Dark Energy Coupled with Dark Matter in the Accelerating Universe

To model the observed Universe containing both dark energy and dark matter, we study the effective Yang-Mills condensate model of dark energy and add a non-relativistic matter component as the dark matter, which is generated out of the decaying dark energy at a constant rate Г, a parameter of our model. For the Universe driven by these two components, the dynamic evolution still has asymptotic behaviour: the expansion of the Universe is accelerating with an asymptotically constant rate H, and the densities of both components approach to finite constant values. Moreover, Ω△～ 0.7 for dark energy and Ωm～0.3 for dark matter are achieved if the decay rate Г is chosen such that Г/H ～ 1.     

Dark Energy Coupled with Relativistic Dark Matter in Accelerating Universe

Recent observations favour an accelerating Universe dominated by the dark energy. We take the effective Yang-Mills condensate as the dark energy and couple it to a relativistic matter which is created by the decaying condensate. The dynamic evolution has asymptotic behaviour with finite constant energy densities, and the fractional densities Ω∧ ～0.7 for dark energy and Ωm ～ 0.3 for relativistic matter are achieved at proper values of the decay rate. The resulting expansion of the Universe is in the de Sitter acceleration.     

Is a co-rotating Dark Disk a threat to Dark Matter directional detection?

Recent N-body simulations are in favor of the presence of a co-rotating Dark Disk that might contribute significantly (10%–50%) to the local Dark Matter density. Such substructure could have dramatic effect on directional detection. Indeed, in the case of a null lag velocity, one expects an isotropic WIMP velocity distribution arising from the Dark Disk contribution, which might weaken the strong angular signature expected in directional detection. For a wide range of Dark Disk parameters, we evaluate in this Letter the effect of such dark component on the discovery potential of upcoming directional detectors. As a conclusion of our study, using only the angular distribution of nuclear recoils, we show that Dark Disk models as suggested by recent N-body simulations will not affect significantly the Dark Matter reach of directional detection, even in extreme configurations.     

Dark halo or bigravity?

Observations show that about the of the Universe iscomposed by invisible (dark) matter (DM), for which manycandidates have been proposed. In particular, the anomalousbehavior of rotational curves of galaxies (i.e. theflattening at large distance instead of the Keplerian fall)requires thatthis matter is distributed in an extended halo around the galaxy. In order to reproduce this matter density profiles in Newtonian gravity and in cold dark matter (CDM) paradigm (in which theDM particles are collisionless), many ad-hoc approximations are required.The flattening of rotational curves can be explained by asuitable modification of gravitational force in bigravity theories, together with mirror matter model that predicts the existenceof a dark sector in which DM has the same physical properties of visible matter.As an additional result, the Newton constant is different at distances much less and much greater than 20 kpc.     

A Modified Generalized Chaplygin Gas as the Unified Dark Matter–Dark Energy Revisited

A modified generalized Chaplygin gas (MGCG) is considered as the unified dark matter–dark energy revisited. The character of MGCG is endued with the dual role, which behaves as matter at early times and as a quiessence dark energy at late times. The equation of state for MGCG is p?=???αρ/(1?+?α)????(z)ρ ^???α /(1?+?α) , where $\vartheta(z)=-[\,\rho_{\,\rm 0c}(1+z)^{3}]\,^{(1+\alpha)}(1-\Omega_{\,\rm 0B})^{\alpha}\{\alpha\Omega_{\,\rm 0DM}+ \Omega_{\,\rm 0DE}[\,\omega_{\,\rm DE}+\alpha(1+\omega_{\rm DE})](1+z)^{3\omega_{\rm DE}(1+\alpha)}\}$ . Some cosmological quantities, such as the densities of different components of the universe Ω _i (i, respectively, denotes baryons, dark matter, and dark energy) and the deceleration parameter q, are obtained. The present deceleration parameter q ₀, the transition redshift z _T, and the redshift z _eq, which describes the epoch when the densities in dark matter and dark energy are equal, are also calculated. To distinguish MGCG from others, we then apply the Statefinder diagnostic. Later on, the parameters (α and ω _DE) of MGCG are constrained by combination of the sound speed $c^{2}_{\rm s}$ , the age of the universe t ₀, the growth factor m, and the bias parameter b. It yields $\alpha=-3.07^{+5.66}_{-4.98}\times10^{-2}$ and $\omega_{\rm DE}=-1.05^{+0.06}_{-0.11}$ . Through the analysis of the growth of density perturbations for MGCG, it is found that the energy will transfer from dark matter to dark energy which reach equal at z _eq～0.48 and the density fluctuations start deviating from the linear behavior at z～0.25 caused by the dominance of dark energy.     

A Possible Origin of Dark Energy

We discuss the possibility that the existence of dark energy may be due to the presence of a spin zero field φ(χ),either elementary or composite. In the presence of other matter field, the transformation φ(χ)→φ(χ)＋constant can generate a negative pressure, like the cosmological constant. In this picture, our universe can be thought asa very large bag, similar to the much smaller MIT bag model for a single nucleon.     

Dark energy in the Swampland Ⅱ

正We have recently pointed out [1] that the string swampland conjectures [2], if true, provide important constraints on dark energy models. The constraints apply to the field range of a scalar field?described by an effective field theory, and to the slope of the potential Ⅴ of such fields. Specifically, we considered the consequence of the constraint     

Is Dark Energy an illusion?

Much evidence has accumulated that within the context of general relativistic Friedmann-Robertson-Walker (FRW) cosmology there must exist a new, and gravitationally repulsive, substance in the Universe. The effect of this new type of energy density on the expansion of the Universe is to cause its acceleration, and the name that is given to it is ‘Dark Energy’. To say whether or not Dark Energy really exists, however, requires a definite model for the Universe. That is, to be sure of the existence of Dark Energy, and the cosmological acceleration it causes, we must first be sure of the cosmological model we are using to interpret our observations. This is the subject of the present contribution, which will concentrate on the observational status of the Copernican Principle, which is at the heart of the FRW model. In particular, we will outline recent progress that has been made toward answering the question ‘can the observations usually requiring the existence of Dark Energy be accounted for without introducing any new and exotic types of energy density, if we are prepared to give up some of the assumptions of the standard cosmological model?’, or, alternatively, ‘is Dark Energy an illusion?’.     

Dark Energy Density in Brane World

We present a possible explanation to the tiny positive cosmological constant under the frame of AdS5 spacetime embedded by a dS4 brahe. We calculate the dark energy density by summing the zero point energy of massive scalar fields in AdS5 spacetime. Under the assumption that the radius of AdS5 spacetime is of the same magnitude as the radius of observable universe, the dark energy density in dS4 brahe is obtained, which is smaller than the observational value. The reasons are also discussed.     

Cosmological Constant or Variable Dark Energy？

Selection statics of the Akaike information criterion （AIC） model and the Bayesian information criterion （BIC） model are applied to the A-cold dark matter （ACDM） cosmological model, the constant equation of state of dark energy, w =constant, and the parametrized equation of state of dark energy, w（z） = wo ＋ wlz/（1 ＋ z）, to determine which one is the better cosmological model to describe the evolution of the universe by combining the recent cosmic observational data including She Ia, the size of baryonic acoustic oscillation （BAO） peak from SDSS, the three-year WMAP CMB shift parameter. The results show that AIC, BIC and current datasets are not powerful enough to discriminate one model from the others, though odds suggest differences between them.     

A Parametric Dynamic Model of Dark Energy

The double complex symmetric gravitational theory is extended to the parametric symmetric gravitational theory by introducing a parameter β. Hence parametric Friedmann-Robertson-Walker equations are obtained and some characters of dark energy in corresponding spaces are discussed by taking different values of β. In our method some previous results can be included as the special case of our results. It is worth noting that some characters of dark energy can be more intuitively described in our model. By analysis, we can predict that the fate of universe would be a Big Rip in the future, and also find that the state parameters for the two different constraint conditions wФ are consistent with the present cosmological observations.     

Extra Dimensions and Vacuum Dark Energy Models

The role of vacuum energy or cosmological constant in cosmology is discussed in a kind of nontrivial higherdimensional model. Under the framework of Einstein‘s gravity, we obtain the corresponding equations of motion and find that the cosmological constant and vacuum energy in the full regime does not drive its acceleration, but decelerates the expansion of the universe. The dimension of space is required to be n = 3 if we regard vacuum energy or cosmological constant as the candidate to drive the accelerated expansion of the universe.     

Interaction between Two-Dimensional White-Light Photovoltaic Dark Spatial Solitons

Using fully incoherent white light emitted from an incandescent bulb （a line source） and amplitude mask, we study experimentally the interaction between two 21） white-light photovoltaic dark spatial solitons with three different separations （40μm, 50μm and 60μm） and arrangement directions （parallel to, perpendicular to and tilted at 45° with respect to the crystalline c axis） propagating in parallel in close proximity in seff-defocusing LiNbO3：Fe crystal. Experimental results reveal that a 21） white-light dark soliton pair only experiences attractive forces when their mutual separation is sufflciently small （〈 60 μm）, and the degree of the attraction depends on their mutual separation and their arrangement direction. When the separation is larger than 60 μm, the interaction is not evident.     

Dark Lump Excitations in Bose—Einstein Condensates

We investigate the dynamics of two-dimensional matter-wave pulses in a Bose-Einstein condensate with diskshaped traps.For the case of repulsive atom-atom interactions,a Kadomtsev-Petviashvili equation with positive dipersion is derived using the method of multiple scales.The results show that it is possible to excite dark lump-like two-dimensional nonlinear excitations in the Bose-Einstein condensate.     

Do We Really Need Cold Dark Matter?

It is shown that a negative cosmological constant –10^–56 cm^–2 can completely replace cold dark matter in galaxy clusters. The consequences of such a constant are discussed.     

Little Perturbations Grow up…Without Dark Matter

One of the main problems of Cosmology is to conciliate the initial homogeneity and isotropy of the Universe with the subsequent formation of galaxies. In order to find a solution, the Cosmic Microwave Radiation was deeply investigated and a very small anisotropy was finally detected and indicated as the cause of the structure formation. In the standard cosmological theory it is often demonstrated that the linear perturbations do not evolve in a way able to explain the large scale structure of the today observed Universe. In our paper we want to give a simple counterexample showing that it would be possible the formation of a clustered structure in the Universe without the help of the existence of Dark Matter.     

Comparison of Supernovae Datasets Constraints on Dark Energy

Cosmological measurements suggest that our universe contains a dark energy component. In order to study the dark energy evolution, we constrain a parameterized dark energy equation of state ω（z） = ω0 ＋ ω1 1＋z/z using the recent observational datasets： 157 Gold type Ia supernovae and the newly released 182 Gold type Ia supernovae by the maximum likelihood method. It is found that the best fit ω（z） crosses -1 in the past and the present best fit value of ω（0） 〈 -1 obtained from 157 Gold-type Ia supernovae. The crossing of-1 is not realized and ω0 = -1 is not ruled out in 1σ confidence level for the 182 Gold-type Ia supernovae. It is also found that the range of parameter ω0 is wide even in 1σ confidence level and the best fit ω（z） is sensitive to the prior of Ωm.     

Genesis of Dark Energy: Dark Energy as Consequence of Release and Two-Stage Tracking of Cosmological Nuclear Energy

Recent observations on Type-Ia supernovae and low density (Ω _m =0.3) measurement of matter including dark matter suggest that the present-day universe consists mainly of repulsive-gravity type ‘exotic matter’ with negative-pressure often said ‘dark energy’ (Ω _x =0.7). But the nature of dark energy is mysterious and its puzzling questions, such as why, how, where and when about the dark energy, are intriguing. In the present paper the authors attempt to answer these questions while making an effort to reveal the genesis of dark energy and suggest that ‘the cosmological nuclear binding energy liberated during primordial nucleo-synthesis remains trapped for a long time and then is released free which manifests itself as dark energy in the universe’. It is also explained why for dark energy the parameter w=-\frac23w=-\frac{2}{3} . Noting that w=1 for stiff matter and w=\frac13w=\frac{1}{3} for radiation; w=-\frac23w=-\frac{2}{3} is for dark energy because “−1” is due to ‘deficiency of stiff-nuclear-matter’ and that this binding energy is ultimately released as ‘radiation’ contributing “ +\frac13+\frac{1}{3} ”, making w=-1+\frac13=-\frac23w=-1+\frac{1}{3}=-\frac{2}{3} . When dark energy is released free at Z=80, w=-\frac23w=-\frac{2}{3} . But as on present day at Z=0 when the radiation-strength-fraction (δ), has diminished to δ→0, the w=-1+d\frac13=-1w=-1+\delta\frac{1}{3}=-1 . This, almost solves the dark-energy mystery of negative pressure and repulsive-gravity. The proposed theory makes several estimates/predictions which agree reasonably well with the astrophysical constraints and observations. Though there are many candidate-theories, the proposed model of this paper presents an entirely new approach (cosmological nuclear energy) as a possible candidate for dark energy.     

A Conjecture on the Origin of Dark Energy

A conjecture on the origin of the dark energy in our universe is proposed. The analysis indicates that the dark energy may originate from the quantum fluctuations of space-time limited in our universe. Applying both the uncertainty principle and the holographic principle, the author finds that the density of such quantum fluctuation energy is pv = 3c^4/32GL^2H, where LH is the size of the event horizon of our universe and G is the gravitational constant. Using this dark energy model which contains no adjustable parameters, we obtain the current fraction ΩA ≡ ρv/ρc ≈ π/4 and the corresponding equation of state w(z) ≈ -1 (1 - π/4)z with ρc being the critical energy density. These theoretical results are perfectly consistent with the recent cosmological observations. The striking coincidence implies that the quantum fluctuation energy of space-time may be the only source of dark energy. In addition, the analysis shows that the vacuum fluctuation energy does exist, but it comes from spacetime rather than matter. This may have some deep implications for discrete space-time and quantum gravity.