Letter: The Energy Density of the Quaternionic Field as Dark Energy in the Universe

In this article we describe a model of the universe consisting of a mixture of the ordinary matter and a so-called cosmic quaternionic field. The basic idea here consists in an attempt to interpret as the energy density of the quaternionic field whose source is any form of energy including the proper energy density of this field. We set the energy density of this field to and show that the ratio of ordinary dark matter energy density assigned to is constant during the cosmic evolution. We investigate the interaction of the quaternionic field with the ordinary dark matter and show that this field exerts a force on the moving dark matter which might possible create the dark matter in the early universe. Such determined fulfils the requirements asked from the dark energy. In this model of the universe, the cosmological constant, the fine-tuning and the age problems might be solved. Finally, we sketch the evolution of the universe with the cosmic quaternionic field and show that the energy density of the cosmic quaternionic field might be a possible candidate for the dark energy.     

A power-law coupled three-form dark energy model

We consider a field theory model of coupled dark energy which treats dark energy as a three-form field and dark matter as a spinor field. By assuming the effective mass of dark matter as a power-law function of the three-form field and neglecting the potential term of dark energy, we obtain three solutions of the autonomous system of evolution equations, including a de Sitter attractor, a tracking solution and an approximate solution. To understand the strength of the coupling, we confront the model with the latest Type Ia Supernova, Baryon Acoustic Oscillations and Cosmic Microwave Background radiation observations, with the conclusion that the combination of these three databases marginalized over the present dark matter density parameter \(\Omega _{m0}\) and the present three-form field \(\kappa X_{0}\) gives stringent constraints on the coupling constant, \(-\,0.017< \lambda <0.047\) (\(2\sigma \) confidence level), by which we present the model’s applicable parameter range.     

Letter: Electromagnetic Mass-Models in General Relativity Reexamined

The problem of constructing a model of an extended charged particle within the context of general relativity has a long and distinguished history. The distinctive feature of these models is that, in some way or another, they require the presence of negative mass in order to maintain stability against Coulomb's repulsion. Typically, the particle contains a core of negative mass surrounded by a positive-mass outer layer, which emerges from the Reissner-Nordström field. In this work we show how the Einstein-Maxwell field equations can be used to construct an extended model where the mass is positive everywhere. This requires the principal pressures to be unequal inside the particle. The model is obtained by setting the effective matter density, rather than the rest matter density, equal to zero. The Schwarzschild mass of the particle arises from the electrical and gravitational field (Weyl tensor) energy. The model satisfies the energy conditions of Hawking and Ellis. A particular solution that illustrates the results is presented.     

Dynamics of anisotropic cosmological model with ultrarelativistic matter,magnetic field,and fluxes of free isotropic particles

Within the framework of general relativity a dynamics of homogeneous anistropic axially symmetric model of the Bianchi type I is considered for the case when sources of gravitational field are ultrarelativistic matter, homogeneous magnetic field, and fluxes of free particles. Qualitative analysis of the field equations on a phase plane is given. All solutions of a considered type for large values of proper time asymptotically approach the flat Friedmann model while the value of energy density of free particles approaches the double value of magnetic field energy density. Near a singular state the solution exhibits oscillating behavior with successive interchange of Kasner singularities of pancake-like and filament-like types. It is also shown that in the absence of matter a solution retains its character.     

A dilemma in the physics of gravitational fields

It is shown that improper use of local quantities for nonlocal situations in fields leads to traditional errors. Nonlocal theoretical quantities referred to standards in a fixed field are defined in order to obtain reliable results. Nonlocal properties of gravitational fields and matter located in it are deduced with the help of physical principles and an electromagnetic model for matter. In spite of the fact that the local velocity of light should be constant, the field is a space of variable nonlocal velocity of light, which accounts for its properties. Matter and light virtually propagate themselves without exchanging energy with the external field, in disagreement with traditional assumptions. Matter becomes contracted by the field. The results are self-consistent and consistent with the observed facts. Bodies withr2GM would be different from black holes and they may account for the peak of highest energy of cosmic radiation and other astronomical facts.     

The mass of a static charged sphere

The field of a static, charged sphere is investigated using general relativity. InNordström's exterior solution the parametersm ande, referring to mass and charge, are unrelated, and indeedm can be put equal to zero. It is shown that, if the interior solution is considered,m cannot be put zero unless the matter density is negative. The contribution of the electric field energy to the gravitational mass is estimated using certain special models. A model is given in which the gravitational attraction of the charged matter balances its electrical repulsion. If the radius is allowed to tend to zero, this gives a model of a point charge with finite and non-zero mass and charge.     

Floating and sinking: the imprint of massive scalars around rotating black holes

We study the coupling of massive scalar fields to matter in orbit around rotating black holes. It is generally expected that orbiting bodies will lose energy in gravitational waves, slowly inspiraling into the black hole. Instead, we show that the coupling of the field to matter leads to a surprising effect: because of superradiance, matter can hover into "floating orbits" for which the net gravitational energy loss at infinity is entirely provided by the black hole's rotational energy. Orbiting bodies remain floating until they extract sufficient angular momentum from the black hole, or until perturbations or nonlinear effects disrupt the orbit. For slowly rotating and nonrotating black holes floating orbits are unlikely to exist, but resonances at orbital frequencies corresponding to quasibound states of the scalar field can speed up the inspiral, so that the orbiting body sinks. These effects could be a smoking gun of deviations from general relativity.     

Noether symmetry of <Emphasis Type="Italic">F</Emphasis>(<Emphasis Type="Italic">T</Emphasis>) cosmology with quintessence and phantom scalar fields

In this paper, we investigate the Noether symmetries of F(T) cosmology involving matter and dark energy. In this model, the dark energy is represented by a canonical scalar field with a potential. Two special cases for dark energy are considered, including phantom energy and quintessence. We obtain F(T)～T ^3/4, and the scalar potential V(?)～? ² for both models of dark energy and discuss quantum picture of this model. Some astrophysical implications are also discussed.     

Tree level gravity—scalar matter interactions in analogy with Fermi theory of weak interactions using only a massive vector field

In this paper we work in perturbative quantum gravity coupled to scalar matter at tree level and we introduce a new effective model in analogy with the Fermi theory of weak interaction and in relation with a previous work where we have studied only the gravity and its self-interaction. This is an extension of the I.T.B. model (Intermediate-Tensor-Boson) for gravity also to gravitationally interacting scalar matter. We show that in a particular gauge the infinite series of interactions containing n gravitons and two scalars could be rewritten in terms of only two Lagrangians containing a massive field, the graviton and, obviously, the scalar field. Using the S-matrix we obtain that the low energy limit of the amplitude reproduces the local Lagrangian for the scalar coupled to gravity.     

Exact scaling solutions and fixed points for general scalar field

We show that the most general dark energy model that possesses a scaling solution ρ?∝an${\rho }_{?}\propto a^n$ is the k-essence model, which includes both of the quintessence and tachyon models. The exact scaling solutions are then derived. The potential that gives the tracking solution in which dark energy exactly tracks the background matter field is the inverse squared potential. The quintessence field with exponential potential can be obtained from the k-essence field with the inverse squared potential. We also find the fixed points and study their main properties, whereby the scalar field dominant fixed point is identified.     

Relativistic theory of gravitation

In the present paper a relativistic theory of gravitation (RTG) is unambiguously constructed on the basis of the special relativity and geometrization principle. In this a gravitational field is treated as the Faraday-Maxwell spin-2 and spin-0 physical field possessing energy and momentum. The source of a gravitational field is the total conserved energy-momentum tensor of matter and of a gravitational field in Minkowski space. In the RTG the conservation laws are strictly filfilled for the energy-momentum and for the angular momentum of matter and a gravitational field. The theory explains the whole available set of experiments on gravity. By virtue of the geometrization principle, the Riemannian space in our theory is of field origin, since it appears as an effective force space due to the action of a gravitational field on matter. The RTG leads to an exceptionally strong prediction: The universe is not closed but just flat. This suggests that in the universe a missing mass should exist in a form of matter.     

An Effective Field Theory Model to Describe Nuclear Matter in Heavy-Ion Collisions

Relativistic mean field theory with mesons , , and mediating interactions and nucleons as basic fermions has been very successful in describing nuclear matter and finite nuclei. However, in heavy-ion collisions, where the c. m. energy of two colliding nucleons will be in the hundreds of GeV region, nucleons are not expected to behave as point-like particles. Analyses of elastic pp and ¯pp scattering data in the relevant c. m. energy range show that the nucleon is a composite object—a topological soliton or Skyrmion embedded in a condensed quark-antiquark ground state. Against this backdrop, we formulate an effective field theory model of nuclear matter based on the gauged linear -model where quarks are the basic fermions, but the mesons still mediate the interactions. The model describes the nucleon as a Skyrmion and produces a q¯q ground state analogous to a superconducting ground state. Quarks are quasi-particles in this ground state. When the temperature exceeds a critical value, the scalar field in the ground state vanishes, quarks become massless, and a chiral phase transition occurs leading to chiral symmetry restoration. We explore the possibility of a first order phase transition in this model by introducing suitable self-interactions of the scalar field. Internal structures of the Skyrmions are ignored, and they are treated as point-like fermions.     

Bulk Viscous Cosmological Model with Interacting Dark Fluids

We study a cosmological model for a spatially flat Universe whose constituents are a dark energy field and a matter field comprising baryons and dark matter. The constituents are assumed to interact with each other, and a non-equilibrium pressure is introduced to account for irreversible processes. We take the non-equilibrium pressure to be proportional to the Hubble parameter within the framework of a first-order thermodynamic theory. The dark energy and matter fields are coupled by their barotropic indexes, which depend on the ratio between their energy densities. We adjust the free parameters of the model to optimize the fits to the Hubble parameter data. We compare the viscous model with the non-viscous one, and show that the irreversible processes cause the dark-energy and matter-density parameters to become equal and the decelerated–accelerated transition to occur at earlier times. Furthermore, the density and deceleration parameters and the distance modulus have the correct behavior, consistent with a viable scenario of the present status of the Universe.     

Unified model of baryonic matter and dark components

We investigate an interacting two-fluid cosmological model and introduce a scalar field representation by means of a linear combination of the individual energy densities. Applying the integrability condition to the scalar field equation we show that this “exotic quintessence” is driven by an exponential potential and the two-fluid mixture can be considered as a model of three components. These components are associated with baryonic matter, dark matter and dark energy respectively. We use the Simon, Verde and Jimenez [J. Simon, L. Verde, R. Jimenez, Phys. Rev. D 71 (2005) 123001] determination of the redshift dependence of the Hubble parameter to constrain the current density parameters of this model. With the best fit density parameters we obtain the transition redshift between non-accelerated and accelerated regimes zacc=0.66$z_{\mathrm{acc}}=0.66$ and the time elapsed since the initial singularity t₀=19.8 Gyr$t_0=19.8\text{Gyr}$. We study the perturbation evolution of this model and find that the energy density perturbation decreases with the cosmological time.     

Non-minimal quintessence: Dynamics and coincidence problem

Brans–Dicke scalar–tensor theory provides a conformal coupling of the scalar field with gravity in Einstein’s frame. This model is equivalent to an interacting quintessence in which dark matter is coupled to dark energy. This provides a natural mechanism to alleviate the coincidence problem. We investigate the dynamics of this model and show that it leads to comparable dark energy and dark matter densities today.     

Letter: A Model of Dark Energy for the Accelerating Universe

Recent observations of large scale structure of the Universe, especially that of Type Ia supernovae, indicate that the Universe is flat and is accelerating, and that the dominant energy density in the Universe is the cosmic dark energy. We propose a model in which the cosmic effective Yang-Mills condensate familiar in particle physics plays the role of the dark energy that causes the acceleration of the Universe. Since the quantum effective Yang-Mills field in certain states has the equation of state p _y = – _y , when employed as the cosmic matter source, it naturally results in an accelerating expansion of the Universe. With the matter components ( _m 1/3) being added into the model, the composition of YM condensate and matter components can give rise to the desired equation of state w –2/3 for the Universe.     

Dynamics of a spherically symmetric inhomogeneous coupled dark energy model with coupling term proportional to non relatvistic matter

The quasi-local scalar variables approach is applied to a spherically symmetric inhomogeneous Lemaître–Tolman–Bondi metric containing a mixture of non-relativistic cold dark matter and coupled dark energy with constant equation of state. The quasi-local coupling term considered is proportional to the quasi-local cold dark matter energy density and a quasi-local Hubble factor-like scalar via a coupling constant \(\alpha \). The autonomous numerical system obtained from the evolution equations is classified for different choices of the free parameters: the adiabatic constant of the dark energy w and \(\alpha \). The presence of a past attractor in a non-physical region of the energy densities phase-space of the system makes the coupling term non physical when the energy flows from the matter to the dark energy in order to avoid negative values of the dark energy density in the past. On the other hand, if the energy flux goes from dark energy to dark matter, the past attractor lies in a physical region. The system is also numerically solved for some interesting initial profiles leading to different configurations: an ever expanding mixture, a scenario where the dark energy is completely consumed by the non-relativistic matter by means of the coupling term, a scenario where the dark energy disappears in the inner layers while the outer layers expand as a mixture of both sources, and, finally, a structure formation toy model scenario, where the inner shells containing the mixture collapse while the outer shells expand.     

A dynamical approach to the cosmological constant

This Letter presents an exact analytic solution of a simple cosmological model in presence of both nonrelativistic matter and scalar field where Einstein's cosmological constant Λ appears as an integration constant. Unlike Einstein's cosmological constant ascribed to vacuum energy, the dark energy density and the energy density of the ordinary matter decrease at the same rate during the expansion of the universe. Therefore the model is free of the coincidence problem. Comparing the predictions using this model with the current cosmological observations shows that the results are consistent.     

Cosmological model with interactions in the dark sector

A cosmological model for the present Universe is analyzed whose constituents are a non-interacting baryonic matter field and interacting dark matter and dark energy fields. The dark energy and dark matter are coupled through their effective barotropic indexes, which are considered as functions of the ratio of their energy densities. Two asymptotically stable cases are investigated for the ratio of the dark energy densities which have their parameters adjusted by considering best fits to Hubble function data. It is shown that the deceleration parameter, the density parameters, and the luminosity distance have the correct behavior which is expected for a viable present scenario of the Universe.