Scalar field as dark energy accelerating expansion of the Universe

The features of a homogeneous scalar field ϕ with classical Lagrangian L = ϕ_;i ϕ^;i /2 − V(ϕ) and tachyon field Lagrangian L = −V(ϕ)√1 − ϕ_;i ϕ^;i causing the observable accelerated expansion of the Universe are analyzed. The models with constant equation-of-state parameter w _de = p _de/ρ_de < −1/3 are studied. For both cases the fields ϕ(a) and potentials V(a) are reconstructed for the parameters of cosmological model of the Universe derived from the observations. The effect of rolling down of the potential V(ϕ) to minimum is shown. Published in Ukrainian in Kinematika i Fizika Nebesnykh Tel, 2008, Vol. 24, No. 5, pp. 345–359. The article was translated by the authors.     

A local void and the accelerating Universe

RCW 114 is a filamentary nebula of about 250 arcmin diameter. Based on its large diameter-to-filament-width ratio, the expansion velocity, distance and size of the shell, it has been suggested that RCW 114 is a supernova remnant in its momentum-conserving phase. Confirmation of this identification is important, as the large angular size and extensive optical emission of this object will allow for detailed study to improve our knowledge of supernova remnants and their interaction with the interstellar medium.
We have used the FLAIR instrument on the UK Schmidt Telescope to obtain optical spectra of several filaments in RCW 114. These confirm that the emission is being produced by the interaction of the shock wave of a supernova remnant with the surrounding interstellar medium. We also obtained narrow-band H α +[N  ii ] and [S  ii ] images to examine the spatial variation in ionization structure.     

The limits of bound structures in the accelerating Universe

According to the latest evidence, the Universe is entering an era of exponential expansion, where gravitationally bound structures will get disconnected from each other, forming isolated 'island universes'. In this scenario, we present a theoretical criterion to determine the boundaries of gravitationally bound structures and a physically motivated definition of superclusters as the largest bound structures in the Universe. We use the spherical collapse model self-consistently to obtain an analytical condition for the mean density enclosed by the last bound shell of the structure (2.36 times the critical density in the present Universe, assumed to be flat, with 30 per cent matter and 70 per cent cosmological constant, in agreement with the previous, numerical result of Chiueh & He). N -body simulations extended to the future show that this criterion, applied at the present cosmological epoch, defines a sphere that encloses ≈99.7 per cent of the particles that will remain bound to the structure at least until the scale parameter of the Universe is 100 times its present value. On the other hand, (28 ± 13) per cent of the enclosed particles are in fact not bound, so the enclosed mass overestimates the bound mass, in contrast with the previous, less rigorous criterion of, e.g. Busha and collaborators, which gave a more precise mass estimate. We also verify that the spherical collapse model estimate for the radial infall velocity of a shell enclosing a given mean density gives an accurate prediction for the velocity profile of infalling particles, down to very near the centre of the virialized core.     

Future evolution of bound superclusters in an accelerating Universe

The evolution of marginally bound supercluster-like objects in an accelerating Λ cold dark matter (ΛCDM) Universe is followed, by means of cosmological simulations, from the present time to an expansion factor   a = 100  . The objects are identified on the basis of the binding density criterion introduced by Dünner et al. Superclusters are identified with the ones whose mass   M > 10¹⁵  h ⁻¹ M_⊙  , the most massive one with   M ∼ 8 × 10¹⁵  h ⁻¹ M_⊙  , comparable to the Shapley supercluster. The spatial distribution of the superclusters remains essentially the same after the present epoch, reflecting the halting growth of the cosmic web as Λ gets to dominate the expansion of the Universe. The same trend can be seen in the stagnation of the development of the mass function of virialized haloes and bound objects. The situation is considerably different when looking at the internal evolution, quantified in terms of their shape, compactness and density profile, and substructure in terms of their multiplicity function. We find a continuing evolution from a wide range of triaxial shapes at   a = 1  to almost perfect spherical shapes at   a = 100  . We also find a systematic trend towards a higher concentration. Meanwhile, we see their substructure gradually disappearing, as the surrounding subclumps fall in and merge to form one coherent, virialized system.     

A simple Newtonian model for the fractal accelerating Universe

We propose a simple mechanism for driving the recently detected non-decelerating expansion of the observable Universe. It is based on the assumption of the predominance of the vacuum energy over the inert matter and a fractal structuring of the latter. The Newton’s universal gravitational force ensures the attraction between celestial bodies at small and medium astronomical distances, but gives rise to the repulsive interaction between mutually very separated cosmic subsystems, like clusters and superclusters, and thus to the observed accelerating expansion and the fractal structuring.     

Acceleration of the Universe in presence of tachyonic field

In this letter, we have assumed that the Universe is filled in tachyonic field with potential, which gives the acceleration of the Universe. For certain choice of potential, we have found the exact solutions of the field equations. We have shown the decaying nature of potential. From recently developed statefinder parameters, we have investigated the role of tachyonic field in different stages of the evolution of the Universe.     

Slow rotation of radiating viscous-fluid Universe coupled with scalar field

One of the models which have stable limit cycles but are very close to the transition of the type I intermittency is examined in some detail. The work integrals are calculated for nonlinear oscillations with various amplitudes. The model reaches its limit cycle by saturation of the driving forces due to the ionized helium (He⁺) ionization. By increasing amplitudes damping becomes superior to the driving forces and so the limit cycle is stable. However, with even larger amplitudes the model becomes pulsational unstable indicating a large positive contribution to the work integral at rather deep interior. Strong luminosity drops are observed in this region during contraction phase. It is shown that the drops come from the neutral helium and hydrogen (He and H) ionization zones moved down to the deep interior at contraction phase with increasing amplitudes. A shock wave is generated by the radiation pressure at the ionization zones and propagates outwards at the phase. The zone between the ionization zones and the detached shock front is compressed locally. Thus, subsequent contraction leads the pressure at the zone becomes very high, causing remarkable enhancement of the opacities. Thus the driving becomes to work efficiently. This is a main driving force with finite amplitudes beyond the limit cycle, and makes the model to have an unstable fixed point beyond it.     

Rotational perturbations of cosmological viscous-fluid Universe with zero-mass scalar field

Certain new analytic solutions for the rotational perturbations of the Robertson-Walker universe are found out to substantiate the possibility of the existence of a rotating viscous universe with zero-mass scalar field. The values for (r, t) which is related to the local dragging of inertial frames are investigated. In all the cases the rotational velocity is found to decay with time. Except for perfect dragging the scalar field is found to have a damping effect on the rotation of matter. The damping effect is found to be roughly analogous to viscosity. In some solutions it is found that the scalar field may exist only during a time period in the course of evolution of the Universe.     

Coupled perfect fluid and zero-mass scalar field in an expanding Universe

A spatially flat expanding cosmological model filled with interacting perfect fluid and zero-mass scalar field is obtianed under a specific law for Hubble's parameter. It is shown that this model avoids the big-bang singularity.     

Entropy and statefinder diagnosis in chameleon cosmology

In this paper, the generalized second law (GSL) of thermodynamics and entropy is revisited in the context of cosmological models with bouncing behavior such as chameleon cosmology where the boundary of the universe is assumed to be enclosed by the dynamical apparent horizon. From a thermodynamic point of view, to link between thermodynamic and geometric parameters in cosmological models, we introduce “entropy rate of change multiplied by the temperature” as a model independent thermodynamic state parameter together with the well known {r,s} statefinder to differentiate the dark energy models.     

Dynamics of the self-interacting chameleon cosmology

In this article we study the properties of the flat FRW chameleon cosmology in which the cosmic expansion of the Universe is affected by the chameleon field and dark energy. In particular, we perform a detailed examination of the model in the light of numerical analysis. The results illustrate that the interacting chameleon filed plays an important role in late time universe acceleration and phantom crossing.     

Holographic dark energy in chameleon tachyon cosmology

We propose in this paper an interacting holographic dark energy (IHDE) model in chameleon–tachyon cosmology by interaction between the components of the dark sectors. In the formalism, the interaction term emerges from the scalar field coupling matter Lagrangian in the model rather than being inserted into the formalism as an external source for the interaction. The correspondence between the tachyon field and the holographic dark energy (HDE) densities allows to reconstruct the tachyon scalar field and its potential in a flat FRW universe. The model can show the accelerated expansion of the universe and satisfies the observational data.     

Nonlinear spinor field in Bianchi type-I Universe filled with viscous fluid: numerical solutions

We consider a system of nonlinear spinor and a Bianchi type I gravitational fields in presence of viscous fluid. The nonlinear term in the spinor field Lagrangian is chosen to be λ F, with λ being a self-coupling constant and F being a function of the invariants I an J constructed from bilinear spinor forms S and P. Self-consistent solutions to the spinor and BI gravitational field equations are obtained in terms of τ, where τ is the volume scale of BI universe. System of equations for τ and ε, where ε is the energy of the viscous fluid, is deduced. This system is solved numerically for some special cases.      

Impact of neutron star oscillations on the accelerating electric field in the polar cap of pulsar

Pulsar “standard model”, that considers a pulsar as a rotating magnetized conducting sphere surrounded by plasma, is generalized to the case of oscillating star. We developed an algorithm for calculation of the Goldreich-Julian charge density for this case. We consider distortion of the accelerating zone in the polar cap of pulsar by neutron star oscillations. It is shown that for oscillation modes with high harmonic numbers (l,m) changes in the Goldreich-Julian charge density caused by pulsations of neutron star could lead to significant altering of an accelerating electric field in the polar cap of pulsar. In the moderately optimistic scenario, that assumes excitation of the neutron star oscillations by glitches, it could be possible to detect altering of the pulsar radioemission due to modulation of the accelerating field. This work was partially supported by RFBR grant 04-02-16720, and by the grants N.Sh.-5218.2006.2 and RNP-2.1.1.5940.     

On the holographic dark energy in chameleon scalar-tensor cosmology

We study the holographic dark energy (HDE) model in generalized Brans-Dicke scenario with a non-minimal coupling between the scalar field and matter Lagrangian namely Chameleon Brans Dicke (CBD) mechanism. In this study we consider the interacting and non-interacting cases for two different cutoffs. The physical quantities of the model such as, equation of state (EoS) parameter, deceleration parameter and the evolution equation of dimensionless parameter of dark energy are obtained. We shall show that this model can describe the dynamical evolution of fraction parameter of dark energy in all epochs. Also we find the EoS parameter can cross the phantom divide line by suitable choices of parameters without any mines kinetic energy term.     

Planets in the early Universe

Several planets have recently been discovered around stars that are old and metal-poor, implying that these planets are also old, formed in the early Universe together with their hosts. The canonical theory suggests that the conditions for their formation could not have existed at such early epochs. In this paper we argue that the required conditions, such as sufficiently high dust-to-gas ratio, could in fact have existed in the early Universe immediately following the first episode of metal production in Pop. III stars, both in metal-enhanced and metal-deficient environments. Metal-rich regions may have existed in multiple isolated pockets of enriched and weakly-mixed gas close to the massive Pop. III stars. Observations of quasars at redshifts z～5, and gamma-ray bursts at z～6, show a very wide spread of metals in absorption from [X/H]??3 to ??0.5. This suggests that physical conditions in the metal-abundant clumps could have been similar to where protoplanets form today. However, planets could have formed even in low-metallicity environments, where formation of stars is expected to proceed due to lower opacity at higher densities. In such cases, the circumstellar accretion disks are expected to rotate faster than their high-metallicity analogues. This in turn can result in the enhancement of dust particles at the disk periphery, where they can coagulate and start forming planetesimals. In conditions with the low initial specific angular momentum of the cloud, radiation from the central protostar can act as a trigger to drive small-scale instabilities with typical masses in the Earth to Jupiter mass range. Discoveries of planets around old metal-poor stars (e.g. HIP 11952, [Fe/H]～?1.95, ～13 Gyr) show that planets did indeed form in the early Universe and this may require modification of our understanding of the physical processes that produce them. This work is an attempt to provide one such heuristic scenario for the physical basis for their existence.     

Early Universe in Self-Creation Cosmology

Spatially homogeneous and isotropic Robertson-Walker model of the universe is studied in Barber's second self-creation theory of gravitation in the presence of perfect fluid by using gamma-law equation of state p =(-1). The parameter gamma varies continuously with cosmological time. Exact solutions of the field equations are obtained for inflationary period and radiation-dominated era by using the power law relation R^n-3 = B. Some physical properties of the models are also discussed.     

Dark Energy in the Universe

I 24 Studying the Nature of Dark Energy with Galaxy Clusters I 50 Constraining Dark Energy via Baryon Acoustic Oscillations I 65 Constraining Dark Energy with Redshift Surveys I 103 Dark Energy: Necessity, Models and Expectations I 177 Searching for galaxy clusters through weak lensing, X‐rays and the SZ observations I 181 SNIa and Dark Energy