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In this paper, we use the latest observations of quasars covering the redshift range of 0.04 < z < 5.1 to investigate a series of Chaplygin gas models as candidates for unified dark matter and dark energy. Based on different combinations of available standard candle and standard ruler data, we put constraints on the generalized Chaplygin gas (GCG), modified Chaplygin gas (MCG), new generalized Chaplygin gas (NGCG) and viscous generalized Chaplygin gas (VGCG) models. Moreover, we apply Jensen–Shannon divergence (JSD), statefinder diagnostics, and the deviance information criterion (DIC) to distinguish these CG models, based on the statistical results derived from Markov chain Monte Carlo method. The results show that (1) The standard ruler data could provide more stringent constraints on the cosmological parameters of different CG models considered in this analysis. Interestingly, the matter density parameter Ω m and Hubble constant H 0 derived from the available data are well consistent with those from the Planck 2018 results; (2) Based on the statistical criteria JSD, our findings demonstrate the well consistency between Chaplygin gas and the concordance Λ CDM model. However, in the framework of statefinder diagnostics, the GCG and NGCG models cannot be distinguished from Λ CDM, while MCG and VGCG models show significant deviation from Λ CDM in the present epoch; (3) According to the the statistical criteria DIC, we show that the MCG and VGCG models have substantial observational support from high-redshift quasars, whereas the GCG and NGCG models miss out on the less observational support category but can not be ruled out. [ABSTRACT FROM AUTHOR]

It has been shown that time delay of strong gravitational lensing is not only an effective cosmological probe to constrain the Hubble constant, the matter density, and the curvature of the universe but also useful for breaking the degeneracy of the dark energy equation of state and thus provide complementarity with other popular probes, such as Type Ia supernovae, baryon acoustic oscillations, and cosmic microwave background radiation. Interestingly, compared to traditional strong lensing systems where quasars act as sources, strongly lensed gravitational waves (GWs) from compact binary coalescence and their electromagnetic (EM) counterpart systems have been recently proposed to be more powerful for studying cosmology since GWs and their EM counterparts are transients. Short durations of GWs and their EM counterparts are very advantageous to time delay measurement and lens profile modelling. Here, in the framework of Chevalier–Polarski–Linder parametrization, we investigate improvement of constraining power on the dark energy equation of state due to including time delay measurements of strong lensed GW systems. It is suggested that, on the basis of the third generation ground-based detector, e.g. the Einstein Telescope, adding time delay of only 30 strong lensed GW systems to Type Ia supernovae and cosmic microwave background radiation can improve the dark energy figure of merit by a factor of 2. For the traditional standard siren method where the uncertainties of luminosity distances of GWs are |${\sim }10{{\ \rm per\ cent}}$|⁠ , a few × 104 events are expected to present similar constraints. In the precision cosmology era, this progress is of great significance for studying the nature of dark energy. [ABSTRACT FROM AUTHOR]

Abstract: In this Letter, we propose a new generalized Ricci dark energy (NGR) model to unify Ricci dark energy (RDE) and XCDM. Our model can distinguish between RDE and XCDM by introducing a parameter β called weight factor. When , NGR model becomes the usual RDE model. The XCDM model is corresponding to . Moreover, NGR model permits the situation where neither nor . We then perform a statefinder analysis on NGR model to see how β effects the trajectory on the plane. In order to know the value of β, we constrain NGR model with latest observations including type Ia supernovae (SNe Ia) from Union2 set (557 data), baryonic acoustic oscillation (BAO) observation from the spectroscopic Sloan Digital Sky Survey (SDSS) data release 7 (DR7) galaxy sample and cosmic microwave background (CMB) observation from the 7-year Wilkinson Microwave Anisotropy Probe (WMAP7) results. With Markov Chain Monte Carlo (MCMC) method, the constraint result is , which manifests the observations prefer a XCDM universe rather than RDE model. It seems RDE model is ruled out in NGR scenario within 2σ regions. Furthermore, we compare it with some of successful cosmological models using AIC information criterion. NGR model seems to be a good choice for describing the universe. [Copyright &y& Elsevier]

Abstract: gravity is thought to be an alternative to dark energy which can explain the acceleration of the universe. It has been tested by different observations including type Ia supernovae (SNIa), the cosmic microwave background (CMB), the baryon acoustic oscillations (BAO) and so on. In this Letter, we use the Hubble constant independent ratio between two angular diameter distances to constrain model in Palatini approach . These data are from various large systematic lensing surveys and lensing by galaxy clusters combined with X-ray observations. We also combine the lensing data with CMB and BAO, which gives a stringent constraint. The best-fit results are or using lensing data only. When combined with CMB and BAO, the best-fit results are or . If we further fix (corresponding to ΛCDM), the best-fit value for α is for the lensing analysis and for the combined data, respectively. Our results show that ΛCDM model is within 1σ range. [Copyright &y& Elsevier]

ABSTRACT In order to test the possible interaction between dark energy and dark matter, we investigate observational constraints on a phenomenological scenario, in which the ratio between the dark energy and matter densities is proportional to the power-law case of the scalefactor, r≡ (ρ X/ρm) ∝ aξ. By using the Markov chain Monte Carlo method, we constrain the phenomenological interacting dark energy model with the newly revised H( z) data, as well as the cosmic microwave background (CMB) observation from the 7-year Wilkinson Microwave Anisotropy Probe ( WMAP7) results, the baryonic acoustic oscillation (BAO) observation from the spectroscopic Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) galaxy sample and the Type Ia supernovae (SNe Ia) from the Union2 set. The best-fitting values of the model parameters are Ωm0= 0.27+0.02−0.02 (1σ)+0.04−0.03 (2σ), ξ= 3.15+0.48−0.50(1σ)+0.72−0.71(2σ) and w X=−1.05+0.15−0.14(1σ)+0.21−0.21(2σ), which are more stringent than previous results. These results show that the standard Λ cold dark matter (ΛCDM) model without any interaction remains a good fit to the recent observational data; however, the interaction that the energy transferring from dark matter to dark energy is slightly favoured over the interaction from dark energy to dark matter. It is also shown that the H( z) data can give more stringent constraints on the phenomenological interacting scenario when combined with CMB and BAO observations, and the confidence regions of H( z)+BAO+CMB, SNe+BAO+CMB and H( z)+SNe+BAO+CMB combinations are consistent with each other. [ABSTRACT FROM AUTHOR]

BSTRACT We apply the Union2 compilation of 557 supernova Ia data, the baryon acoustic oscillation measurement of distance, the cosmic microwave background radiation data from the 7-year Wilkinson Microwave Anisotropy Probe, and the Hubble parameter data to study the geometry of the Universe and the property of dark energy by using models and parametrizations with different high-redshift behaviours of w( z). We find that the Λ cold dark matter model is consistent with current data, that the Dvali-Gabadadze-Porrati model is excluded by the data at more than the 3σ level, that the Universe is almost flat and that the current data are unable to distinguish between models with different behaviours of w( z) at high redshift. We also add the growth factor data to constrain the growth index of the Dvali-Gabadadze-Porrati model and find that it is more than 1σ away from its theoretical value. [ABSTRACT FROM AUTHOR]

Abstract: Motivated by the fact that any nonzero Λ can introduce a length scale or a time scale into Einsteinʼs theory, . Conversely, any cosmological length scale or time scale can introduce a , . In this Letter, we investigate the time varying corresponding to the length scales, including the Hubble horizon, the particle horizon and the future event horizon, and the time scales, including the age of the universe and the conformal time. It is found out that, in this scenario, the model can be taken as the unified origin of the holographic and agegraphic dark energy models with interaction between the matter and the dark energy, where the interacting term is determined by . We place observational constraints on the models originating from different cosmological length scales and time scales with the recently compiled “Union2 compilation” which consists of 557 Type Ia supernovae (SNIa) covering a redshift range . In conclusion, an accelerating expansion universe can be derived in the cases taking the Hubble horizon, the future event horizon, the age of the universe and the conformal time as the length scale or the time scale. [Copyright &y& Elsevier]

Inspired by the holographic principle, we suggest that the density of dark energy is proportional to the spatial Ricci scalar curvature (SRDE). Such a model is phenomenologically viable. The best fit values of its parameters at 68% confidence level are found to be Ωm 0 = 0.259±0.016 and α = 0.261±0.0122, constrained from the Union+CFA3 sample of 397 SNIa and the BAO measurement. We find that the equation of state of SRDE crosses -1 at z ≃ -0.14. The present value of the deceleration parameter q(z) for SRDE is found to be qz = 0~-0.85. The phase transition from deceleration to acceleration of the Universe for SRDE occurs at the redshift zq = 0~0.4. After studying the perturbation of each component of the Universe, we show that the matter power spectra and cosmic microwave background temperature anisotropy are slightly affected by SRDE, compared with ΛCDM. [ABSTRACT FROM AUTHOR]

Hybrid Chaplygin gas model is put forward, in which the gases play the role of dark energy. For this model the coincidence problem is greatly alleviated. The effective equation of state of the dark energy may cross the phantom divide w = -1. Furthermore, the crossing behavior is decoupled from any gravity theories. In the present model, w < -1 is only a transient behavior. There is a de Sitter attractor in the future infinity. Hence, the big rip singularity, which often afflicts the models with matter whose effective equation of state less than -1, naturally disappears. There exist stable scaling solutions, both at the early universe and the late universe. We discuss the perturbation growth of this model. We find that the index is consistent with observations. [ABSTRACT FROM AUTHOR]

By using the comoving distance, we derive an analytic expression for the optical depth of gravitational lensing, which depends on the redshift to the source and the cosmological model characterized by the cosmic mass density parameter Ω[sub m], the dark energy density parameter Ω[sub x]. and its equation of state ω[sub x] = p[sub x]/ρ[sub x]. It is shown that, the larger the dark energy density and the more negative its pressure, the higher is the gravitational lensing probability. This fact can provide an independent constraint for dark energy. [ABSTRACT FROM AUTHOR]

Direct measurements of Hubble parameters H(z) are very useful for cosmological model parameters inference. Based on them, Sahni, Shafieloo and Starobinski introduced a two-point diagnostic Omh2(zi,zj) as an interesting tool for testing the validity of the ΛCDM model. Applying this test they found a tension between observations and predictions of the ΛCDM model. We use the most comprehensive compilation H(z) data from baryon acoustic oscillations (BAO) and differential ages (DA) of passively evolving galaxies to study cosmological models using the Hubble parameters itself and to distinguish whether ΛCDM model is consistent with the observational data with statistical analysis of the corresponding Omh2(zi,zj) two-point diagnostics. Our results show that presently available H(z) data significantly improve the constraints on cosmological parameters. The corresponding statistical Omh2(zi,zj) two-point diagnostics seems to prefer the quintessence with w>-1 over the ΛCDM model. Better and more accurate prior knowledge of the Hubble constant, will considerably improve the performance of the statistical Omh2(zi,zj) method. [ABSTRACT FROM AUTHOR]

We study the redshift distribution of two samples of early-type gravitational lenses, extracted from a larger collection of 122 systems, to constrain the cosmological constant in the ΛCDM model and the parameters of a set of alternative dark energy models (XCDM, Dvali-Gabadadze-Porrati, and Ricci dark energy models), in a spatially flat universe. The likelihood is maximized for ΩΛ = 0.70 ± 0.09 when considering the sample excluding the Sloan Lens ACS systems (known to be biased toward large image-separation lenses) and no-evolution, and ΩΛ = 0.81 ± 0.05 when limiting to gravitational lenses with image separation Δθ > 2″ and no-evolution. In both cases, results accounting for galaxy evolution are consistent within 1σ. The present test supports the accelerated expansion, by excluding the null hypothesis (i.e., ΩΛ = 0) at more than 4σ, regardless of the chosen sample and assumptions on the galaxy evolution. A comparison between competitive world models is performed by means of the Bayesian information criterion. This shows that the simplest cosmological constant model—that has only one free parameter—is still preferred by the available data on the redshift distribution of gravitational lenses. We perform an analysis of the possible systematic effects, finding that the systematic errors due to sample incompleteness, galaxy evolution, and model uncertainties approximately equal the statistical errors, with present-day data. We find that the largest sources of systemic errors are the dynamical normalization and the high-velocity cutoff factor, followed by the faint-end slope of the velocity dispersion function. [ABSTRACT FROM AUTHOR]

Abstract: In this Letter, we find suitable potentials in the multiple scalar fields scenario by using the Noether symmetry approach. We discussed three models with multiple scalar fields: N-quintessence with positive kinetic terms, N-phantom with negative kinetic terms and N-quintom with both positive and negative kinetic terms. In the N-quintessence case, the exponential potential which could be derived from several theoretic models is obtained from the Noether conditions. In the N-phantom case, the potential , which could be derived from the Pseudo Nambu–Goldstone boson model, is chosen as the Noether conditions required. In the N-quintom case, we derive a relation between the potential forms for the quintessence-like fields and the phantom-like fields by using the Noether symmetry. [Copyright &y& Elsevier]

Abstract: The cosmic coincidence problem is a serious challenge to dark energy model. We suggest a quantitative criteria for judging the severity of the coincidence problem. Applying this criteria to three different interacting models, including the interacting quintessence, interacting phantom, and interacting Chaplygin gas models, we find that the interacting Chaplygin gas model has a better chance to solve the coincidence problem. Quantitatively, we find that the coincidence index C for the interacting Chaplygin gas model is smaller than that for the interacting quintessence and phantom models by six orders of magnitude. [Copyright &y& Elsevier]

Abstract: We use the new gamma-ray bursts (GRBs) data, combined with the baryon acoustic oscillation (BAO) observation from the spectroscopic Sloan digital sky survey (SDSS) data release, the newly obtained A parameter at from the WiggleZ dark energy survey, the cosmic microwave background (CMB) observations from the 7-year Wilkinson microwave anisotropy probe (WMAP7) results, and the type Ia supernovae (SNeIa) from Union2 set, to constrain a phenomenological model describing possible interactions between dark energy and dark matter, which was proposed to alleviate the coincidence problem of the standard ΛCDM model. By using the Markov chain Monte Carlo (MCMC) method, we obtain the marginalized 1σ constraints , , and . We also consider other combinations of these data for comparison. These results show that: (1) the energy of dark matter is slightly transferring to that of dark energy; (2) even though the GRBs+BAO+CMB data present less stringent constraints than SNe+BAO+CMB data do, the GRBs can help eliminate the degeneracies among parameters. [Copyright &y& Elsevier]

Abstract: We constrain a unified dark matter (UDM) model from the latest observational data. This model assumes that the dark sector is degenerate. Dark energy and dark matter are the same component. It can be described by an affine equation of state . Our data set contains the newly revised data, type Ia supernovae (SNe Ia) from Union2 set, baryonic acoustic oscillation (BAO) observation from the spectroscopic Sloan Digital Sky Survey (SDSS) data release 7 (DR7) galaxy sample, as well as the cosmic microwave background (CMB) observation from the 7-year Wilkinson Microwave Anisotropy Probe (WMAP7) results. By using the Markov Chain Monte Carlo (MCMC) method, we obtain the results in a flat universe: , , . Moreover, when considering a non-flat universe, , , , . These give a more stringent results than before. We also give the results from other combinations of these data for comparison. The observational Hubble parameter data can give a more stringent constraint than SNe Ia. From the constraint results, we can see the parameters α and are very close to zero, which means a flat universe is strongly supported and the speed of sound of the dark sector seems to be zero. [Copyright &y& Elsevier]