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A brief review of the recent astronomical data, indicating that the universe is abundantly populated by heavy black holes (BH), is presented. Conventional astrophysics and cosmology cannot explain such a high population of BHs. A mechanism of the paper of 1963 is described, which at least qualitatively explained the observational data. In particular, the prediction that massive primordial BHs can be cosmological dark matter "particles" is discussed. [ABSTRACT FROM AUTHOR]

This paper deals with the cosmological study of the Bianchi type I model made of dust fluid in the background of dark energy. Cosmological solutions are obtained for the following four cases: (i) only with dark matter; (ii) only with dark energy; (iii) both dark matter and dark energy are present but without interaction; (iv) interactive dark matter and dark energy. [ABSTRACT FROM AUTHOR]

A comparative study of various parametrizations of the dark energy equation of state is made. Astrophysical constraints from LSS, CMB and BBN are laid down to test the physical viability and cosmological compatibility of these parametrizations. A critical evaluation of the four-index parametrizations reveals that Hannestad–Mörtsell as well as Lee parametrizations are simple and transparent in probing the evolution of the dark energy during the expansion history of the universe and they satisfy the LSS, CMB and BBN constraints on the dark energy density parameter. [ABSTRACT FROM AUTHOR]

Introducing non-commutative variables in the minisuperspace, we obtain the non-commutative quantum solutions to the Wheeler–DeWitt equation with an arbitrary factor ordering, for the anisotropic Bianchi type I cosmological model, coupled to barotropic matter and cosmological term Λ. From the probability density, the inclusion of non-commutativity seems to retard the isotropization of the universe. [ABSTRACT FROM AUTHOR]

We discuss the exact solutions of brane universes and the results indicate that the Friedmann equations on the branes are modified with a new density term. Then, we assume the new term as the density of dark energy. Using Wetterich's parametrization equation of state (EOS) of dark energy, we obtain that the new term varies with the redshift z. Finally, the evolutions of the mass density parameter Ω2, dark energy density parameter Ωx and deceleration parameter q2 are studied. [ABSTRACT FROM AUTHOR]

A new exact solution of the coupled Einstein–Maxwell equations is given and studied. It is found using the soliton method, adding one soliton to the Schwarzschild background. The solution is stationary and axial-symmetric, and has five physical parameters. The physical interpretation we give is that it describes a Kerr–Newman (KN) naked singularity linked by a "strut" to a charged black hole. Indeed, on the axis, between the two bodies an unavoidable anomaly region is present (gφφ < 0 and a conic singularity). The solution is stationary also in the case with zero total angular momentum. Finally, we give the force lines of the electrical field in a general case, and in the case in which the KN singularity has a much smaller mass than the nearby black hole; we also considered the behavior at different distances of the charge. In spite of the naive interpretation suggested by the mathematical construction of the solution, what we expected to be a "Schwarzschild" black hole appears to be charged and rotating; we interpret this fact as a parameter-mixing phenomenon. [ABSTRACT FROM AUTHOR]

We compute the quantum string entropy Ss(m, H) from the microscopic string density of states ρs(m, H) of mass m in de Sitter space–time. We find for high m (high Hm → c/α') a new phase transition at the critical string temperature Ts = (1/2πkB)Lcl c2/α', higher than the flat space (Hagedorn) temperature ts (Lcl = c/H, the Hubble constant H acts at the transition, producing a smaller string constant α' and thus, a higher tension). Ts is the precise quantum dual of the semiclassical (QFT Hawking–Gibbons) de Sitter temperature Tsem = ħ c/(2πkBLcl). By precisely identifying the semiclassical and quantum (string) de Sitter regimes, we find a new formula for the full de Sitter entropy Ssem(H), as a function of the usual Bekenstein–Hawking entropy $S_{\rm sem}^{(0)}(H)$. For Lcl ≫ ℓPlanck, i.e. for low $H\ll c/\ell_{\rm Planck}, S_{\rm sem}^{(0)}(H)$ is the leading term, but for high H near c/ℓPlanck, a new phase transition operates and the whole entropy Ssem (H) is drastically different from the Bekenstein–Hawking entropy $S_{\rm sem}^{(0)}(H)$. We compute the string quantum emission cross-section σstring by a black hole in de Sitter (or asymptotically de Sitter) space–time (bhdS). For Tsem bhdS ℓ Ts (early evaporation stage), it shows the QFT Hawking emission with temperature Tsem bhdS (semiclassical regime). For Tsem bhdS → Ts, σstring exhibits a phase transition into a string de Sitter state of size $L_{\rm s} = \ell_{\rm s}^{2}/L_{\rm cl}$, $(\ell_{\rm s}= \sqrt{\hbar \alpha'/c})$, and string de Sitter temperature Ts. Instead of featuring a single pole singularity in the temperature (Carlitz transition), it features a square root branch point (de Vega–Sanchez transition). New bounds on the black hole radius rg emerge in the bhdS string regime: it can become rg = Ls/2, or it can reach a more quantum value, rg = 0.365 ℓs. [ABSTRACT FROM AUTHOR]

We consider the anomaly induced effective action in $\mathcal{N} = 4$ super Yang–Mills theory in interaction with the Brans–Dicke (BD) field. The generalization of the BD theory so as to permit an energy exchange between the scalar field and ordinary matter fields, was recently worked out by T. Clifton and J. D. Barrow [Phys. Rev. D73, 104022 (2006)]. We derive the scalar field equations for the dilaton field, and the BD field, and discuss the Friedmann equation in the general case. The present paper is a continuation of an investigation few years ago dealing with the case of conformal anomaly plus ordinary classical gravity [I. Brevik and S. D. Odintsov, Phys. Lett. B455, 104 (1999)]. [ABSTRACT FROM AUTHOR]

We study the Non-Linear Born–Infeld (NLBI) scalar field model and quintessence model with two different potentials (V(ϕ) = -sϕ and $\frac{1}{2}m^2 \phi^2$). We investigate the differences between these two models. We explore the equation of state parameter w and the evolution of scale factor a(t) in both the NLBI scalar field and quintessence model. The present age of universe and the transition redshift are also obtained. We use the Gold dataset of 157 SN-Ia to constrain the parameters of the two models. All the results show that the NLBI model is slightly superior to the quintessence model. [ABSTRACT FROM AUTHOR]

We study the effects of the gapless color–flavor locked (gCFL) phase on the equation of state of strongly interacting matter in the range of baryonic chemical potential involved in a compact star. We analyze the possibility of a phase transition from hadronic matter to gCFL quark matter and we discuss, for different values of the strange quark mass and diquark coupling strength, the existence of a gCFL phase in quark or hybrid stars. The mass–radius relation and the structure of compact stars containing the gCFL phase are shown and the physical relevance of this superconducting phase inside a stellar object is also discussed. [ABSTRACT FROM AUTHOR]

In this paper, we study the cosmological dynamics of dilatonic dark energy model and its phantom model — with negative kinetic energy. When the potential is taken as the form $[1+A\sin(B\sigma)]e^{-\sqrt{8\pi G} \sigma}$, we investigate the existence of a late time attractor solution, and find out the sufficient condition. One interesting feature found by us is that the evolutions of components of comic density are locally fluctuating on the way to the late time attractor. But this local fluctuation cannot hold the trend that the equation of state ω evolves to -1 and the cosmic density parameter Ωσ evolves to 1, which are important features for a dark energy model that can meet the current observations. [ABSTRACT FROM AUTHOR]

We study a set of static solutions of the Einstein equations in the presence of a massless scalar field and establish their connection to the Kantowski–Sachs cosmological solutions based on some kind of duality transformations. The physical properties of the limiting case of an empty hyperbolic space–time (pseudo-Schwarzschild geometry) are analyzed in some detail. [ABSTRACT FROM AUTHOR]

We study the problem of all-sky search in reference to a continuous gravitational wave (CGW) whose wave-form is known in advance. We employ the concept of fitting factor and study the variation in the bank of search templates with different Earth azimuth at t = 0. We found that the number of search templates varies significantly. Hence, accordingly, the computational demand for the search may be reduced up to two orders by time shifting the data. [ABSTRACT FROM AUTHOR]

We consider a nonlinear sigma model coupled to the metric of a conic space. We obtain restrictions for a nonlinear sigma model to be a source of the conic space. We then study a nonlinear sigma model in the conic space background. We find coordinate transformations which reduce the chiral fields equations in the conic space background to field equations in Minkowski space–time. This enables us to apply the same methods for obtaining exact solutions in Minkowski space–time to the case of a conic space–time. In the case the solutions depend on two spatial coordinates we employ Ivanov's geometrical ansatz. We give a general analysis and also present classes of solutions in which there is dependence on three and four coordinates. We discuss with special attention the intermediate, instanton and meron solutions and their analogous in the conic space. We find differences in the total actions and topological charges of these solutions and discuss the role of the deficit angle. [ABSTRACT FROM AUTHOR]

Limits are imposed upon the possible rate of change of extra spatial dimensions in a decrumpling model Universe with time variable spatial dimensions (TVSD) by considering the time variation of (1+3)-dimensional Newton's constant. Previous studies on the time variation of (1+3)-dimensional Newton's constant in TVSD theory had not include the effects of the volume of the extra dimensions and the effects of the surface area of the unit sphere in D-space dimensions. Our main result is that the absolute value of the present rate of change of spatial dimensions to be less than about 10-14 yr-1. Our results would appear to provide a prima facie case for ruling the TVSD model out. We show that based on observational bounds on the present variation of Newton's constant, one would have to conclude that the spatial dimension of the Universe when the Universe was "at the Planck scale" to be less than or equal to 3.09. If the dimension of space when the Universe was "at the Planck scale" is constrained to be fractional and very close to 3, then the whole edifice of TVSD model loses credibility. [ABSTRACT FROM AUTHOR]

It is argued that the accretion of phantom energy onto a wormhole in a universe with a positive cosmological constant leads also to a gradual increase of the wormhole throat radius, which eventually overtakes the super-accelerated expansion of the universe and becomes infinite at a time in the future before the big rip. Thus, the universe as a whole can time travel toward the past or the future. [ABSTRACT FROM AUTHOR]

We consider a five-dimensional Ricci flat Bouncing cosmology and assume that the four-dimensional universe is permeated smoothly by three minimally coupled matter components: CDM + baryons ρm, radiation ρr and dark energy ρx. Evolutions of these three components are studied and it is found that dark energy dominates before the bounce, and pulls the universe contracting. In this process, dark energy decreases while radiation and the matter increases. After the bounce, the radiation and matter dominates alternatively and then decreases with the expansion of the universe. At present, the dark energy dominates again and pushes the universe accelerating. In this model, we also obtain that the equation of state (EOS) of dark energy at present time is wx0≈-1.05 and the redshift of the transition from decelerated expansion to accelerated expansion is zT≈0.37, which are compatible with the current observations. [ABSTRACT FROM AUTHOR]

The Cosmic no hair theorem is studied in anisotropic Bianchi brane models which admit power law inflation with a scalar field. We note that all Bianchi models except Bianchi type IX transit to an inflationary regime and the anisotropy washes out at a later epoch. It is found that in the brane world, the anisotropic universe approaches the isotropic phase via inflation much faster than that in the general theory of relativity. The modification in the Einstein field equations on the brane is helpful for a quick transition to an isotropic era from the anisotropic brane. We note a case where the curvature term in the field equation initially drives power law inflation on the isotropic brane which is however not permitted without the brane framework. [ABSTRACT FROM AUTHOR]

The type Ia supernovae (SNeIa) observational data are used to estimate the parameters of a cosmological model with cold dark matter and the generalized Chaplygin gas model (GCGM). The GCGM depends essentially on five parameters: the Hubble constant, the parameter $\bar A$ related to the velocity of the sound, the equation of state parameter α, the curvature of the Universe and the fraction density of the generalized Chaplygin gas (or the cold dark matter). The parameter α is allowed to take negative values and to be greater than one. The Bayesian parameter estimation yields $\alpha = - 0.86^{+6.01}_{-0.15}$, $H_0 = 62.0^{+1.32}_{-1.42} \ {\rm km/Mpc.s}$, $\Omega_{k0} = -0.74_{-1.32}^{+1.42}$, $\Omega_{m0} = 0.00^{+0.86}_{-0.00}$, $\Omega_{c0} = 1.39^{+1.21}_{-1.25}$, $\bar A =1.00^{+0.00}_{-0.39}$, $t_0 = 15.3^{+4.2}_{-3.2}$ and $q_0 = -0.80^{+0.86}_{-0.62}$, where t0 is the age of the Universe and q0 is the value of the deceleration parameter today. Our results indicate that a Universe completely dominated by the generalized Chaplygin gas is favored, which reinforces the idea that the this gas may unify the description for dark matter and dark energy, at least as far as the SNeIa data is concerned. A closed and accelerating Universe is also favored. The traditional Chaplygin gas model (CGM), α = 1 is not ruled out, even if it does not give the best-fitting. Particular cases with four or three independent free parameters are also analyzed. [ABSTRACT FROM AUTHOR]

Vacuum and dark energy energy problems are reviewed. Cosmology with non-zero vacuum energy is discussed. The astronomical data which indicate that the universe is filled with an anti-gravitating state of matter are described. The mechanisms which may lead to cancellation of almost infinite vacuum energy down to the astronomically observed value are enumerated with an emphasis to dynamical adjustment. [ABSTRACT FROM AUTHOR]

Entropy production by a dilaton decay is studied in the model where the dilaton acquires potential via gaugino condensation in the hidden gauge group. Its effect on the Affleck-Dine baryogenesis is investigated. It is shown that the baryon asymmetry produced by this mechanism with the higher-dimensional terms is diluted by the dilaton decay and can be regulated to the observed value. [ABSTRACT FROM AUTHOR]

We study the consequences of the existence of spacelike conformal Killing vectors (SpCKV) parallel to x[SUPa] for cosmic strings and string fluid in the context of general relativity. The inheritance symmetries of the cosmic strings and string fluid are discussed in the case of SpCKV. Furthermore we examine proper homothetic spacelike Killing vectors for the cosmic strings and string fluid. [ABSTRACT FROM AUTHOR]

In this paper we have revisited the research work of Rahman and Bera[SUP22] on Kaluza-Klein cosmological model within the framework of Lyra Geometry. It has been shown that the empty universe model yields a power law relation without any assumption. The role of bulk viscosity on five-dimensional cosmological model is discussed. The physical behaviour of the models is examined in all cases. [ABSTRACT FROM AUTHOR]

In recent years it is generally believed that we should consider positive vacuum energy density or cosmological constant. Also as higher dimensional theory is important at the early stages of the Universe, so it will be interesting to study classical tests of cosmology in a higher dimensional generalized Kantowski-Sachs model. For matter field, we consider dust and a cosmological constant and examine which are physically permissible. [ABSTRACT FROM AUTHOR]

We have examined cosmic no-hair theorem for anisotropic Bianchi-models in Einstein-Cartan theory and it is possible to have inflationary solution with a scalar field. It has been noted that during the inflationary era, the evolution of the Universe does not depend on the form of the potential but the late time behaviour is dependent on the potential for the inflation field. [ABSTRACT FROM AUTHOR]

A plane-symmetric non-static cosmological model representing a bulk viscous fluid distribution has been obtained which is inhomogeneous and anisotropic and a particular case of which is gravitationally radiative. Without assuming any ad hoc law, we obtain a cosmological constant as a decreasing function of time. The physical and geometric features of the models are also discussed. [ABSTRACT FROM AUTHOR]

We study the evolution of a flat Friedmann-Robertson{Walker Universe, filled with a bulk viscous cosmological fluid, in the presence of time varying "constants." The dimensional analysis of the model suggests a proportionality between the bulk viscous pressure of the dissipative fluid and the energy density. Using this assumption and with the choice of the standard equations of state for the bulk viscosity coefficient, temperature and relaxation time, the general solution of the field equations can be obtained, with all physical parameters having a power-law time dependence. The symmetry analysis of this model, performed using Lie group techniques, confirms the uniqueness of the solution for this functional form of the bulk viscous pressure. In order to fin another possible solution we relax the hypotheses and assume a concrete functional dependence for the "constants." [ABSTRACT FROM AUTHOR]

On the mass neutrino phase calculations along both the particle geodesic line and the photon null line, there exists a double counting, a factor of 2, when comparing the geodesic phase with the null phase. Moreover, we compare the phase calculations with the same-energy description and the same-momentum description by means of the Minkowski diagram, and obtain the practical equivalence of these two descriptions. On the same-velocity description, although not corresponding any real physical process, we still indicate its phase calculation in the Minkowski diagram, which has the same result as those of same-energy and same-momentum cases. Furthermore, in the curved space-time, we also prove the existence of the double counting of the geodesic phase to the null phase. Our conclusions are the same as others' results by the different methods. [ABSTRACT FROM AUTHOR]

Predictions of the CMB spectrum from a bimetric gravity theory (BGT)[SUP1] are presented. The initial inflationary period in BGT is driven by a vanishingly small speed of gravitational waves v[SUBg] in the very early universe. This initial inflationary period is insensitive to the choice of scalar field potential and initial values of the scalar field. After this initial period of inflation, v[SUBg] will increase rapidly and the effects of a potential will become important. We show that a quadratic potential introduced into BGT yields an approximately at spectrum with inflation parameters: n[SUBs] = 0.98, n[SUBt] = -0.027, α[SUBs] = -3.2 × 10[SUP-4] and α[SUBt] = -5.0 × 10[SUP-4], with r ≥ 0.014. [ABSTRACT FROM AUTHOR]

In this paper we discuss once more the zero mode contribution to the vacuum energy density. We show that a careful treatment of the zero modes leads to the conclusion that domain walls may be ferromagnetic, and could generate a magnetic field of cosmological interest. [ABSTRACT FROM AUTHOR]

The cosmological problem is studied here starting from a metric with a variable scale factor for time and two different ones for space, noted R for ordinary space and S for the extra dimensions. Once R has been obtained from physical principles, one is left for S with a differential equation which can be solved through Lie group analysis and dynamical systems theory, and a frequent use of computer algebra has been made in the ensuing calculations. This leads to a good agreement with the recent observational results relating to distant supernovae. However, the solution obtained for S is particularly simple if spacetime has a dimension at least equal to ten, a result to be compared with those given by superstring or M theories. [ABSTRACT FROM AUTHOR]

We report on the study of the mass-radius (M - R) relation and the radial oscillations of proto strange stars. For the quark matter we have employed the very recent modification, the temperature and density dependent quark mass model of the well known density dependent quark mass model. We find that the maximum mass the star can support increases significantly with the temperature of the star in this model which implies that transition to a black hole at the early stage of formation of the star is inhibited. As for the effect of the neutrinos, we find, contrary to expectation, that the values of mass, radius and oscillation frequencies are almost independent of the neutrino chemical potentials. [ABSTRACT FROM AUTHOR]