The sign-changeable interaction between variable generalized Chaplygin gas and dark matter

In this paper, we study a cosmological model with the sign-changeable interaction between variable generalized Chaplygin gas (VGCG) and dark matter. The dynamical analysis indicates that there exists a stable scaling attractor, which can help to alleviate the coincidence problem. Furthermore, when the parameters of the model take some fixed values, the attractor corresponds to the phase w=?0.939 and the equation of state of VGCG approaches it from either w>?1 or w<?1 depending on the choice of its initial cosmic density parameter and the ratio of pressure to critical energy density. So, the phantom divide can be crossed. We find the interaction term Q can change its sign from Q<0 to Q>0 as the universe expands, which is different from the usual interaction. Also, we place constraints on the parameters from the point of view of dynamics.     

New agegraphic dark energy model with the sign-changeable interaction

In this paper, we investigate a cosmological model with the sign-changeable interaction between new agegraphic dark energy (NADE) and dark matter. By analysis it is shown that the equation of state (EoS) of NADE can cross the phantom divide under the condition of the model parameter β<0. In addition, we plot the trajectories of the interacting NADE model for different values of the parameters n and β in the statefinder plane. It is found that the statefinder trajectories can be distinguished by both n and β. Furthermore, we study the interacting NADE model by means of the w?w′ analysis.     

Observational constraints on G-corrected holographic dark energy using a Markov chain Monte Carlo method

We constrain holographic dark energy (HDE) with time varying gravitational coupling constant in the framework of the modified Friedmann equations using cosmological data from type Ia supernovae, baryon acoustic oscillations, cosmic microwave background radiation and X-ray gas mass fraction. Applying a Markov Chain Monte Carlo (MCMC) simulation, we obtain the best fit values of the model and cosmological parameters within 1σ confidence level (CL) in a flat universe as: $\varOmega_{b}h^{2}=0.0222^{+0.0018}_{-0.0013}$ , $\varOmega_{c}h^{2}=0.1121^{+0.0110}_{-0.0079}$ , $\alpha_{G}\equiv \dot{G}/(HG) =0.1647^{+0.3547}_{-0.2971}$ and the HDE constant $c=0.9322^{+0.4569}_{-0.5447}$ . Using the best fit values, the equation of state of the dark component at the present time w _d0 at 1σ CL can cross the phantom boundary w=?1.     

Cosmological evolution of pilgrim dark energy

We study pilgrim dark energy model by taking IR cut-offs as particle and event horizons as well as conformal age of the universe. We derive evolution equations for fractional energy density and equation of state parameters for pilgrim dark energy. The phantom cosmic evolution is established in these scenarios which is well supported by the cosmological parameters such as deceleration parameter, statefinder parameters and phase space of ω _? and \(\omega'_{\vartheta}\) . We conclude that the consistent value of parameter μ is μ<0 in accordance with the current Planck and WMAP9 results.     

Bianchi type-V universe with wet dark fluid

The Bianchi type-V universe filled with dark energy from a wet dark fluid has been considered. A new equation of state for the dark energy component of the universe has been used. It is modeled on the equation of state p=γ(ρ?ρ _? ) which can describe a liquid, for example water. The exact solutions to the corresponding field equations are obtained in quadrature form. The solution for constant deceleration parameter have been studied in detail for power-law and exponential forms both. The case $\gamma =\frac{1}{3}$ has been also analysed.     

Tachyonic (phantom) power-law cosmology

Tachyonic scalar field-driven late universe with dust matter content is considered. The cosmic expansion is modeled with power-law and phantom power-law expansion at late time, i.e. z?0.45. WMAP7 and its combined data are used to constraint the model. The forms of potential and the field solution are different for quintessence and tachyonic cases. Power-law cosmology model (driven by either quintessence or tachyonic field) predicts unmatched equation of state parameter to the observational value, hence the power-law model is excluded for both quintessence and tachyonic field. In the opposite, the phantom power-law model predicts agreeing valued of equation of state parameter with the observational data for both quintessence and tachyonic cases, i.e. $w_{\phi, 0} = -1.49^{+11.64}_{-4.08}$ (WMAP7+BAO+H ₀) and $w_{\phi, 0} = -1.51^{+3.89}_{-6.72} $ (WMAP7). The phantom-power law exponent β must be less than about ?6, so that the ?2<w _?,0<?1. The phantom power-law tachyonic potential is reconstructed. We found that dimensionless potential slope variable Γ at present is about 1.5. The tachyonic potential reduced to V=V ₀ ? ^?2 in the limit Ω _m,0→0.     

Statefinder diagnostic and w–w′ analysis for the agegraphic dark energy model with the sign-changeable interaction

The statefinder diagnostic and w–w′ analysis are useful methods for distinguishing different dark energy models. In this paper, we study the agegraphic dark energy (ADE) model with the sign-changeable interaction by using the statefinder diagnostic and w–w′ analysis. The evolution trajectories of this model in the r–s and w–w′ planes are plotted for different model parameters. It is shown that the model parameters significantly affect the evolution trajectories in the r–s and w–w′ planes. Furthermore, we can differentiate the ADE model with the sign-changeable interaction from the LCDM model by means of the statefinder diagnostic and w–w′ analysis.     

A simple derivation of level spacing of quasinormal frequencies for a black hole with a deficit solid angle and quintessence-like matter

In this paper, we investigate analytically the level space of the imaginary part of quasinormal frequencies for a black hole with a deficit solid angle and quintessence-like matter by the Padmanabhan’s method (Padmanabhan in Class. Quantum Gravity 21:L1, 2004). Padmanabhan presented a method to study analytically the imaginary part of quasinormal frequencies for a class of spherically symmetric spacetimes including Schwarzschild-de Sitter black holes which has an evenly spaced structure. The results show that the level space of scalar and gravitational quasinormal frequencies for this kind of black holes only depend on the surface gravity of black-hole horizon in the range of $-1<w<-\frac{1}{3}$ , respectively. We also extend the range of w to w≤?1, the results of which are similar to that in $-1<w<-\frac{1}{3}$ case. Particularly, a black hole with a deficit solid angle in accelerating universe will be a Schwarzschild-de Sitter black hole, fixing w=?1 and ε ²=0. And a black hole with a deficit solid angle in the accelerating universe will be a Schwarzschild black hole,when ρ ₀=0 and ε ²=0. In this paper, w is the parameter of state equation, ε ² is a parameter relating to a deficit solid angle and ρ ₀ is the density of static spherically symmetrical quintessence-like matter at r=1.     

Interacting holographic dark energy models: a general approach

Dark energy models inspired by the cosmological holographic principle are studied in homogeneous isotropic spacetime with a general choice for the dark energy density \(\rho_{d}=3(\alpha H^{2}+\beta\dot{H})\) . Special choices of the parameters enable us to obtain three different holographic models, including the holographic Ricci dark energy (RDE) model. Effect of interaction between dark matter and dark energy on the dynamics of those models are investigated for different popular forms of interaction. It is found that crossing of phantom divide can be avoided in RDE models for β>0.5 irrespective of the presence of interaction. A choice of α=1 and β=2/3 leads to a varying Λ-like model introducing an IR cutoff length Λ ^?1/2. It is concluded that among the popular choices an interaction of the form Q∝Hρ _m suits the best in avoiding the coincidence problem in this model.     

Holographic dark energy model with variable deceleration parameter,cosmic coincidence and the future singularity of the Kantowski-Sachs universe

The present work deals with a spatially homogeneous and anisotropic Kantowski-Sachs space time filled with two minimally interacting fluids; dark matter and a hypothetical anisotropic fluid as the holographic dark energy components. To obtain an exact solution of the Einstein’s field equations, we used the assumption of linearly varying deceleration parameter. We have investigated geometric and kinematic properties of the model and the role of the anisotropic holographic dark energy in the evolution of the Kantowski-Sachs universe. Under the suitable condition, it is observed that the anisotropy parameter of the universe and the skewness parameter of the holographic dark energy approaches to zero for large cosmic time and the universe can achieve flatness for some particular moments throughout its entire lifetime. Results show that the coincidence parameter $( \Re= \frac{\rho_{\varLambda}}{\rho_{M}} )$ increases with increasing time and a big rip type future singularity will occur for this model. We have also applied the statefinder diagnostics method to study the behavior of different stages of the universe and to differentiate the proposed dark energy model from the ΛCDM model. Since in this model, the universe has a finite life time and passes through a significant time when the dark energy and the matter energy densities are roughly comparable, so considering $\frac{1}{ \Re_{0}} <\Re < \Re_{0}$ , where ?₀ is any fixed ratio, we have calculated the fraction of total life time of the universe when the universe passes through the coincidental stage for this future singularity. The results are found to be consistent with recent cosmological observations.     

Particle motion in the Schwarzschild-Quintessence space-time

The influence of free static spherically symmetric quintessence on particle motion in the Schwarzschild-quintessence space-time has been studied by numerical calculation. In the Schwarzschild space-time, the particle motion can be determined by an effective potential. However, this potential is dependent on the quintessence’s state parameter w _q . We find that when the quintessence’s state parameter w _q is in the range of $[-\frac{1}{3},0]$ , the massive particle’s motion is just like that in the Schwarzschild space-time. And when $-1\leqslant w_{q}<-\frac{1}{3}$ , a maximum unstable circular orbit exists for every L, and no matter how small L is, the scattering state exists, which leads to the accelerating expansion of our universe. The exists of the maximum orbit can even explain why galaxies is in a ball.     

Multi-Band Spectral Properties of Fermi Blazars

The multi-band data covering optical, X-ray and γ-ray energy regions of 130 Fermi blazars in the First LAT AGN Catalog (1LAC) were collected to investigate the broadband spectral properties. The composite spectral indices show that HBLs have convex optical-to-X-ray continua and concave X-ray-to-γ-ray continua, α _{γX γ} >0 and α _XOX ?<?0, while FSRQs and LBLs have α _{γX γ} ?<?0. The α _XOX distribution of FSRQs and LBLs extends from negative to positive values. We suggest α _{γX γ} ?>?0 and α _XOX ?<?0 could be considered as a criterion for HBLs. Moreover, HBLs have narrow distribution of peak interval of $\log\nu_{\rm p}^{\rm ic}-\log\nu_{\rm p}^{\rm syn}$ , and FSRQs have significant anti-correlation between $\log\nu_{\rm p}^{\rm ic}-\log\nu_{\rm p}^{\rm syn}$ and $\log\nu_{\rm p}^{\rm syn}$ . This indicates that SSC model is responsible for high energy emission of HBLs, while EC for FSRQs. Our results also indicate that FSRQs with larger break energy of electrons have smaller bulk Lorentz factor of dissipation region.     

Transit Time of Coronal Mass Ejections under Different Ambient Solar Wind Conditions

The speed [v(R)] of coronal mass ejections (CMEs) at various distances from the Sun is modeled (as proposed by Vr?nak and Gopalswamy in J. Geophys. Res. 107, 2002, doi: 10.1029/2001/JA000120 ) by using the equation of motion a _drag=γ(v?w) and its quadratic form a _drag=γ(v?w)|v?w|, where v and w are the speeds of the CME and solar wind, respectively. We assume that the parameter γ can be expressed as γ=αR ^β , where R is the heliocentric distance, and α and β are constants. We extend the analysis of Vr?nak and Gopalswamy to obtain a more detailed insight into the dependence of the CME Sun–Earth transit time on the CME speed and the ambient solar-wind speed, for different combinations of α and β. In such a parameter-space analysis, the results obtained confirm that the CME transit time depends strongly on the state of the ambient solar wind. Specifically, we found that: i) for a particular set of values of α and β, a difference in the solar-wind speed causes larger transit-time differences at low CME speeds [v ₀], than at high v ₀; ii) the difference between transit times of slow and fast CMEs is larger at low solar-wind speed [w ₀] than at high w ₀; iii) transit times of fast CMEs are only slightly influenced by the solar-wind speed. The last item is especially important for space-weather forecasting, since it reduces the number of key parameters that determine the arrival time of fast CMEs, which tend to be more geo-effective than the slow ones. Finally, we compared the drag-based model results with the observational data for two CME samples, consisting of non-interacting and interacting CMEs (Manoharan et al. in J. Geophys. Res. 109, 2004). The comparison reveals that the model results are in better agreement with the observations for non-interacting events than for the interacting events. It was also found that for slow CMEs (v ₀<500 km?s^?1), there is a deviation between the observations and the model if slow-wind speeds (≈?300?–?400 km?s^?1) are taken for the model input. On the other hand, the model values and the observed data agree for both the slow and the fast CMEs if higher solar-wind speeds are assumed. It is also found that the quadratic form of the drag equation reproduces the observed transit times of fast CMEs better than the linear drag model.     

Dark energy model with variable q and ω in LRS Bianchi-II space-time

The present study deals with spatial homogeneous and anisotropic locally rotationally symmetric (LRS) Bianchi-II dark energy model in general relativity. The Einstein’s field equations have been solved exactly by taking into account the proportionality relation between one of the components of shear scalar $(\sigma^{1}_{1})$ and expansion scalar (?), which, for some suitable choices of problem parameters, yields time dependent equation of state (EoS) and deceleration parameter (DP), representing a model which generates a transition of universe from early decelerating phase to present accelerating phase. The physical and geometrical behavior of universe have been discussed in detail.     

New holographic dark energy in modified f(R) Horava-Lifshitz gravity

In this paper, we study the new holographic dark energy model in the framework of modified f(R) Horava-Lifshitz Gravity. We apply correspondence scheme to construct model the in underlying scenario using power-law form of scale factor. To explore accelerated expansion of the universe, some well-known cosmological parameters (equation of state parameter and squared speed of sound) and cosmological planes (ω _Λ – \(\omega'_{\varLambda}\) and statefinder) are discussed for reconstructed model. It is interesting to conclude that these parameters represent phantom behavior of the universe with stable configuration. also, the cosmological planes show compatible results with recent observations for accelerated expansion of the universe.     

The Chandra X-ray galaxy clusters at z<1.4: constraints on the evolution of L X −T−M g relations

We analyzed the luminosity-temperature-mass of gas (L _X ?T?M _g ) relations for a sample of 21 Chandra galaxy clusters. We used the standard approach (β?model) to evaluate these relations for our sample that differs from other catalogues since it considers galaxy clusters at higher redshifts (0.4<z<1.4). We assumed power-law relations in the form $L_{X} \sim(1 +z)^{A_{L_{X}T}} T^{\beta_{L_{X}T}}$ , $M_{g} \sim(1 + z)^{A_{M_{g}T}} T^{\beta_{M_{g}T}}$ , and $M_{g} \sim(1 + z)^{A_{M_{g}L_{X}}} L^{\beta_{M_{g}L_{X}}}$ . We obtained the following fitting parameters with 68 % confidence level: $A_{L_{X}T} = 1.50 \pm0.23$ , $\beta_{L_{X}T} = 2.55 \pm0.07$ ; $A_{M_{g}T} = -0.58 \pm0.13$ and $\beta_{M_{g}T} = 1.77 \pm0.16$ ; $A_{M_{g}L_{X}} \approx-1.86 \pm0.34$ and $\beta_{M_{g}L_{X}} = 0.73 \pm0.15$ , respectively. We found that the evolution of the M _g ?T relation is small, while the M _g ?L _X relation is strong for the cosmological parameters Ω _m =0.27 and Ω _Λ =0.73. In overall, the clusters at high-z have stronger dependencies between L _X ?T?M _g correlations, than those for clusters at low-z. For most of galaxy clusters (first of all, from MACS and RCS surveys) these results are obtained for the first time.     

New agegraphic dark energy in <Emphasis Type="Italic">f</Emphasis>(<Emphasis Type="Italic">R</Emphasis>) gravity

In this paper, we study a cosmological application of the new agegraphic dark energy density in the f(R) gravity framework. We employ the new agegraphic model of dark energy to obtain the equation of state for the new agegraphic energy density in a spatially flat universe. Our calculations show, taking n<0, that it is possible to have w _Λ crossing −1. This implies that one can generate a phantom-like equation of state from a new agegraphic dark energy model in a flat universe in the modified gravity cosmology framework. Also, we develop a reconstruction scheme for the modified gravity with f(R) action.     

Did the universe start with the Higgs-field?

In a cosmological model developed by the author in previous articles the universe starts in a geometrical phase transition in Minkowski space. Here the source of the gravitational field is a Higgs-like scalar field $\bar{\phi}$ . A relation of this cosmological field $\bar{\phi}$ with the Higgs-field ? _H in the gauge theory of electroweak interaction is established. This relation leads to two dimensionless constants. One of them is interpreted as a characteristic constant of the phase transition and is connected with the volume of huge bubbles of open universes.     

A new type of interaction between generalized Chaplygin gas and dark matter

In this paper, we investigate the model with a new type of interaction between generalized Chaplygin gas (GCG) and dark matter. It is shown that there exists a stable scaling attractor, which provides the possibility to alleviate the coincidence problem. The equation of state (EoS) of GCG approaches the attractor phase from either w _g >?1 or w _g w _g w _g w _g >?1), and next cross again the phantom divide (the transition from w _g >?1 to w _g Q can change its sign from Q<0 to Q>0 as the universe expands, which is different from the usual interaction. Moreover, we investigate the model from statefinder viewpoint. The statefinder diagnostic can not only discriminate the model with different coupling constant but also distinguish the model from other dark energy models.     

Anisotropic quintessence stars

We propose a relativistic model for: quintessence stars with the combination of an anisotropic pressure corresponding to normal matter and a quintessence dark energy having a characteristic parameter ω _q such that $-1<\omega_{q}< -\frac{1}{3}$ . We discuss various physical features of the model and show that the model satisfies all the regularity conditions and can provide stable equilibrium configurations.