Using Energy Conditions to Distinguish Brane Models and Study Brane Matter

Current universe (assumed here to be normal matter on the brane) is pressureless from observations. In this case the energy condition is ρ₀≥0 and p₀=0. By using this condition, brane models can be distinguished. Then, assuming arbitrary component of matter in DGP model, we use four known energy conditions to study the matter on the brane. If there is nonnormal matter or energy (for example dark energy with w<-1/3) on the brane, the universe is accelerated.     

Abundance of Asymmetric Dark Matter in Brane World Cosmology

Relic abundance of asymmetric Dark Matter particles in brane world cosmological scenario is investigated in this article. Hubble expansion rate is enhanced in brane world cosmology and it affects the relic abundance of asymmetric Dark Matter particles. We analyze how the relic abundance of asymmetric Dark Matter is changed in this model. We show that in such kind of nonstandard cosmological scenario, indirect detection of asymmetric Dark Matter is possible if the cross section is small enough which let the anti-particle abundance kept in the same amount with the particle. We show the indirect detection signal constraints can be used to such model only when the cross section and the 5-dimensional Planck mass scale are in appropriate values.     

Null Geodesics in Brane World Scenarios

We study the null bulk geodesic motion in the brane world inwhich the bulk metric has an un-stabilized extra spatial dimension.We find that the null bulk geodesic motion as observed on the3-brane with Z₂ symmetry would be a timelike geodesic motioneven though there exists an extra non-gravitational force incontrast with the case of the stabilized extra spatial dimension.In other words the presence of the extra non-gravitational forcewould not violate the Z₂ symmetry.     

Brane Cosmology, Varying Speed of Light and Inflation in Models with One or More Extra Dimensions

We summarize the approach to brane cosmology known as mirage cosmology and use it to determine the Friedmann equation on a 3-brane embedded in different bulk spacetimes all with one or more extra dimensions. Usually, when there is more than one extra dimension the junction conditions, central to the usual brane world scenarios, are difficult to apply. This problem does not arise in mirage cosmology because the brane is treated as a test particle in the background spacetime. We discuss in detail the dynamics of a brane embedded in two specific 10D bulk spacetimes, namely Sch-AdS₅ × S₅ and a rotating black hole, and from the dynamics—which are now rather more complicated since the brane can move in all the extra dimensions—determine the new dark fluid terms in the brane Friedmann equation. Some of these, such as the cosmological constant term, are seen to be bulk dependent. We then show explicitly how this mirage cosmology approach matches with the familiar junction condition approach when there is just one extra dimension. The issue of a varying speed of light in mirage cosmology is addressed and we find a scenario in which c _eff always increases, tending asymptotically to c ₀ as the universe expands. Finally some comments are made regarding brane inflation and limitations of the mirage cosmology approach are also discussed.     

Letter: State of Matter for Effective Yang-Mills Fields and Energy Conditions

It has been known that the Penrose-Hawking energy conditions are satisfied by conventional matter. The scalar fields employed in inflationary models can violate the strong energy condition, but not the weak energy condition. Recent observational data of the cosmic microwave background radiation seem to favor a perturbation spectrum index n > 1, which implies that the weak energy condition should be violated by the dominant matter during the inflation. In this paper the state of matter for the effective Yang-Mills fields is examined, and it is found that the quantum Yang-Mills fields in some particular states can violate the weak energy condition naturally.     

A Cosmological Model of Holographic Brane Gravity

A cosmological scenario with two branes (A and B) moving in a 5-dimensional bulk is considered. As in the case of ecpyrotic and born-again braneworld models it is possible that the branes collide. The energy-momentum tensor is taken to describe a perfect barotropic fluid on the A-brane and a phenomenological time-dependent cosmological constant on the B-brane. The A-brane is identified with our Universe and its cosmological evolution in the approximation of a homogeneous and isotropic brane is analysed. The dynamics of the radion (a scalar field on the brane) contains information about the proper distance between the branes. It is demonstrated that the deSitter type solutions are obtained for late time evolution of the braneworld and accelerative behaviour is anticipated at the present time.     

A Study of Brane Solutions in D-Dimensional Coupled Gravity System

We use only the equation.of motion for an interacting system of gravity, dilaton and antisymmetric tensor to study the soliton solutions by making use of a Poincaré-invariant ansatz. We show that the system of equations is completely integrable and the solution is unique with appropriate boundary conditions. Some new classes of solutions are also given explicitly.     

Crossing w=1 in Gauss-Bonnet Brane World with Induced Gravity

Recent type Ia supernovas data seemingly favor a dark energy model whose equation of state w（z） crosses -1 very recently, which is a much more amazing problem than the acceleration of the universe. In this paper we show that it is possible to realize such a crossing without introducing any phantom component in a Gauss-Bonnet brane worm with induced gravity, where a four-dimensional curvature scalar on the brahe and a five-dimensional Gauss-Bonnet term in the bu/k are present. In this realization, the Gauss-Bonnet term and the mass parameter in the bulk play a crucial role.     

Brane Universes Tested by Supernovae Ia

We discuss observational constraints coming from supernovae Ia imposed on the behaviour of the Randall-Sundrum models. In the case of dust matter on the brane, the difference between the best-fit Perlmutter model with a -term and the best-fit brane models becomes detectable for redshifts z > 1.2. It is interesting that brane models predict brighter galaxies for such redshifts which is in agreement with the measurement of the z = 1.7 supernova. We also demonstrate that the fit to supernovae data can also be obtained, if we admit the super-negative dark energy (phantom matter) p = – (4/3) on the brane, where the dark energy in a way mimics the influence of the cosmological constant. It also appears that the dark energy enlarges the age of the universe which is demanded in cosmology. Finally, we propose to check for dark radiation and brane tension by the application of the angular diameter of galaxies minimum value test. We point out the existence of coincidence problem for the brane tension parameter.     

Crossing w = -1 in Gauss-Bonnet Brane World with Induced Gravity

Recent type Ia supernovas data seemingly favor a dark energy model whose equation of state w(z) crosses -1 very recently, which is a much more amazing problem than the acceleration of the universe. In this paper we show that it is possible to realize such a crossing without introducing any phantom component in a Gauss-Bonnet brane world with induced gravity, where a four-dimensional curvature scalar on the brane and a five-dimensional Gauss-Bonnet term in the bulk are present. In this realization, the Gauss-Bonnet term and the mass parameter in the bulk play a crucial role.     

暗能量的理论模型

将现有文献中存在的暗能量模型(严格的说是指解释现在宇宙的加速膨胀模型)分为三类. 第一类是将宇宙加速归因为在宇宙尺度上广义相对论的修改; 第二类是由于宇宙微扰的反作用或宇宙非均匀性的作用; 第三类为宇宙中存在的一类奇异能量成分, 它出现在爱因斯坦方程的右边. 对每一类模型, 举了几个例子, 并给以一些评述.     

Investigation of Homogeneity and Matter Distribution on Large Scales Using Large Quasar Groups

We use 20 large quasar group (LQG) samples in Park et al. (2015) to investigate the homogeneity of the 0.3 z 1.6 Universe (z denotes the redshift). For comparison, we also employ the 12 LQGs samples at 0.5 z 2 in Komberg et al. (1996) to do the analysis. We calculate the bias factor b and the two-point correlation function ξ_LQG for such groups for three different density profiles of the LQG dark matter halos, i.e. the isothermal profile, the Navarro-Frenk-White (NFW) profile, and the (gravitational) lensing profile. We consider the CDM (cold dark matter plus a cosmological constant ) underlying matter power spectrum with Ω_m = 0.28, Ω_Λ= 0.72, the Hubble constant H₀ = 100 h·km·s^-1·Mpc^-1 with h = 0.72. Dividing the samples into three redshift bins, we find that the LQGs with higher redshift are more biased and correlated than those with lower redshift. The homogeneity scale RH of the LQG distribution is also deduced from theory. It is defined as the comoving radius of the sphere inside which the number of LQGs N(< r) is proportional to r³ within 1%, or equivalently above which the correlation dimension of the sample D₂ is within 1% of D₂ = 3. For Park et al.'s samples and the NFW dark matter halo profile, the homogeneity scales of the LQG distribution are R_H ～ 247 h^-1·Mpc for 0.2 < z ≤ 0.6, R_H ～ 360 h^-1·Mpc for 0.6 < z ≤ 1.2, and R_H ～ 480 h^-1·Mpc for 1.2 < z 1.6. The maximum extent of the LQG samples are beyond RH in each bin, showing that the LQG samples are not homogeneously distributed on such a scale, i.e. a length range of ～ 500 h^-1·Mpc and a mass scale of ～ 10¹⁴M_⊙. The possibilities of a top-down structure formation process as was predicted by the hot/warm dark matter (WDM) scenarios and the redshift evolution of bias factor b and correlation amplitude ξ_LQG of the LQGs as a consequence of the cosmic expansion are both discussed. Different results were obtained based on the LQG sample in Komberg et al. (1996) and the possible reasons for such differences were discussed.     

Phantom-Like Effects on Warped DGP Brane

We study phantom-like effect on the DGP brane embedded in a five-dimensional AdS bulk. We show that this effect can be realized without phantom matter on this warped DGP brahe. We investigate the role played by the bulk cosmological constant on the phantom-like effect on the brane and we show that it tends to reduce this effect. Also, warped compactification of the bulk manifold increases the values of the effective and total equation of state parameters of the model relative to the case with Minkowski bulk. We extend our study to the ease that induced curvature on the brahe is modified in the spirit of the f（R）-gravity.     

Brane Formation and Cosmological Constraint on the Number of Extra Dimensions

Special relativity is generalized to extra dimensions and quantized energy levels of particles are obtained. By calculating the probability of particles' motion in extra dimensions at high temperature of the early universe, it is proposed that the branes may have not existed since the very beginning of the universe, but formed later. Meanwhile, before the formation, particles of the universe may have filled in the whole bulk, not just on the branes. This scenario differs from that in the standard big bang cosmology in which all particles are assumed to be in the 4D spacetime. So, in brane models, whether our universe began from a 4D big bang singularity is questionable. A cosmological constraint on the number of extra dimensions is also given which favors N ≥ 7.     

Cosmological Consequences of Scale-Related Comoving Masses for Cosmic Pressure, Mass, and Vacuum Energy Density

According to ideas of Mach, Whitrow, Dirac, or Hoyle, inertial masses of particles should not be a genuine, predetermined quantity; rather they should represent a relational quantity which by its value somehow reflects the deposition and constellation of all other objects in their cosmic environment. In this paper we want to pick up suggestions given by Thirring and by Hoyle of how, due to requirements of the equivalence of rotations and of general relativistic conformal scale invariance, the particle masses of cosmic objects should vary with the cosmic length scale. We study cosmological consequences of comoving cosmic masses which co-evolve by mass with the expansion of the universe. The vanishing of the covariant divergence of the cosmic energy-momentum tensor under the new prerequisite that matter density only falls off with the reciproke of the squared cosmic scale S(t) then leads to the astonishing result that cosmic pressuredoes not fall off adiabatically but rather falls off in a quasi-isothermal behaviour, varying with S(t) as matter density does. Hence, as a new cosmological fact, it arises that, even in the late phases of cosmic expansion, pressure cannot be neglected what concerns its gravitational action on the cosmic dynamics. We then show that under these conditions the cosmological equations can, however, only be solved if, in addition to matter, also pressure and energy density of the cosmic vacuum are included in the calculation. An unaccelerated expansion with a Hubble parameter falling off with S(t)⁻¹ is obtained for a vacuum energy density decay according to S(t)⁻² with a well-tuned proportion of matter and vacuum pressures. As it appears from these results, a universe with particle masses increasing with the cosmic sale S(t) is in fact physically conceivable in an energetically consistent manner, if vacuum energy at the expansion of the universe is converted into mass density of real matter with no net energy loss occuring. This universe in addition also happens to be an economical one which has and keeps a vanishing total energy.     

Cosmology of Brane-Induced Gravity Models

We review various aspects of the cosmology of brane-induced gravity models. After recalling some properties of these models, we give the equations governing the cosmological dynamics in a Z ₂ symmetric case. We then discuss properties of two particular solutions of interest, a self-accelerating solution that has been proposed to provide an alternative explanation to the observed late time acceleration of the universe, and a self-flattening solution. The latter is also discussed in relation with the van Dam–Veltman–Zakharov discontinuity.     

Anisotropic Cosmological Models with Variable G and Decaying Vacuum Energy

We present Bianchi type-I cosmological models with a perfect fluid source and time-dependent gravitational and cosmological constants based on new exact solutions of Einstein’s equations. The perfect fluid is chosen to obey a barotropic equation of state. The models obtained represent a radiation dominated phase and a dust era. In some of the models the expansion changes from a decelerating phase to an accelerating one and these models asymptotically tend to the de Sitter universe. The paper is dedicated to late Prof. S.R. Roy, Ex-Head, Department of Mathematics, Banaras Hindu University, Varanasi, India.     

Reconstruction of Scalar Field Dark Energy Models in Kaluza-Klein Universe

This paper is devoted to study the modified holographic dark energy model by taking its different aspects in the flat Kaluza-Klein universe.We construct the equation of state parameter which evolutes the universe from quintessence region towards the vacuum.It is found that the modified holographic model exhibits instability against small perturbations in the early epoch of the universe but becomes stable in the later times.We also develop its correspondence with some scalar field dark energy models.It is interesting to mention here that all the results are consistent with the present observations.     

Asymmetric Brane-World Scenarios and Simulated Gravity

I review the main aspects of the simulated gravity that arises in brane-world models in which the reflection symmetry is broken. I recall its main aspects, show how a Newton-like force can be simulated on small scales, and discuss the post-Newtonian constraints as well as the cosmology of this model.     

Reconstruction of Scalar Field Dark Energy Models in Kaluza-Klein Universe

This paper is devoted to study the modified holographic dark energy model by taking its different aspects in the flat Kaluza-Klein universe. We construct the equation of state parameter which evolutes the universe from quintessence region towards the vacuum. It is found that the modified holographic model exhibits instability against small perturbations in the early epoch of the universe but becomes stable in the later times. We also develop its correspondence with some scalar field dark energy models. It is interesting to mention here that all the results are consistent with the present observations.