Qualitative analysis of string cosmology with loop corrections. I

Homogeneous and Isotropic cosmological models of low-energy, string gravitation with loop corrections to the dilaton coupling functions are investigated by methods of the qualitative theory of dynamical systems. An ideal fluid with a barotropic equation of state is considered as the nongravitational source. In the general case of curved models, the cosmological equations are represented in the form of a third-order, autonomous, dynamical system. Phase portraits for different coupling functions are constructed for flat models. The asymptotic behavior of the general solution in limiting regions is investigated. The stabilization of the dilaton is analyzed using the Damour-Polyakov mechanism. Translated from Astrofizika, Vol. 42, No. 1, pp. 117–136, January–March, 1999.     

On string cosmology with a dilaton potential. I

The problems of flat directions and supersymmetry breaking are fundamental problems that are still unsolved in the comparison of string theory with low-energy physics. A possible nonperturbative dilaton potential may play an important role in their solution. In the present paper we consider D-dimensional, low-energy, string cosmological models with a di/aton potential. Exact solutions are written for several simple potentials. The picture of cosmological evolution of a gravi-dilaton model is investigated by methods of the qualitative theory of dynamical systems for different cases of the behavior of the potential in the domain of strong coupling. Features of models having potentials that take negative values in certain ranges of values of the dilaton field are discussed. A specific mechanism of generation of a nonperturbative potential, based on gaugino condensation in the hidden sector of the gauge group, is considered. Translated from Astrofizika, Vol. 40, No. 2, pp. 233–251, April–June, 1997.     

Dilaton stabilization in string cosmology. II

A mechanism for stabilization of the dilaton field within the framework of low-energy string gravitation with loop corrections to the dilaton coupling function was proposed in the first part of this paper. The mechanism is based on the assumption that loop corrections generate a singular dilaton kinetic function for a certain value of the dilaton field. For a nortgravitational source with a constant barotropic index, the system of cosmological equations reduces to an autonomous, third-order dynamical system. The behavior of the general solution in the vicinity of singularities of the dilaton coupling function is investigated by methods of the qualitative theory of dynamical systems for different values of the singularity index. The conditions under which solutions with a constant dilaton are attractors for a general solution with a variable dilaton are determined. The evolution of models is considered, the corresponding phase diagrams are constructed, and the question of the effectiveness of dilaton stabilization is investigated. Translated from Astrofizika, Vol. 43, No. 2, pp. 313–324, April–June, 2000.     

On string cosmology with a dilaton potential. II

Investigations of string cosmology with a nonperturbative dilaton potential, begun in the first part of this work, are continued. The picture of cosmological evolution of an isotropic, gravi-dilaton model for different cases of the behavior of the potential in the region of strong coupling is analyzed by methods of the qualitative theory of dynamical systems. Features of models with potentials that take negative values in certain ranges of values of the dilaton field are discussed. A specific mechanism of generation of the nonperturbative potential, based on gaugino condensation in a hidden sector of the gauge group, is considered. Translated from Astrofizika, Vol. 40, No. 4, pp. 517–534, October-December, 1997.     

qualitative analysis of string cosmology with loop corrections. II

Homogeneous and Isotropic cosmological models of effective string theory with a curved space are investigated by the methods of the qualitative theory of dynamical systems. It is shown that for radiation-dominated models, tthe corresponding dynamical system can be integrated exactly for the general case of dilaton coupling functions. Models in the tree approximation with a two-dimensional phase space are considered separately. In the general case of loop corrections, all possible stationary points are found and their character is determined. The results are illustrated using a specific example. Various cases of fixing the dilaton within the framework of the Damour-Polyakov mechanism are considered. Translated from Astrofizika, Vol. 42, No. 2, pp. 295–310, April–June, 1999.     

Dilaton stabilization in string cosmology. I

A new mechanism is proposed for stabilization of the scalar dilaton field within the framework of lowenergy string gravitation with loop corrections to the dilaton coupling functions. It is based on the assumption that the loop corrections generate a kinetic dilaton function, which is singular for some finite value of the dilaton field. For a nongravitational source of the barotropic type, the system of equations describing the evolution of homogeneous and isotropic cosmological models is represented in the form of a thirdorder, autonomous, dynamical system. The behavior of the general solution in the vicinity of singularities of the dilaton coupling function is investigated by methods of the qualitative theory of dynamical systems. It is shown that there is a class of solutions, different from solutions of the general theory of relativity, with a constant dilaton. The conditions under which these solutions are an attractor for a general solution with a variable dilaton are determined. Translated from Astrofizika, Vol. 43, No. 1, pp. 123-136, January–March, 2000.     

Accelerating universe with time variation of G and Λ

We study a gravitational model in which scale transformations play the key role in obtaining dynamical G and Λ. We take a non-scale invariant gravitational action with a cosmological constant and a gravitational coupling constant. Then, by a scale transformation, through a dilaton field, we obtain a new action containing cosmological and gravitational coupling terms which are dynamically dependent on the dilaton field with Higgs type potential. The vacuum expectation value of this dilaton field, through spontaneous symmetry breaking on the basis of anthropic principle, determines the time variations of G and Λ. The relevance of these time variations to the current acceleration of the universe, coincidence problem, Mach’s cosmological coincidence and those problems of standard cosmology addressed by inflationary models, are discussed. The current acceleration of the universe is shown to be a result of phase transition from radiation toward matter dominated eras. No real coincidence problem between matter and vacuum energy densities exists in this model and this apparent coincidence together with Mach’s cosmological coincidence are shown to be simple consequences of a new kind of scale factor dependence of the energy momentum density as ρ∼a ⁻⁴. This model also provides the possibility for a super fast expansion of the scale factor at very early universe by introducing exotic type matter like cosmic strings.     

Cosmological Evolution of String Effective Gravitation with a Dilaton Potential and Higher-Loop Corrections. I

Cosmological evolution is investigated within the framework of low-energy string gravitation with higher-loop corrections to the dilaton coupling functions in the presence of a dilaton potential and a nongravitational source. It is shown that for homogeneous and isotropic models with a flat space, the cosmological system of equations reduces to an autonomous, third-order, dynamical system. Subclasses of models with a constant dilaton, which provide the basis for various cosmological mechanisms of dilaton stabilization, are considered. A class of solutions is distinguished with asymptotic scaling behavior of the energy density of the dilaton field.     

On string cosmology with loop corrections

Homogeneous cosmological models are investigated within the framework of low- energy string gravitation with loop corrections. Various conformai representations of the effective action are considered. Without specifying the correction functions in the Lagrangian, cosmological solutions are found with an arbitrary curvature and with dilaton fields, moduli fields, and Kalb- Ramond fields corresponding to a source with an extremely stiff equation of state. They generalize previously known solutions of the tree approximation. The behavior of the solutions in different asymptotic domains is investigated. Translated from Astrofizika, Vol. 41, No. 2, pp. 277–295, April-June, 1998.     

Cosmological model with a conformally coupled scalar field

Modern concepts of the universe support the assumed existence of a nongravitational source, known as dark energy, for which ε + 3 P < 0 (where ε is the energy density and P is the pressure). This ensures accelerated expansion of the universe. This paper examines a tensor-scalar variant of the theory of gravitation with a conformally coupled scalar field. Various cosmological models are examined and the possible evolutionary development of the universe with accelerated expansion is discussed. Translated from Astrofizika, Vol. 51, No. 4, pp. 653–661 (November 2008).     

Effect of <Emphasis Type="Italic">q</Emphasis>-nonextensive distribution of electrons on electron acoustic solitons

Ion acoustic shock waves (IASWs) are studied in a plasma consisting of electrons, positrons and ions. Boltzmann distributed positrons and superthermal electrons are considered in the plasma. The dissipation is taken into account the kinematic viscosity among the plasma constituents. The Korteweg–de Vries–Burgers (KdV–Burgers) equation is derived by reductive perturbation method. Shock waves are solutions of KdV–Burgers equation. It is observed that an increasing positron concentration decreases the amplitude of the waves. Furthermore, in the existence of the kinematic viscosity among the plasma, the shock wave structure appears. The effects of ion kinematic viscosity (η ₀) and the superthermal parameter (k) on the ion acoustic waves are found.     

Interacting Ricci dark energy with logarithmic correction

Motivated by the holographic principle, it has been suggested that the dark energy density may be inversely proportional to the area A of the event horizon of the universe. However, such a model would have a causality problem. In this work, we consider the entropy-corrected version of the holographic dark energy model in the non-flat FRW universe and we propose to replace the future event horizon area with the inverse of the Ricci scalar curvature. We obtain the equation of state (EoS) parameter ω _Λ, the deceleration parameter q and W_D￠\Omega_{D}' in the presence of interaction between Dark Energy (DE) and Dark Matter (DM). Moreover, we reconstruct the potential and the dynamics of the tachyon, K-essence, dilaton and quintessence scalar field models according to the evolutionary behavior of the interacting entropy-corrected holographic dark energy model.     

Ion acoustic solitons and double layers in electron–positron–ion plasmas with dust particulates

Propagation of small but finite amplitude ion acoustic solitons and double layers are investigated in electron–positron–ion plasmas in presence of highly negatively charged impurities or dust. The presence of negatively charged dust particulates can result in existence of two critical concentrations of ion–electron density ratio α. One of them α _D decides the existence of double layers, whereas the other one α _R decides the nature of the solitons and double layers. The system supports both compressive and rarefactive solitons as well as double layers. The parameter regimes of transitions from compressive to rarefactive solitons and double layers are also specified.     

Do Quasi-Regular Structures Really Exist in the Solar Photosphere? I. Observational Evidence

Two series of solar-granulation images – the La Palma series of 5 June 1993 and the SOHO MDI series of 17 – 18 January 1997 – are analysed both qualitatively and quantitatively. New evidence is presented for the existence of long-lived, quasi-regular structures (first reported by Getling and Brandt, Astron. Astrophys. 382, L5 (paper I), 2002), which no longer appear unusual in images averaged over 1 – 2-hour time intervals. Such structures appear as families of light and dark concentric rings or families of light and dark parallel strips (“ridges” and “trenches” in the brightness distributions). In some cases, rings are combined with radial “spokes” and can thus form “web” patterns. The characteristic width of a ridge or trench is somewhat larger than the typical size of granules. Running-average movies constructed from the series of images are used to seek such structures. An algorithm is developed to obtain, for automatically selected centres, the radial distributions of the azimuthally averaged intensity, which highlight the concentric-ring patterns. We also present a time-averaged granulation image processed with a software package intended for the detection of geological structures in aerospace images. A technique of running-average-based correlations between the brightness variations at various points of the granular field is developed and indications are found for a dynamical link between the emergence and sinking of hot and cool parcels of the solar plasma. In particular, such a correlation analysis confirms our suggestion that granules – overheated blobs – may repeatedly emerge on the solar surface. Based on our study, the critical remarks by Rast (Astron. Astrophys. 392, L13, 2002) on the original paper by Getling and Brandt (paper I) can be dismissed.     

Meteoroid ablation models

The fate of entering meteoroids in atmosphere is determined by their size, velocity and substance properties. Material from ablation of small-sized meteors (roughly R≤0.01–1 cm) is mostly deposited between 120 and 80 km altitudes. Larger bodies (up to meter sizes) penetrate deeper into the atmosphere (down to 20 km altitude). Meteoroids of cometary origin typically have higher termination altitude due to substance properties and higher entry velocity. Fast meteoroids (V>30–40 km/s) may lose a part of their material at higher altitudes due to sputtering. Local flow regime realized around the falling body determines the heat transfer and mass loss processes. Classic approach to meteor interaction with atmosphere allows describing two limiting cases: – large meteoroid at relatively low altitude, where shock wave is formed (hydrodynamical models); – small meteoroid/or high altitudes – free molecule regime of interaction, which assumes no collisions between evaporated meteoroid particles. These evaporated particles form initial train, which then spreads into an ambient air due to diffusion. Ablation models should make it possible to describe physical conditions that occur around meteor body. Several self-consistent hydrodynamical models are developed, but similar models for transition and free molecule regimes are still under study. This paper reviews existing ablation models and discusses model boundaries.     

Om diagnostic for dilaton dark energy

Om diagnostic can differentiate between different models of dark energy without the accurate current value of matter density. We apply this geometric diagnostic to dilaton dark energy (DDE) model and differentiate DDE model from LCDM. We also investigate the influence of coupled parameter α on the evolutive behavior of Om with respect to redshift z. According to the numerical result of Om, we get the current value of equation of state ω _σ0=−0.952 which fits the WMAP5+BAO+SN very well.     

Search for Spatial Structures at Scales Z∼ 1. III. The Effect of Lensing on QSO ?

We carried out a search for peak in homogeneities in the distribution of matter – namely clumps and voids, within the range Z ∼ 1-3. We used a new method, based on the lensing of quasars by a combination of lenses, belonging to the above sought in homogeneities in the matter distribution. This work confirms the evidence of the existence of in homogeneities found by us earlier – of a clump (superattractor N.1), and of a void(supervoid). Besides, the existence of a new gigantic clump (superattractorN.2) was also discovered at Z ∼ 3. These clumps could well serve ascenters of the Bose-condensation in the early Universe; in particular –as Anselm's arion condensate, which leads to the formation of quasiperiodic structures with a period p ∼ 100-200 Mpc. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evolution of Population II stars

The current knowledge of the evolution of Population II stars, as observed in galactic halos and globular clusters, is outlined. The recent theoretical results provided by an improved physical understanding of the stars are reported, with a particular emphasis upon those stellar evolutionary phases which are the keystones of the interpretation of globular cluster stars. Within the up–to–date theoretical scenario, the luminosity of the turn–off stars and horizontal branch stars, which are of fundamental importance for distance and age determinations, turns out to be fainter and brighter, respectively, in comparison with previous theoretical computations. The predicted absolute magnitude of RR Lyrae stars is consistent with the bright values suggested from the long distance scale (Sandage 1993), but the slope of the relation between and metallicity ([Fe/H]) agrees with that suggested from the short distance scale (see, e.g., Carney et al. 1992, Clementini et al. 1995), at least with [Fe/H]-1.3. As to the globular cluster ages, the new computations provide younger ages ( 10 Gyr–13 Gyr), weakening the conflict with current cosmological estimates. The recent results derived from the fitting of HIPPARCOS parallaxes of field subdwarfs to the fiducial main sequence of globular clusters are consistent with the up–to–date theoretical models. However, the direct determinations of , which are based on the HIPPARCOS proper motions and trigonometric parallaxes of field RR Lyrae stars, give fainter absolute magnitudes than those obtained from the improved evolutionary models. This seems to suggest that the problem of the globular cluster distance and age is still open. Received 6 April 1998     

Effects of the Polarity States of the Heliospheric Magnetic Field and Particle Drifts in Cosmic Radiation

Forbush decrease (FD) events recorded at the ground-based neutron monitors (NMs) during the period 1961 – 1999, have been selected and recovery characteristic of these events have been analyzed. The average profile of FDs observed during different polarity states of the heliosphere is obtained by superposed epoch analysis separately for the periods 1961 – 1969 (A < 0), 1971 – 1979 (A > 0), 1981 – 1989 (A < 0) and 1991 – 1999 (A > 0). Hourly count rate of neutron monitors of different cut-off rigidities have been utilized. The results are compared with model predictions including drifts. No marked difference is observed in the amplitudes of FDs during A < 0 and A > 0. Rigidity spectrum fitted with a power law yields the values of spectral exponent that are closer to values predicted by two-dimensional models including drifts. The recovery rate of FDs varies with the polarity of HMF and the rate is higher (recovery time smaller) during A > 0 than during A < 0 epoch, consistent with the model predictions including the drift effects in the HMF. This difference in recovery time of FDs during A > 0 and A < 0 polarity conditions provides experimental evidence that drift plays an important role in cosmic ray modulation.     

Models of strange stars having a crust

Models of strange quark stars with a crust consisting of atomic nuclei and degenerate electrons, maintained by an electrostatic barrier at the surface of the strange quark matter, are investigated for a realistic range of parameters of the MIT bag model. The density at which neutrons escape from nuclei, ρ = ρ_drip, is taken as the maximum possible boundary density of the crust. Series of strange stars are calculated as a function of central density. Configurations with masses of 1.44 and 1.77 M{ie330-1} and a gravitational redshift Z_s = 0.23, corresponding to the best-known observational data, are investigated. The presence of a crust results in the existence of a minimum mass for strange stars, and also helps to explain the glitch phenomenon of pulsars within the framework of the existence of strange quark matter. Translated from Astrofizika, Vol. 42, No. 3, pp. 439–448, July–September, 1999.