arXiv subjects

Lixin Xu

Publications and source records attributed to Lixin Xu.

At least 127 records · Page 7Linked to original sources

arXiv2010

Time Variable Cosmological Constants from Cosmological Horizons

In this paper, motivated from the fact that a de Sitter cosmological boundary corresponds to a positive cosmological constant, we consider time variable cosmological constants, dubbed {\it horizon cosmological constants}. The horizon cosmological constants correspond to Hubble horizon, future event horizon and particle horizon are discussed respectively. When the Hubble horizon is taken as a cosmological length scale, the effective equation of state of horizon cosmological constant is quintessence-like. The values of model parameter $c$ will determine the current status of our universe. When particle horizon is taken as the cosmological length scale, non viable cosmological model can be obtained for the requirement of $Ω_Λ<1/3$ which conflicts with current comic observations. When the future event horizon is taken as the role of cosmological length scale, the forms of effective equation of state of horizon cosmological constants are the the same as the holographic ones. But, their evolutions are different because of the effective interaction with cold dark matter.

astro-ph.CO↗

arXiv2010

Scattering of scalar perturbations with cosmological constant in low-energy and high-energy regimes

We study the absorption and scattering of massless scalar waves propagating in spherically symmetric spacetimes with dynamical cosmological constant both in low-energy and high-energy zones. In the former low-energy regime, we solve analytically the Regge-Wheeler wave equation and obtain an analytic absorption probability expression which varies with $M\sqrtΛ$, where $M$ is the central mass and $Λ$ is cosmological constant. The low-energy absorption probability, which is in the range of $[0, 0.986701]$, increases monotonically with increase in $Λ$. In the latter high-energy regime, the scalar particles adopt their geometric optics limit value. The trajectory equation with effective potential emerges and the analytic high-energy greybody factor, which is relevant with the area of classically accessible regime, also increases monotonically with increase in $Λ$, as long $Λ$ is less than or of the order of $10^4$. In this high-energy case, the null cosmological constant result reduces to the Schwarzschild value $27πr_g^2/4$.

gr-qc↗

arXiv2010

Cosmic Constraint to DGP Brane Model: Geometrical and Dynamical Perspectives

In this paper, the Dvali-Gabadadze-Porrati (DGP) brane model is confronted by current cosmic observational data sets from geometrical and dynamical perspectives. On the geometrical side, the recent released Union2 $557$ of type Ia supernovae (SN Ia), the baryon acoustic oscillation (BAO) from Sloan Digital Sky Survey and the Two Degree Galaxy Redshift Survey (transverse and radial to line-of-sight data points), the cosmic microwave background (CMB) measurement given by the seven-year Wilkinson Microwave Anisotropy Probe observations (shift parameters $R$, $l_a(z_\ast)$ and redshift at the last scatter surface $z_\ast$), ages of high redshifts galaxies, i.e. the lookback time (LT) and the high redshift Gamma Ray Bursts (GRBs) are used. On the dynamical side, data points about the growth function (GF) of matter linear perturbations are used. Using the same data sets combination, we also constrain the flat $Λ$CDM model as a comparison. The results show that current geometrical and dynamical observational data sets much favor flat $Λ$CDM model and the departure from it is above $4σ$($6σ$) for spatially flat DGP model with(without) SN systematic errors. The consistence of growth function data points is checked in terms of relative departure of redshift-distance relation.

astro-ph.CO↗

arXiv2010

Observational Constraints to Ricci Dark Energy Model by Using: SN, BAO, OHD, fgas Data Sets

In this paper, we perform a global constraint on the Ricci dark energy model with both the flat case and the non-flat case, using the Markov Chain Monte Carlo (MCMC) method and the combined observational data from the cluster X-ray gas mass fraction, Supernovae of type Ia (397), baryon acoustic oscillations, current Cosmic Microwave Background, and the observational Hubble function. In the flat model, we obtain the best fit values of the parameters in $1σ, 2σ$ regions: $Ω_{m0}=0.2927^{+0.0420 +0.0542}_{-0.0323 -0.0388}$, $α=0.3823^{+0.0331 +0.0415}_{-0.0418 -0.0541}$, $Age/Gyr=13.48^{+0.13 +0.17}_{-0.16 -0.21}$, $H_0=69.09^{+2.56 +3.09}_{-2.37 -3.39}$. In the non-flat model, the best fit parameters are found in $1σ, 2σ$ regions:$Ω_{m0}=0.3003^{+0.0367 +0.0429}_{-0.0371 -0.0423}$, $α=0.3845^{+0.0386 +0.0521}_{-0.0474 -0.0523}$, $Ω_k=0.0240^{+0.0109 +0.0133}_{-0.0130 -0.0153}$, $Age/Gyr=12.54^{+0.51 +0.65}_{-0.37 -0.49}$, $H_0=72.89^{+3.31 +3.88}_{-3.05 -3.72}$. Compared to the constraint results in the $Λ\textmd{CDM}$ model by using the same datasets, it is shown that the current combined datasets prefer the $Λ\textmd{CDM}$ model to the Ricci dark energy model.

astro-ph.CO↗

arXiv2010

Current Observational Constraints to Holographic Dark Energy Model with New Infrared cut-off via Markov Chain Monte Carlo Method

In this paper, the holographic dark energy model with new infrared (IR) cut-off for both the flat case and the non-flat case are confronted with the combined constraints of current cosmological observations: type Ia Supernovae, Baryon Acoustic Oscillations, current Cosmic Microwave Background, and the observational hubble data. By utilizing the Markov Chain Monte Carlo (MCMC) method, we obtain the best fit values of the parameters with $1σ, 2σ$ errors in the flat model: $Ω_{b}h^2=0.0233^{+0.0009 +0.0013}_{-0.0009 -0.0014}$, $α=0.8502^{+0.0984 +0.1299}_{-0.0875 -0.1064}$, $β=0.4817^{+0.0842 +0.1176}_{-0.0773 -0.0955}$, $Ω_{de0}=0.7287^{+0.0296 +0.0432}_{-0.0294 -0.0429}$, $Ω_{m0}=0.2713^{+0.0294 +0.0429}_{-0.0296 -0.0432}$, $H_0=66.35^{+2.38 +3.35}_{-2.14 -3.07}$. In the non-flat model, the constraint results are found in $1σ, 2σ$ regions: $Ω_{b}h^2=0.0228^{+0.0010 +0.0014}_{-0.0010 -0.0014}$, $Ω_k=0.0305^{+0.0092 +0.0140}_{-0.0134 -0.0176}$, $α=0.8824^{+0.2180 +0.2213}_{-0.1163 -0.1378}$, $β=0.5016^{+0.0973 +0.1247}_{-0.0871 -0.1102}$, $Ω_{de0}=0.6934^{+0.0364 +0.0495}_{-0.0304 -0.0413}$, $Ω_{m0}=0.2762^{+0.0278 +0.0402}_{-0.0320 -0.0412}$, $H_0=70.20^{+3.03 +3.58}_{-3.17 -4.00}$. In the best fit holographic dark energy models, the equation of state of dark energy and the deceleration parameter at present are characterized by $w_{de0}=-1.1414\pm0.0608, q_0=-0.7476\pm0.0466$ (flat case) and $w_{de0}=-1.0653\pm0.0661, q_0=-0.6231\pm0.0569$ (non-flat case). Compared to the $Λ\textmd{CDM}$ model, it is found the current combined datasets do not favor the holographic dark energy model over the $Λ\textmd{CDM}$ model.

astro-ph.CO↗

arXiv2010

Constraints on modified Chaplygin gas from recent observations and a comparison of its status with other models

In this Letter, a modified Chaplygin gas (MCG) model of unifying dark energy and dark matter with the exotic equation of state $p_{MCG}=Bρ_{MCG} -\frac A{ρ_{MCG}^α}$ 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 ($B$,$B_{s}$,$α$) 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 $-0.946\leq w(0)\leq-1.282$. 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.

astro-ph.CO↗

arXiv2010

Cosmological constraints on generalized Chaplygin gas model: Markov Chain Monte Carlo approach

We use the Markov Chain Monte Carlo method to investigate a global constraints on the generalized Chaplygin gas (GCG) model as the unification of dark matter and dark energy from the latest observational data: the Constitution 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 non-flat universe, the constraint results for GCG model are, $Ω_{b}h^{2}=0.0235^{+0.0021}_{-0.0018}$ ($1σ$) $^{+0.0028}_{-0.0022}$ $(2σ)$, $Ω_{k}=0.0035^{+0.0172}_{-0.0182}$ ($1σ$) $^{+0.0226}_{-0.0204}$ $(2σ)$, $A_{s}=0.753^{+0.037}_{-0.035}$ ($1σ$) $^{+0.045}_{-0.044}$ $(2σ)$, $α=0.043^{+0.102}_{-0.106}$ ($1σ$) $^{+0.134}_{-0.117}$ $(2σ)$, and $H_{0}=70.00^{+3.25}_{-2.92}$ ($1σ$) $^{+3.77}_{-3.67}$ $(2σ)$, which is more stringent than the previous results for constraint on GCG model parameters. Furthermore, according to the information criterion, it seems that the current observations much support $Λ$CDM model relative to the GCG model.

astro-ph.CO↗

arXiv2010

Reconstructing Dark Energy Potentials From Parameterized Deceleration Parameters

In this paper, the properties of dark energy are investigated according to the parameterized deceleration parameter $q(z)$, which is used to describe the extent of the accelerating expansion of the universe. The potential of dark energy $V(ϕ)$ and the cosmological parameters, such as the dimensionless energy density $Ω_ϕ$, $Ω_{m}$, and the state parameter $w_ϕ$, are connected to it. Concretely, by giving two kinds of parameterized deceleration parameters $q(z)=a+\frac{bz}{1+z}$ and $q(z)=1/2+\frac{az+b}{(1+z)^2}$, the evolution of these parameters and the reconstructed potentials $V(ϕ)$ are plotted and analyzed. It's found that the potentials run away with the evolution of universe.

astro-ph.CO↗

arXiv2010

The Integrated Sachs-Wolfe Effect in Time Varying Vacuum Model

The integrated Sachs-Wolfe (ISW) effect is an important implication for dark energy. In this paper, we have calculated the power spectrum of the ISW effect in the time varying vacuum cosmological model, where the model parameter $β=4.407$ is obtained by the observational constraint of the growth rate. It's found that the source of the ISW effect is not only affected by the different evolutions of the Hubble function $H(a)$ and the dimensionless matter density $Ω_m(a)$, but also by the different growth function $D_+(a)$, all of which are changed due to the presence of matter production term in the time varying vacuum model. However, the difference of the ISW effect in $Λ(t)\textmd{CDM}$ model and $Λ\textmd{CDM}$ model is lessened to a certain extent due to the integration from the time of last scattering to the present. It's implied that the observations of the galaxies with high redshift are required to distinguish the two models.

astro-ph.CO↗

arXiv2010

Observational constraint on generalized Chaplygin gas model

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 five-year WMAP shift parameter. It is obtained that the best fit values of the GCG model parameters with their confidence level are $A_{s}=0.73^{+0.06}_{-0.06}$ ($1σ$) $^{+0.09}_{-0.09}$ $(2σ)$, $α=-0.09^{+0.15}_{-0.12}$ ($1σ$) $^{+0.26}_{-0.19}$ $(2σ)$. 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 $-0.91\geq w_{0de}\geq-1.00$.

astro-ph.CO↗

arXiv2010

Geometrical Diagnostics for Generalized Chaplygin Gas

A new diagnostic method, $Om$ is applied to generalized Chaplygin gas (GCG) model as the unification of dark matter and dark energy. On the basis of the recently observed data: the Union supernovae, the observational Hubble data, the SDSS baryon acoustic peak and the five-year WMAP shift parameter, we show the discriminations between GCG and $Λ$CDM model. Furthermore, it is calculated that the current equation of state of dark energy $w_{0de}=-0.964$ according to GCG model.

astro-ph↗

arXiv2010

Area spectrum of near-extremal SdS black holes via the new interpretation of quasinormal modes

Motivated by the recent work about a new physical interpretation of quasinormal modes by Maggiore, we investigate the quantization of near-extremal Schwarzschild-de Sitter black holes in the four dimensional spacetime. Following Kunstatter's method, we derive the area and entropy spectrum of near-extremal Schwarzschild-de Sitter black holes which differs from Setare's result. Furthermore, we find that the derived a universal area spectrum is $2πn$ which is equally spaced.

astro-ph.CO↗

arXiv2010

Observational constraints on holographic dark energy with varying gravitational constant

We use observational data from Type Ia Supernovae (SN), Baryon Acoustic Oscillations (BAO), Cosmic Microwave Background (CMB) and observational Hubble data (OHD), and the Markov Chain Monte Carlo (MCMC) method, to constrain the cosmological scenario of holographic dark energy with varying gravitational constant. We consider both flat and non-flat background geometry, and we present the corresponding constraints and contour-plots of the model parameters. We conclude that the scenario is compatible with observations. In 1$σ$ we find $Ω_{\Lambda0}=0.72^{+0.03}_{-0.03}$, $Ω_{k0}=-0.0013^{+0.0130}_{-0.0040}$, $c=0.80^{+0.19}_{-0.14}$ and $Δ_G\equiv G'/G=-0.0025^{+0.0080}_{-0.0050}$, while for the present value of the dark energy equation-of-state parameter we obtain $w_0=-1.04^{+0.15}_{-0.20}$.

astro-ph.CO↗

arXiv2009

Holographic Dark Energy Model with Hubble Horizon as an IR Cut-off

The main task of this paper is to realize a cosmic observational compatible universe in the framework of holographic dark energy model when the Hubble horizon $H$ is taken as the role of an IR cut-off. When the model parameter $c$ of a time variable cosmological constant (CC) $Λ(t)=3c^{2}H^{2}(t)$ becomes time or scale dependent, an extra term enters in the effective equation of sate (EoS) of the vacuum energy $w^{eff}_Λ=-c^2-d\ln c^{2}/3d\ln a$. This extra term can make the effective EoS of time variable CC cross the cosmological boundary and be phantom-like at present. For the lack of a first principle and fundamental physics theory to obtain the form $c^2$, we give a simple parameterized form of $c^2$ as an example. Then the model is confronted by the cosmic observations including SN Ia, BAO and CMB shift parameter $R$. The result shows that the model is consistent with cosmic observations.

astro-ph.CO↗

arXiv2009

Time Variable Cosmological Constant from Renormalization Group Equations

In this paper, a time variable cosmological constant (CC) from renormalization group equations (RGEs) is explored, where the renormalization scale $μ^2=R^{-2}_{CC}=Max(\dot{H}+2H^2,-\dot{H})$ is taken. The cosmological parameters, such as dimensionless energy density, deceleration parameter and effective equation of state of CC etc, are derived. Also, the cosmic observational constraints are implemented to test the model's consistence. The results show that it is compatible with cosmic data. So, it would be a viable dark energy model.

astro-ph.CO↗

arXiv2009

Cosmic Constraint on Ricci Dark Energy Model

In this paper, a holographic dark energy model, dubbed Ricci dark energy, is confronted with cosmological observational data from type Ia supernovae (SN Ia), baryon acoustic oscillations (BAO) and cosmic microwave background (CMB). By using maximum likelihood method, it is found out that Ricci dark energy model is a viable candidate of dark energy model with the best fit parameters: $Ω_{m0}=0.34\pm 0.04$, $α=0.38\pm 0.03$ with $1σ$ error. Here, $α$ is a dimensionless parameter related with Ricci dark energy $ρ_{R}$ and Ricci scalar $R$, i.e., $ρ_{R}\propto αR$.

astro-ph↗

arXiv2009

Generalized Holographic and Ricci Dark Energy Models

In this paper, we consider generalized holographic and Ricci dark energy models where the energy densities are given as $ρ_{R}=3c^2M^{2}_{pl}Rf(H^2/R)$ and $ρ_{h}=3c^2M^{2}_{pl}H^2g(R/H^2)$ respectively, here $f(x),g(y)$ are positive defined functions of dimensionless variables $H^2/R$ or $R/H^2$. It is interesting that holographic and Ricci dark energy densities are recovered or recovered interchangeably when the function $f(x)=g(y)\equiv 1$ or$f=g\equiv Id$ is taken respectively (for example $f(x),g(x)=1-ε(1-x)$, $ε=0 \text{or} 1$ respectively). Also, when $f(x)\equiv xg(1/x)$ is taken, the Ricci and holographic dark energy models are equivalents to a generalized one. When the simple forms $f(x)=1-ε(1-x)$ and $g(y)=1-η(1-y)$ are taken as examples, by using current cosmic observational data, generalized dark energy models are researched. As expected, in these cases, the results show that they are equivalent ($ε=1-η=1.312$) and Ricci-like dark energy is more favored relative to the holographic one where the Hubble horizon was taken as an IR cut-off. And, the suggestive combination of holographic and Ricci dark energy components would be $1.312 R-0.312H^2$ which is $2.312H^2+1.312\dot{H}$ in terms of $H^2$ and $\dot{H}$.

astro-ph.CO↗

arXiv2009

Time Variable Cosmological Constants from the Age of Universe

In this paper, time variable cosmological constant, dubbed {\it age cosmological constant}, is investigated motivated by the fact: any cosmological length scale and time scale can introduce a cosmological constant or vacuum energy density into Einstein's theory. The age cosmological constant takes the form $ρ_Λ=3c^2M^2_P/t_Λ^2$, where $t_Λ$ is the age of our universe or conformal time. The effective equation of state of age cosmological constant are $w^{eff}_Λ=-1+{2/3}\frac{\sqrt{Ω_Λ}}{c}$ and $w^{eff}_Λ=-1+{2/3}\frac{\sqrt{Ω_Λ}}{c}(1+z)$ when the age of universe and conformal time are taken as the role of cosmological time scales respectively. They are the same as the so-called agegraphic dark energy models. However, the evolution history are different from the agegraphic ones for their different evolution equations.

astro-ph.CO↗