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Lixin Xu

Publications and source records attributed to Lixin Xu.

At least 109 records · Page 6Linked to original sources
arXiv2013

Unified Dark Fluid with Constant Adiabatic Sound Speed: Including Entropic Perturbations

In this paper, we continue to study a unified dark fluid model with a constant adiabatic sound speed but with the entropic perturbations. When the entropic perturbations are included, an effective sound speed, which reduces to the adiabatic sound speed when the entropic perturbations are zero, has to be specified as an additional free model parameter. Due to the relations between the adiabatic sound speed and equations of state (EoS) $c^2_{s,ad}(a)=w(a)-d\ln(1+w(a))/3 d\ln a$, the equation of state can be determined up to an integration constant in principle when an adiabatic sound speed is given. Then there are two degrees of freedom to describe the linear perturbations for a fluid. Its micro-scale properties are characterized by its EoS or adiabatic sound speed and an effective sound speed. We take the effective sound speed and adiabatic sound speed as free model parameters and then use the currently available cosmic observational data sets, which include type Ia supernova Union 2.1, baryon acoustic oscillation and WMAP 7-year data of cosmic background radiation, to constrain the possible entropic perturbations and the adiabatic sound speed via the Markov Chain Monte Carlo method. The results show that the cosmic observations favor a small effective sound speed $c^2_{s,eff}=0.00155_{- 0.00155}^{+ 0.000319}$ in $1σ$ region.

astro-ph.CO↗
arXiv2013

Constraints on the Holographic Dark Energy Model from Type Ia Supernovae, WMAP7, Baryon Acoustic Oscillation and Redshift-Space Distortion

In this paper, we use the joint measurement of geometry and growth rate from matter density perturbations to constrain the holographic dark energy model. The geometry measurement includes type Ia supernovae (SN Ia) Union2.1, full information of cosmic microwave background (CMB) from WMAP-7yr and baryon acoustic oscillation (BAO). For the growth rate of matter density perturbations, the results $f(z)σ_8(z)$ measured from the redshift-space distortion (RSD) in the galaxy power spectrum are employed. Via the Markov Chain Monte Carlo method, we try to constrain the model parameters space. The jointed constraint shows that $c=0.750_{- 0.0999- 0.173- 0.226}^{+ 0.0976+ 0.215+ 0.319}$ and $σ_8=0.763_{- 0.0465- 0.0826- 0.108}^{+ 0.0477+ 0.0910+ 0.120}$ with $1,2,3σ$ regions. After marginalizing the other irrelevant model parameters, we show the evolution of the equation of state of HDE with respect to the redshift $z$. Though the current cosmic data points favor a phantom like HDE Universe for the mean values of the model parameters in the future, it can behave like quintessence in $3σ$ regions.

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arXiv2012

A New Unified Dark Fluid Model and Its Cosmic Constraint

In this paper, we propose a new unified dark fluid (UDF) model with equation of state (EoS) $w(a)=-α/(βa^{-n}+1)$, which includes the generalized Chaplygin gas model (gGg) as its special case, where $α$, $β$ and $n$ are three positive numbers. It is clear that this model reduces to the gCg model with EoS $w(a)=-B_s/(B_s+(1-B_s)a^{-3(1+α)})$, when $α=1$, $β=(1-B_s)/B_s$ and $n=3(1+α)$. By combination the cold dark matter and the cosmological constant, one can coin a EoS of unified dark fluid in the form of $w(a)=-1/(1+(1-Ω_Λ)a^{-3}/Ω_Λ)$. With this observations, our proposed EoS provides a possible deviation from $Λ$CDM model when the model parameters $α$ and $n$ deviate from 1 and 3 respectively. By using the currently available cosmic observations from type Ia supernovae (SN Ia) Union2.1, baryon acoustic oscillation (BAO) and cosmic microwave background radiation (CMB), we test the viability of this model and detect the possible devotion from the $Λ$CDM model. The results show that the new UDF model fits the cosmic observation as well as that of the $Λ$CDM model and no deviation is found from the $Λ$CDM model in $3σ$ confidence level. However, our new UDF model can give a non-zero sound speed, as a contrast, which is zero for the $Λ$CDM model. We expect the large structure formation information can distinct the new UDF model from the $Λ$CDM model.

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arXiv2012

Revisiting Cardassian Model and Cosmic Constraint

In this paper, we revisit the Cardassian model in which the radiation energy component is included. It is important for early epoch when the radiation cannot be neglected because the equation of state (EoS) of the effective dark energy becomes time variable. Therefore, it is not equivalent to the quintessence model with a constant EoS anymore. This situation was almost overlooked in the literature. By using the recent released Union2 557 of type Ia supernovae (SN Ia), the baryon acoustic oscillation (BAO) from Sloan Digital Sky Survey and the WiggleZ data points, the full information of cosmic microwave background (CMB) measurement given by the seven-year Wilkinson Microwave Anisotropy Probe observation, we constrain the Cardassian model via the Markov Chain Monte Carlo (MCMC) method. A tight constraint is obtained: $n= -0.0479_{- 0.0732- 0.148}^{+ 0.0730+ 0.142}$ in $1,2σ$ regions. The deviation of Cardassian model from quintessence model is shown in CMB anisotropic power spectra at high l's parts due to the evolution of EoS. But it is about the order of 0.1% which cannot be discriminated by current data sets. The Cardassian model is consistent with current cosmic observational data sets.

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arXiv2012

Constraints to Holographic Dark Energy Model via Type Ia Supernovae, Baryon Acoustic Oscillation and WMAP

In this paper, the holographic dark energy (HDE) model, where the future event horizon is taken as an IR cut-off, is confronted by using currently available cosmic observational data sets which include type Ia supernovae, baryon acoustic oscillation and cosmic microwave background radiation from full information of WMAP-7yr. Via the Markov Chain Monte Carlo method, we obtain the values of model parameter $c= 0.696_{- 0.0737- 0.132- 0.190}^{+ 0.0736+ 0.159+ 0.264}$ with $1,2,3σ$ regions. Therefore one can conclude that at lest $3σ$ level the future Universe will be dominated by phantom like dark energy. It is not consistent with positive energy condition, however this condition must be satisfied to derive the holographic bound. It implies that the current cosmic observational data points disfavor the HDE model.

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arXiv2012

Modified Chaplygin Gas as a Unified Dark Matter and Dark Energy Model and Cosmic Constraints

A modified Chaplygin gas model (MCG), $ρ_{MCG}/ρ_{MCG0}=[B_{s}+(1-B_{s})a^{-3(1+B)(1+α)}]^{1/(1+α)}$, as a unified dark matter model and dark energy model is constrained by using current available cosmic observational data points which include type Ia supernovae, baryon acoustic oscillation and the seventh year full WMAP data points. As a contrast to the consideration in the literatures, we {\it do not} separate the MCG into two components, i.e. dark mater and dark energy component, but we take it as a whole energy component-a unified dark sector. By using Markov Chain Monte Carlo method, a tight constraint is obtained: $α= 0.000727_{- 0.00140- 0.00234}^{+ 0.00142+ 0.00391}$, $B=0.000777_{- 0.000302- 0.000697}^{+ 0.000201+ 0.000915}$ and $B_s= 0.782_{- 0.0162- 0.0329}^{+ 0.0163+ 0.0307}$ .}

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arXiv2012

Cosmological Model-independent Gamma-ray Bursts Calibration and its Cosmological Constraint to Dark Energy

As so far, the redshift of Gamma-ray bursts (GRBs) can extend to $z\sim 8$ which makes it as a complementary probe of dark energy to supernova Ia (SN Ia). However, the calibration of GRBs is still a big challenge when they are used to constrain cosmological models. Though, the absolute magnitude of GRBs is still unknown, the slopes of GRBs correlations can be used as a useful constraint to dark energy in a completely cosmological model independent way. In this paper, we follow Wang's model-independent distance measurement method and calculate their values by using 109 GRBs events via the so-called Amati relation. Then, we use the obtained model-independent distances to constrain $Λ$CDM model as an example.

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arXiv2012

Revisiting Generalized Chaplygin Gas as a Unified Dark Matter and Dark Energy Model

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 $ρ_{GCG}/ρ_{GCG0}=[B_{s}+(1-B_{s})a^{-3(1+α)}]^{1/(1+α)}$, where $α$ and $B_s$ 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: $α=0.00126_{- 0.00126- 0.00126}^{+ 0.000970+ 0.00268}$ and $B_s= 0.775_{- 0.0161- 0.0338}^{+ 0.0161+ 0.0307}$.

astro-ph.CO↗
arXiv2012

Unified Dark Fluid with Constant Adiabatic Sound Speed and Cosmic Constraints

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 $c_s^2=α$, which is in the range $[0,1]$, is studied. At first, via the energy conservation equation, its energy density, $ρ_d/ρ_{d0}=(1-B_s)+B_s a^{-3(1+α)}$ where $B_s$ 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.

astro-ph.CO↗
arXiv2011

CMB Temperature and Matter Power Spectrum in a Decay Vacuum Dark Energy Model

In this paper, a decay vacuum model $\barρ_Λ=3σM_p^2H_0 H$ is revisited by detailed analysis of background evolution and perturbation equations. We show the imprints on CMB temperature and matter power spectrum from the effective coupling terms between dark sectors by comparing to the standard cosmological constant model and observational data points (WMAP7 and SDSS DR7). We find that the decay vacuum model can describe the expansion rate at late times as well as the standard cosmological constant model but it fails to simultaneously reproduce the observed CMB and matter power spectrum. Its generalization $\barρ_Λ=3M_p^2(ξ_1 H_0 H+ξ_2 H^2)$ is also discussed. Detailed analysis of the background evolution shows that the dimensionless parameter $ξ_{2}$ would be zero to avoid the unnatural 'fine tuning' and to keep the positivity of energy density of dark matter and dark energy in the early epoch.

astro-ph.CO↗
arXiv2011

Probing Ricci dark energy model with perturbations by using WMAP seven-year cosmic microwave background measurements, BAO and Type Ia supernovae

In this paper, we investigate the Ricci dark energy model with perturbations through the joint constraints of current cosmological data sets from dynamical and geometrical perspectives. We use the full cosmic microwave background information from WMAP seven-year data, the baryon acoustic oscillations from the Sloan Digital Sky Survey and the Two Degree Galaxy Redshift Survey, and type Ia supernovae from the Union2 compilation of the Supernova Cosmology Project Collaboration. A global constraint is performed by employing the Markov chain Monte Carlo method. With the best-fitting results, we show the differences of cosmic microwave background power spectra and background evolutions for the cosmological constant model and Ricci dark energy model with perturbations.

astro-ph.CO↗
arXiv2011

Does accelerating universe indicates Brans-Dicke theory

The evolution of universe in Brans-Dicke (BD) theory is discussed in this paper. Considering a parameterized scenario for BD scalar field $ϕ=ϕ_{0}a^α$ which plays the role of gravitational "constant" $G$, we apply the Markov Chain Monte Carlo method to investigate a global constraints on BD theory with a self-interacting potential according to the current observational data: Union2 dataset of type supernovae Ia (SNIa), high-redshift Gamma-Ray Bursts (GRBs) data, observational Hubble data (OHD), the cluster X-ray gas mass fraction, the baryon acoustic oscillation (BAO), and the cosmic microwave background (CMB) data. It is shown that an expanded universe from deceleration to acceleration is given in this theory, and the constraint results of dimensionless matter density $Ω_{0m}$ and parameter $α$ are, $Ω_{0m}=0.286^{+0.037+0.050}_{-0.039-0.047}$ and $α=0.0046^{+0.0149+0.0171}_{-0.0171-0.0206}$ which is consistent with the result of current experiment exploration, $\midα\mid \leq 0.132124$. In addition, we use the geometrical diagnostic method, jerk parameter $j$, to distinguish the BD theory and cosmological constant model in Einstein's theory of general relativity.

astro-ph.CO↗
arXiv2011

Cosmography: Supernovae Union2, Baryon Acoustic Oscillation, Observational Hubble Data and Gamma Ray Bursts

In this paper, a parametrization describing the kinematical state of the universe via cosmographic approach is considered, where the minimum input is the assumption of the cosmological principle, i.e. the Friedmann-Robertson-Walker metric. A distinguished feature is that the result does not depend on any gravity theory and dark energy models. As a result, a series of cosmographic parameters (deceleration parameter $q_0$, jerk parameter $j_0$ and snap parameter $s_0$) are constrained from the cosmic observations which include type Ia supernovae (SN) Union2, the Baryon Acoustic Oscillation (BAO), the observational Hubble data (OHD), the high redshift Gamma ray bursts (GRBs). By using Markov Chain Monte Carlo (MCMC) method, we find the best fit values of cosmographic parameters in $1σ$ regions: $H_0=74.299^{+4.932}_{-4.287}$, $q_0=-0.386^{+0.655}_{-0.618}$, $j_0=-4.925^{+6.658}_{-7.297}$ and $s_0=-26.404^{+20.964}_{-9.097}$ which are improved remarkably. The values of $q_0$ and $j_0$ are consistent with flat $Λ$CDM model in $1σ$ region. But the value of $s_0$ of flat $Λ$CDM model will go beyond the $1σ$ region.

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arXiv2011

Constraints on the generalized Chaplygin gas model including gamma-ray bursts via a Markov Chain Monte Carlo approach

We investigate observational constraints on the generalized Chaplygin gas (GCG) model including the gamma-ray bursts (GRBs) at high redshift obtained directly from the Union2 type Ia supernovae (SNe Ia) set. By using the Markov Chain Monte Carlo method, we constrain the GCG model with the cosmology-independent GRBs, as well as the Union2 set, the cosmic microwave background (CMB) observation from the Wilkinson Microwave Anisotropy Probe (WMAP7) result, and the baryonic acoustic oscillation (BAO) observation from the spectroscopic Sloan Digital Sky Survey (SDSS) data release 7 (DR7) galaxy sample. The best-fit values of the GCG model parameters are $A_S$=$0.7475_{-0.0539}^{+0.0556}(1σ)_{-0.0816}^{+0.0794}(2σ)$, $α$=$-0.0256_{-0.1326}^{+0.1760}(1σ)_{-0.1907}^{+0.2730}(2σ)$, and the effective matter density $Ω_{m}=0.2629_{-0.0153}^{+0.0155}(1σ)_{-0.0223}^{+0.0236}(2σ)$, which are more stringent than the previous results for constraining on GCG model parameters.

astro-ph.CO↗
arXiv2011

Combined constraints on modified Chaplygin gas model from cosmological observed data: Markov Chain Monte Carlo approach

We use the Markov Chain Monte Carlo method to investigate a global constraints on the modified Chaplygin gas (MCG) model as the unification of dark matter and dark energy from the latest observational data: the Union2 dataset of type supernovae Ia (SNIa), the observational Hubble data (OHD), the cluster X-ray gas mass fraction, the baryon acoustic oscillation (BAO), and the cosmic microwave background (CMB) data. In a flat universe, the constraint results for MCG model are, $Ω_{b}h^{2}=0.02263^{+0.00184}_{-0.00162}$ ($1σ$) $^{+0.00213}_{-0.00195}$ $(2σ)$, $B_{s}=0.7788^{+0.0736}_{-0.0723}$ ($1σ$) $^{+0.0918}_{-0.0904}$ $(2σ)$, $α=0.1079^{+0.3397}_{-0.2539}$ ($1σ$) $^{+0.4678}_{-0.2911}$ $(2σ)$, $B=0.00189^{+0.00583}_{-0.00756}$ ($1σ$) $^{+0.00660}_{-0.00915}$ $(2σ)$, and $H_{0}=70.711^{+4.188}_{-3.142}$ ($1σ$) $^{+5.281}_{-4.149}$ $(2σ)$.

astro-ph.CO↗
arXiv2011

Constraints on kinematic models from the latest observational data

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 $q_{0}$, jerk parameter $j_{0}$ and transition redshift $z_{T}$, are obtained. Furthermore, we show the departures for two parameterized kinematical models from $Λ$CDM 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.

astro-ph.CO↗
arXiv2011

$Λ(t)$CDM Model as a Unified Origin of Holographic and Agegraphic Dark Energy Models

Motivated by the fact that any nonzero $Λ$ can introduce a length scale or a time scale into Einstein's theory, $r_Λ=ct_Λ=\sqrt{3/|Λ|}$. Conversely, any cosmological length scale or time scale can introduce a $Λ(t)$, $Λ(t)=3/r^2_Λ(t)=3/(c^2t^2_Λ(t))$. In this letter, we investigate the time varying $Λ(t)$ 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 $Λ(t)$CDM 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 $Q=-\dotρ_Λ$. We place observational constraints on the $Λ(t)$CDM 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 $0.015\leq z \leq 1.4$. 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.

astro-ph.CO↗
arXiv2010

Testing the Consistence of Gamma Ray Burst Data-set and Supernovae Union2

In this paper, we test the consistency of Gamma Ray Bursts (GRBs) Data-set and Supernovae Union2 (SNU2) via the so-called {\it multi-dimensional consistency test} under the assumption that $Λ$CDM model is a potentially correct cosmological model. We find that the probes are inconsistent with $1.456σ$ and $85.47%$ in terms of probability. With this observation, it is concluded that GRBs can be combined with SNU2 to constrain cosmological models.

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