from a joint analysis involving the most recent radio sources gravitational lensing sample, namely, the Cosmic all Sky Survey (CLASS) statistical data and the recently published gold SNe Ia sample. It is shown that this particular combination of observational data restricts considerably the dark energy parameter space, which enables possible distinctions between time-dependent and constant EOSs. [Copyright &y& Elsevier]

Abstract: Recent discoveries in the field of observational cosmology have provided increasing evidence that the Universe is undergoing a late time acceleration, which has also stimulated speculations on the nature of the dark component responsible for such a phenomenon. Among several candidates discussed in the current literature, phantom fields, an exotic scalar field with a negative kinetic term and that violates most of the classical energy conditions, appear as a real possibility according to recent observational analysis. In this Letter we examine the invariant characterization for the energy–momentum tensor of phantom fields through the Segre algebraic classification in the framework of general relativity. We also discuss some constraints which are imposed on the values of from the classical energy conditions. [Copyright &y& Elsevier]

Abstract: We propose a new class of accelerating world models unifying the cosmological dark sector (dark matter and dark energy). All the models are described by a simplified version of the Chaplygin gas quartessence cosmology. It is found that even for , this quartessence scenario depends only on a pair of parameters which can severely be constrained by the cosmological tests. As an example we perform a joint analysis involving the latest SNe type Ia data and the recent Sloan Digital Sky Survey measurement of baryon acoustic oscillations. In our analysis we have considered the SNe type Ia Union sample compiled by Kowalski et al. [M. Kowalski et al., Astrophys. J. 686 (2008) 749, arXiv:0804.4142]. At 95.4% (c.l.), we find for BAO+Union sample, and . The best-fit for this simplified quartessence scenario is a spatially closed Universe and its reduced is exactly the same of the flat concordance model (ΛCDM). [Copyright &y& Elsevier]

Abstract: We study the cosmological constraints on the two purely phenomenological models of oscillating dark energy. In these oscillating models, the equation of state of dark energy varies periodically. The periodic equation of state may provide the natural way to unify the early acceleration (inflation) and the late time acceleration of the Universe. These models give the effective way to tackle the cosmic coincidence problem. We examine the observational constraints on the oscillatory models from the latest observational data including the gold sample of 182 SNe type Ia, the shift parameter, , given by the WMAP and the BAO measurements from the SDSS. [Copyright &y& Elsevier]

Abstract: We investigate cosmological consequences of a class of exponential gravity in the Palatini formalism. By using the current largest type Ia Supernova sample along with determinations of the cosmic expansion at intermediary and high-z we impose tight constraints on the model parameters. Differently from other models, we find solutions of transient acceleration, in which the large-scale modification of gravity will drive the Universe to a new decelerated era in the future. We also show that a viable cosmological history with the usual matter-dominated era followed by an accelerating phase is predicted for some intervals of model parameters. [Copyright &y& Elsevier]