Testing the cosmic coincidence problem and the nature of dark energy

Dark energy models which alter the relative scaling behavior of dark energy and matter could provide a natural solution to the cosmic coincidence problem-why the densities of dark energy and dark matter are comparable today. A generalized class of dark energy models is introduced which allows noncanonical scaling of the ratio of dark matter and dark energy with the Robertson-Walker scale factor a(t). We show that determining whether there is a coincidence problem, and the extent of cosmic coincidence, can be addressed by several forthcoming experiments.     

A tracker solution to the cold dark matter cosmic coincidence problem

Recently, we introduced the notion of “tracker fields,” a form of quintessence which has an attractor-like solution. Using this concept, we showed how to construct models in which the ratio of quintessence to matter densities today is independent of initial conditions. Here we apply the same idea to the standard cold dark matter component in cases where it is composed of oscillating fields. Combining these ideas, we can construct a model in which quintessence, cold dark matter, and ordinary matter all contribute comparable amounts to the total energy density today irrespective of initial conditions.     

Phantom dark energy with varying-mass dark matter particles: Acceleration and cosmic coincidence problem

We investigate several varying-mass dark matter particle models in the framework of phantom cosmology. We examine whether there exist late-time cosmological solutions, corresponding to an accelerating universe and possessing dark energy and dark matter densities of the same order. Imposing exponential or power-law potentials and exponential or power-law mass dependence, we conclude that the coincidence problem cannot be solved or even alleviated. Thus, if dark energy is attributed to the phantom paradigm, varying-mass dark matter models cannot fulfill the basic requirement that led to their construction.     

Charged cosmic strings interacting with gravitational and electromagnetic waves

Under a particular choice of the Ernst potential, we solve analytically the Einstein–Maxwell equations to derive a new exact solution depending on five parameters: the mass, the angular-momentum (per unit mass), α, the electromagnetic-field strength, k, the parameter-p and the Kerr-NUT parameter, l. This (Petrov Type D) solution is cylindrically symmetric and represents the curved background around a charged, rotating cosmic string, surrounded by gravitational and electromagnetic waves, under the influence of the Kerr-NUT parameter. A C-energy study in the radiation zone suggests that both the incoming and the outgoing radiation is gravitational, strongly focused around the null direction and preserving its profile. In this case, the absence of the k-parameter from the C-energy implies that, away from the linear defect the electromagnetic field is too weak to contribute to the energy-content of the cylindrically symmetric space-time under consideration. In order to explain this result, we have evaluated the Weyl and the Maxwell scalars near the axis of the linear defect and at the spatial infinity. Accordingly, we have found that the electromagnetic field is concentrated (mainly) in the vicinity of the axis, while falling-off prominently at large radial distances. However, as long as k ≠ 1, the non-zero Kerr-NUT parameter enhances those scalars, both near the axis and at the spatial infinity, introducing some sort of gravitomagnetic contribution.     

Observations favor the crossing of phantom divide lines

Using three different parameterized dark energy models, we reconstruct the properties of dark energy from the latest 397 Sne Ia, CMB and BAO with the present matter density, Ωm0, given prior. We find that, when Ωm0 is not small, for example, Ωm0 = 0.28 or 0.32, an evolving dark energy with a crossing of phantom divide line is favored and this conclusion seems to be model independent. We also find that the evolving properties of dark energy become more and more evident with the increase of Ωm0 given prior.     

On the consistency problem in simple fluids

Exact conditions are given for the sum of elementary graphs in order to obtain a fully consistent approximation in the theory of simple fluids. An approximation has been suggested which has the property of Ornstein-Zernike behaviour along the spinodal curve.     

Dark energy, cosmic coincidence and future singularity of the universe

Dark energy model with the equation of state $p_{DE} =-\rho _{DE} -A\rho _{DE}^\alpha $ , is characterised by four finite life time future singularity of the universe for different values of the parameter $A$ and $\alpha $ [Nojiri et al. in Phys Rev D 71:063004, 2005]. Since from the matter dominated era to the dark energy dominated era the ratio of the dark energy density to the matter energy density increases as the universe expand for these future singularities, the universe passes through a significant time when the dark energy density and the matter energy density are nearly comparable. Considering $\frac{1}{r_0 }<r=\frac{\rho _{DE} }{\rho _M }<r_0 $ , where $r_0$ is any fixed ratio, we calculate the fraction of total life time of the universe when the universe passes through the coincidental stage for these singularities. It has been found that the fractional time varies as $\alpha $ varies within the range for which these finite life time future singularities occur and the fraction is smaller for smaller values of $r_0 $ . Importance of the fractional time and observational limits onto the values of the parameter $A$ and $\alpha $ has also been discussed.     

On the growth of perturbations in interacting dark energy and dark matter fluids

The covariant generalizations of the background dark sector coupling suggested in Mangano, Miele and Pettorino (Mod Phys Lett A 18:831, 2003) are considered. The evolution of perturbations is studied with detailed attention to interaction rate that is proportional to the product of dark matter and dark energy densities. It is shown that some classes of models with coupling of this type do not suffer from early time instabilities in strong coupling regime.     

An interacting scalar field and the recent cosmic acceleration

In this paper it is shown that the Brans – Dicke scalar field itself can serve the purpose of providing an early deceleration and a late time acceleration of the universe without any need of quintessence field if one considers an interaction, i.e., transfer of energy between the dark matter and the Brans – Dicke scalar field.     

Stability and emergence of gravitational waves in the quantum cosmic phantom models

We provide an alternative interpretation of the quantum cosmic phantom models, in which the current acceleration of the universe is due to the existence of an entropy of entanglement. Subsequently, we study their stability as well as the emergence of gravitational waves.     

Semi-local cosmic strings and the cosmological constant problem

We study the cosmological constant problem in a three-dimensional N = 2 supergravity theory with gauge groupSU (2)_global × U(1)_local. The model we consider is known to admit string-like configurations, the so-called semi-local cosmic strings. We show that the stability of these solitonic solutions is provided by supersymmetry through the existence of a lower bound for the energy, even though the manifold of the Higgs vacuum does not contain non-contractible loops. Charged Killing spinors do exist over configurations that saturate the Bogomol'nyi bound, as a consequence of an Aharonov-Bohm-like effect. Nevertheless, there are no physical fermionic zero modes on these backgrounds. The exact vanishing of the cosmological constant does not imply, then, Bose-Fermi degeneracy. This provides a non-trivial example of the recent claim made by Witten on the vanishing of the cosmological constant in three dimensions without unphysical degeneracies.     

A photoelectron-ion-ion triple coincidence technique for the study of double photoionization and its consequences

A new technique in which two photoions are detected in coincidence with a photoelectron is described, and its advantages in the study of double ionization are explored. The power of the technique to distinguish decay routes is demonstrated by proof that the equal mass fragmentation of SO₂²⁺produces predominantly O₂⁺ + S⁺and not O⁺ + O⁺. The dissociation of CH₃I²⁺ following He(II) ionization is shown to involve at least twelve distinct pathways, instead of the six previously known.A major potential of the new technique is to elucidate the dynamics of three-fragment decays: it is shown that the formation of C⁺ + S⁺ from CS₂²⁺ can be modelled as a sequential, rather than a simultaneous explosion. The new technique gives the first demonstration, in the case of NO₂, of angular anisotropy in the electrons ejected in double photoionization. The counting statistics of the new method are shown to allow absolute calibration of the detection efficiencies for both electrons and ions. Finally, the technique offers a new method for the detection and investigation of slowly dissociating doubly-charged ions.     

Visualization of interacting crossing vortex lattices in the presence of quenched disorder

We have imaged interacting crossing pancake vortex (PV) and Josephson vortex (JV) lattices in highly anisotropic Bi2Sr2CaCu2O(8+delta) single crystals under tilted magnetic fields. The dependence of vortex structures on in-plane field is in good quantitative agreement with theoretical predictions, yielding an almost temperature-independent anisotropy parameter of gamma=640+/-25. We directly confirm that the PV/JV attraction arises from small PV displacements in the presence of JV supercurrents and demonstrate how the existence of quenched disorder leads to indirect JV pinning and dynamic vortex fragmentation.     

The features of the EPR spectrum in the vicinity of accidental coincidence of interacting transitions

The mechanisms of the inhomogeneous broadening of the EPR spectra of exchange-coupled copper dimers, high-spin iron centers in lithium germanate, and off-center Tl²⁺ ions in potassium sulfate are analyzed. It is shown that the additional EPR signals observed for these materials when two EPR lines are nearly coincident can be due to averaging of a portion of the spin packets associated with these lines.     

On the crossing of thin shells

The crossing of two dust shells is considered as a simplified model of a collapsing thick layer of dust. We use the Israel's formalism to describe the development of two shells of dust which interact only gravitationally. The formalism is developed in both Schwarzschild and Kruskal coordinates.     

On the resolution of time problem in quantum gravity induced from unconstrained membranes

The relativistic theory of unconstrained p-dimensional membranes (p-branes) is further developed and then applied to the embedding model of induced gravity. Space-time is considered as a 4-dimensional unconstrained membrane evolving in an N-dimensional embedding space. The parameter of evolution or the evolution time is a distinct concept from the coordinate time t=x⁰. Quantization of the theory is also discussed. A covariant functional Schrödinger equation has a solution for the wave functional such that it is sharply localized in a certain subspace P of space-time, and much less sharply localized (though still localized) outside P. With the passage of evolution the region P moves forward in space-time. Such a solution we interpret as incorporating two seemingly contradictory observations: (i) experiments clearly indicate that space-time is a continuum in which events are existing; (ii) not the whole 4-dimensional space-time, but only a 3-dimensional section which moves forward in time, is accessible to our immediate experience. The notorious problem of time is thus resolved in our approach to quantum gravity. Finally we include sources into our unconstrained embedding model.     

两库柏对相互作用问题的研究

在文中给出了库柏对的两种不同表达形式。发现用由对算符组成的哈密顿量来计算更方便,用这些对算符对易性质,而不是把它们看成费米子算符的组合产物。这样一来,研究的库柏对相互作用问题也把库柏对看成对算符。文中解决了两个库柏对相互作用问题,并讨论了推广到多库柏系统的情况。     

Perturbation theory for fluids of rod-like molecules interacting via the Kihara potential

For systems of molecules interacting via the Kihara core potential a first-order perturbation theory is proposed. As a reference system soft convex bodies are employed with interactions given by the entire repulsive part of the original pair potential (i.e. for surface-to-surface distances smaller than that of the potential minimum). Their equilibrium behaviour is interpreted on the basis of the representative hard convex bodies-parallel convex bodies to the assumed cores with temperature-dependent thickness. The shape of the distribution function was approximated by the Verlet-Weis form. Theoretical expressions were used for the determination of the thermodynamic functions of the Kihara-molecule systems at several reduced temperatures and compared with experimental data for nitrogen.     

A resolution of the classical wave-particle problem

The classical wave-particle problem is resolved in accord with Newton's concept of the particle nature of light by associating particle density and flux with the classical wave energy density and flux. Point particles flowing along discrete trajectories yield interference and diffraction patterns, as illustrated by Young's double pinhole interference. Bound particle motion is prescribed by standing waves. Particle motion as a function of time is presented for the case of a particle in a box. Initial conditions uniquely determine the subsequent motion. Some discussion regarding quantum theory is preseted.