arXiv subjects

T. Harko

Publications and source records attributed to T. Harko.

At least 73 records · Page 4Linked to original sources

arXiv2007

Minimum mass-radius ratio for charged gravitational objects

We rigorously prove that for compact charged general relativistic objects there is a lower bound for the mass-radius ratio. This result follows from the same Buchdahl type inequality for charged objects, which has been extensively used for the proof of the existence of an upper bound for the mass-radius ratio. The effect of the vacuum energy (a cosmological constant) on the minimum mass is also taken into account. Several bounds on the total charge, mass and the vacuum energy for compact charged objects are obtained from the study of the Ricci scalar invariants. The total energy (including the gravitational one) and the stability of the objects with minimum mass-radius ratio is also considered, leading to a representation of the mass and radius of the charged objects with minimum mass-radius ratio in terms of the charge and vacuum energy only.

gr-qc↗

arXiv2007

Dynamical instability of fluid spheres in the presence of a cosmological constant

The equations describing the adiabatic, small radial oscillations of general relativistic stars are generalized to include the effects of a cosmological constant. The generalized eigenvalue equation for the normal modes is used to study the changes in the stability of the homogeneous sphere induced by the presence of the cosmological constant. The variation of the critical adiabatic index as a function of the central pressure is studied numerically for different trial functions. The presence of a large cosmological constant significantly increases the value of the critical adiabatic index. The dynamical stability condition of the homogeneous star in the Schwarzschild-de Sitter geometry is obtained and several bounds on the maximum allowable value for a cosmological constant are derived from stability considerations.

gr-qc↗

arXiv2007

Dark energy as a massive vector field

We propose that the Universe is filled with a massive vector field, non-minimally coupled to gravitation. The field equations of the model are consistently derived and their application to cosmology is considered. The Friedmann equations acquire an extra dark-energy component, which is proportional to the mass of the vector particle. This leads to a late-time accelerated de Sitter type expansion. The free parameters of the model (gravitational coupling constants and initial value of the cosmological vector field) can be estimated by using the PPN solar system constraints. The mass of the cosmological massive vector particle, which may represent the main component of the Universe, is of the order of 10^-63 g.

gr-qc↗

arXiv2006

Bounds on the basic physical parameters for anisotropic compact general relativistic objects

We derive upper and lower limits for the basic physical parameters (mass-radius ratio, anisotropy, redshift and total energy) for arbitrary anisotropic general relativistic matter distributions in the presence of a cosmological constant. The values of these quantities are strongly dependent on the value of the anisotropy parameter (the difference between the tangential and radial pressure) at the surface of the star. In the presence of the cosmological constant, a minimum mass configuration with given anisotropy does exist. Anisotropic compact stellar type objects can be much more compact than the isotropic ones, and their radii may be close to their corresponding Schwarzschild radii. Upper bounds for the anisotropy parameter are also obtained from the analysis of the curvature invariants. General restrictions for the redshift and the total energy (including the gravitational contribution) for anisotropic stars are obtained in terms of the anisotropy parameter. Values of the surface redshift parameter greater than two could be the main observational signature for anisotropic stellar type objects.

gr-qc↗

arXiv2006

Electron-positron pair production in the electrosphere of quark stars

We study Schwinger pair creation of charged particles due to the inhomogeneous electric field created by the thin electron layer at the surface of quark stars (the electrosphere). As suggested earlier, due to the low photon emissivity of the quark-gluon plasma and of the electrosphere, electron-positron pair emission could be the main observational signature of quark stars. To obtain the electron-positron pair creation rate we use the tunnelling approach. Explicit expressions for the fermion creation rate per unit time per unit volume are derived, which generalize the classical Schwinger result. The finite size effects in pair production, due to the presence of a boundary (the surface of the quark star), are also considered in the framework of a simple approach. It is shown that the boundary effects induce large quantitative and qualitative deviations of the particle production rate from what one deduces with the Schwinger formula and its generalization for the electric field of the electrosphere. The electron-positron pair emissivity and flux of the electrosphere of quark stars due to pair creation is considered, and the magnitude of the boundary effects for this parameters is estimated. Due to the inhomogeneity of the electric field distribution in the electrosphere and of the presence of the boundary effects, at high temperatures ($T\geq T_{cr}\approx 0.1 $ MeV) we find a lower electron-positron flux as previously estimated. The numerical value of the critical temperature $T_{cr}$ depends on the surface potential of the star. We briefly consider the effect of the magnetic field on the pair creation process and show that the magnetic field can enhance drastically the pair creation rate.

astro-ph↗

arXiv2005

Does the cosmological constant imply the existence of a minimum mass?

We show that in the framework of the classical general relativity the presence of a positive cosmological constant implies the existence of a minimal mass and of a minimal density in nature. These results rigorously follow from the generalized Buchdahl inequality in the presence of a cosmological constant.

gr-qc↗

arXiv2005

Galactic metric, dark radiation, dark pressure and gravitational lensing in brane world models

In the braneworld scenario, the four dimensional effective Einstein equation has extra terms which arise from the embedding of the 3-brane in the bulk. These non-local effects, generated by the free gravitational field of the bulk, may provide an explanation for the dynamics of the neutral hydrogen clouds at large distances from the galactic center, which is usually explained by postulating the existence of the dark matter. We obtain the exact galactic metric, the dark radiation and the dark pressure in the flat rotation curves region in the brane world scenario. Due to the presence of the bulk effects, the flat rotation curves could extend several hundred kpc. The limiting radius for which bulk effects are important is estimated and compared with the numerical values of the truncation parameter of the dark matter halos, obtained from weak lensing observations. There is a relatively good agreement between the predictions of the model and observations. The deflection of photons is also considered and the bending angle of light is computed. The bending angle predicted by the brane world models is much larger than that predicted by standard general relativistic and dark matter models. The angular radii of the Einstein rings are obtained in the small angles approximation. The predictions of the brane world model for the tangential shear are compared with the observational data obtained in the weak lensing of galaxies in the Red-Sequence Cluster Survey. Therefore the study of the light deflection by galaxies and the gravitational lensing could discriminate between the different dynamical laws proposed to model the motion of particles at the galactic level and the standard dark matter models.

astro-ph↗

arXiv2005

Chaplygin gas dominated anisotropic brane world cosmological models

We present exact solutions of the gravitational field equations in the generalized Randall-Sundrum model for an anisotropic brane with Bianchi type I geometry, with a generalized Chaplygin gas as matter source. The generalized Chaplygin gas, which interpolates between a high density relativistic era and a non-relativistic matter phase, is a popular dark energy candidate. For a Bianchi type I space-time brane filled with a cosmological fluid obeying the generalized Chaplygin equation of state the general solution of the gravitational field equations can be expressed in an exact parametric form, with the comoving volume taken as parameter. In the limiting cases of a stiff cosmological fluid, with pressure equal to the energy density, and for a pressureless fluid, the solution of the field equations can be expressed in an exact analytical form. The evolution of the scalar field associated to the Chaplygin fluid is also considered and the corresponding potential is obtained. The behavior of the observationally important parameters like shear, anisotropy and deceleration parameter is considered in detail.

gr-qc↗

arXiv2005

Radiation transport equations in non-Riemannian space-times

The transport equations for polarized radiation transfer in non-Riemannian, Weyl-Cartan type space-times are derived, with the effects of both torsion and non-metricity included. To obtain the basic propagation equations we use the tangent bundle approach. The equations describing the time evolution of the Stokes parameters, of the photon distribution function and of the total polarization degree can be formulated as a system of coupled first order partial differential equations. As an application of our results we consider the propagation of the cosmological gamma ray bursts in spatially homogeneous and isotropic spaces with torsion and non-metricity. For this case the exact general solution of the equation for the polarization degree is obtained, with the effects of the torsion and non-metricity included. The presence of a non-Riemannian geometrical background in which the electromagnetic fields couple to torsion and/or non-metricity affect the polarization of photon beams. Consequently, we suggest that the observed polarization of prompt cosmological gamma ray bursts and of their optical afterglows may have a propagation effect component, due to a torsion/non-metricity induced birefringence of the vacuum. A cosmological redshift and frequency dependence of the polarization degree of gamma ray bursts also follows from the model, thus providing a clear observational signature of the torsional/non-metric effects. On the other hand, observations of the polarization of the gamma ray bursts can impose strong constraints on the torsion and non-metricity and discriminate between different theoretical models.

gr-qc↗

arXiv2005

Relativistic Compact Objects in Isotropic Coordinates

We present a matrix method for obtaining new classes of exact solutions for Einstein's equations representing static perfect fluid spheres. By means of a matrix transformation, we reduce Einstein's equations to two independent Riccati type differential equations for which three classes of solutions are obtained. One class of the solutions corresponding to the linear barotropic type fluid with an equation of state $p=γρ$ is discussed in detail.

gr-qc↗

arXiv2005

Conformally symmetric vacuum solutions of the gravitational field equations in the brane-world models

A class of exact solutions of the gravitational field equations in the vacuum on the brane are obtained by assuming the existence of a conformal Killing vector field, with non-static and non-central symmetry. In this case the general solution of the field equations can be obtained in a parametric form in terms of the Bessel functions. The behavior of the basic physical parameters describing the non-local effects generated by the gravitational field of the bulk (dark radiation and dark pressure) is also considered in detail, and the equation of state satisfied at infinity by these quantities is derived. As a physical application of the obtained solutions we consider the behavior of the angular velocity of a test particle moving in a stable circular orbit. The tangential velocity of the particle is a monotonically increasing function of the radial distance and, in the limit of large values of the radial coordinate, tends to a constant value, which is independent on the parameters describing the model. Therefore a brane geometry admitting a one-parameter group of conformal motions may provide an explanation for the dynamics of the neutral hydrogen clouds at large distances from the galactic center, which is usually explained by postulating the existence of the dark matter.

gr-qc↗

arXiv2004

Photon emissivity of the electrosphere of bare strange stars

We consider the spectrum, emissivity and flux of the electromagnetic radiation emitted by the thin electron layer (the electrosphere) at the surface of a bare strange star. In particular, we carefully consider the effect of the multiple and uncorrelated scattering on the radiation spectrum (the Landau-Pomeranchuk-Migdal effect), together with the effect of the strong electric field at the surface of the star. The presence of the electric field strongly influences the radiation spectrum emitted by the electrosphere. All the radiation properties of the electrons in the electrosphere essentially depend on the value of the electric potential at the quark star surface. The effect of the multiple scattering, which strongly suppresses radiation emission, is important only for the dense layer of the electrosphere situated near the star's surface and only for high values of the surface electric potential of the star. Hence a typical bremsstrahlung radiation spectrum, which could extend to very low frequencies, could be one of the main observational signatures even for low temperature quark stars.

astro-ph↗

arXiv2004

Structure of the electrospheres of bare strange stars

We consider a thin ($\sim 10^2-10^3$ fm) layer of electrons (the electrosphere) at the quark surface of a bare strange star, taking into account the surface effects at the boundary with the vacuum. The quark surface holds the electron layer by an extremely strong electric field, generated in the electrosphere to prevent the electrons from escaping to infinity by counterbalancing the degeneracy and thermal pressure. Because of the surface tension and depletion of $s$ quarks a very thin (a few fm) charged layer of quarks forms at the surface of the star. The formation of this layer modifies the structure of the electrosphere, by significantly changing the electric field and the density of the electrons, in comparison with the case when the surface effects are ignored. Some consequences of the modification of the electrosphere structure on the properties of strange stars are briefly discussed.

astro-ph↗

arXiv2004

Constraints on extra-dimensions and variable constants from cosmological gamma ray bursts

The observation of the time delay between the soft emission and the high-energy radiation from cosmological gamma ray bursts can be used as an important observational test of multi-dimensional physical theories. The main source of the time delay is the variation of the electromagnetic coupling, due to dimensional reduction, which induces an energy dependence of the speed of light. For photons with energies around 1 TeV, the time delay could range from a few seconds in the case of Kaluza-Klein models to a few days for models with large extra-dimensions. Based on these results we suggest that the detection of the 18-GeV photon $ \sim $4500 s after the keV/MeV burst in GRB 940217 provides a strong evidence for the existence of extra-dimensions. The time delay of photons, if observed by the next generation of high energy detectors, like, for example, the SWIFT and GLAST satellite based detectors, or the VERITAS ground-based TeV gamma-ray instrument, could differentiate between the different models with extra-dimensions.

astro-ph↗

arXiv2004

Is there any evidence for extra-dimensions or quantum gravity effects from the delayed MeV-GeV photons in GRB 940217?

The discovery of X-ray afterglows of GRBs, and the identification of host galaxies of GRBs, confirm the cosmological origin of GRBs. However, the discovery of the delayed MeV-GeV photons in GRB940217 imposes serious challenges for the standard emission model of GRB. Although the delayed MeV-GeV photons might be explained by some radiation emission mechanisms, the mystery of detecting an 18 GeV photon still remains unsolved. We suggest that the detection of the 18-GeV photon $ \sim $4500 s after the keV/MeV burst in GRB 940217 provides a strong evidence for the existence of extra-dimensions and/or quantum gravity effects. The delay scale of the 18-GeV photon leads to an estimation of the fundamental energy scale, associated with the linear energy dependence of the speed of light, of the order of $2.1\times 10^{15}$ GeV, which is consistent with the results obtained by another independent analysis on the data of OSSE and BATSE.

astro-ph↗

arXiv2004

Time delay of photons of different energies in multi-dimensional cosmological models

We consider the general expressions for the time delay of photons of different energies in the framework of multi-dimensional cosmological models. In models with compactified extra-dimensions (Kaluza-Klein type models), the main source of the photon time delay is the time variation of the electromagnetic coupling, due to dimensional reduction, which induces an energy-dependence of the speed of light. A similar relation between the fine structure constant and the multi-dimensional gauge couplings also appears in models with large (non-compactified) extra-dimensions. For photons of energies around 1 TeV, propagating on cosmological distances in an expanding Universe, the time delay could range from a few seconds in the case of Kaluza-Klein models to a few days for models with large extra-dimensions. As a consequence of the multi-dimensional effects, the intrinsic time profiles at the emitter rest frame differ from the detected time profiles. The formalism developed in the present paper allows the transformation of the predicted light curves of various energy ranges of the emitter into the frame of the observer, for comparison with observations. Therefore the study of energy and redshift dependence of the time delay of photons, emitted by astrophysical sources at cosmological distances, could discriminate between the different multi-dimensional models and/or quantum gravity effects.

astro-ph↗

arXiv2004

Can the galactic rotation curves be explained in brane world models?

We consider solutions with conformal symmetry of the static, spherically symmetric gravitational field equations in the vacuum in the brane world scenario. By assuming that the vector field generating the symmetry is non-static, the general solution of the field equations on the brane can be obtained in an exact parametric form, with the conformal factor taken as parameter. As a physical application of the obtained solutions we consider the behavior of the angular velocity of a test particle moving in a stable circular orbit. In this case the tangential velocity can be expressed as a function of the conformal factor and some integration constants only. For a specific range of the integration constants, the tangential velocity of the test particle tends, in the limit of large radial distances, to a constant value. This behavior is specific to the galactic rotation curves, and is explained usually by invoking the hypothesis of the dark matter. The limiting value of the angular velocity of the test particle can be obtained as a function of the baryonic mass and radius of the galaxy. The behavior of the dark radiation and dark pressure terms is also considered in detail, and it is shown that they can be expressed in terms of the rotational velocity of a test particle. Hence all the predictions of the present model can be tested observationally. Therefore the existence of the non-local effects, generated by the free gravitational field of the bulk in a conformally symmetric brane, may provide an explanation for the dynamics of the neutral hydrogen clouds at large distances from the galactic center.

gr-qc↗

arXiv2004

Nucleation of quark matter in neutron stars cores

We consider the general conditions of quark droplets formation in high density neutron matter. The growth of the quark bubble (assumed to contain a sufficiently large number of particles) can be described by means of a Fokker-Planck equation. The dynamics of the nucleation essentially depends on the physical properties of the medium it takes place. The conditions for quark bubble formation are analyzed within the frameworks of both dissipative and non-dissipative (with zero bulk and shear viscosity coefficients) approaches. The conversion time of the neutron star to a quark star is obtained as a function of the equation of state of the neutron matter and of the microscopic parameters of the quark nuclei. As an application of the obtained formalism we analyze the first order phase transition from neutron matter to quark matter in rapidly rotating neutron stars cores, triggered by the gravitational energy released during the spinning down of the neutron star. The endothermic conversion process, via gravitational energy absorption, could take place, in a very short time interval, of the order of few tens seconds, in a class of dense compact objects, with very high magnetic fields, called magnetars.

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