arXiv subjects

A. A. Saharian

Publications and source records attributed to A. A. Saharian.

At least 37 records · Page 2Linked to original sources

arXiv2024

Surface Casimir densities on branes orthogonal to the boundary of anti-de Sitter spacetime

We investigate the vacuum expectation value of the surface energy-momentum tensor (SEMT) for a scalar field with general curvature coupling in the geometry of two branes orthogonal to the boundary of anti-de Sitter (AdS) spacetime. For Robin boundary conditions on the branes, the SEMT is decomposed into the contributions corresponding to the self-energies of the branes and the parts induced by the presence of the second brane. The renormalization is required for the first parts only and for the corresponding regularization the generalized zeta function method is employed. The induced SEMT is finite and is free from renormalization umbiguities. For an observer living on the brane, the corresponding equation of state is of the cosmological constant type. Depending on the boundary conditions and on the separation between the branes, the surface energy densities can be either positive or negative. The energy density induced on the brane vanishes in special cases of Dirichlet and Neumann boundary conditions on that brane. The effect of gravity on the induced SEMT is essential at separations between the branes of the order or larger than the curvature radius for AdS spacetime. In the large separation limit the decay of the SEMT, as a function of the proper separation, follows a power law for both massless and massive fields. For parallel plates in Minkowski bulk and for massive fields the fall-off of the corresponding expectation value is exponential.

hep-th↗

arXiv2024

Vacuum currents for a scalar field in models with compact dimensions

This paper reviews the investigations on the vacuum expectation value of the current density for a charged scalar field in spacetimes with toroidally compactified spatial dimensions. As background geometries locally Minkowskian (LM), locally de Sitter (LdS) and locally anti-de Sitter (LAdS) spacetimes are considered. Along compact dimensions quasiperiodicity conditions are imposed on the field operator and the presence of a constant gauge field is assumed. The vacuum current has non-zero components only along compact dimensions. Those components are periodic functions of the magnetic flux enclosed by compact dimensions with the period equal to the flux quantum. For LdS and LAdS geometries and for small values of the length of a compact dimension, compared with the curvature radius, the leading term in the expansion of the the vacuum current along that dimension coincides with that for LM bulk. In this limit the dominant contribution to the mode sum for the current density comes from the vacuum fluctuations with wavelength smaller than the curvature radius and the influence of the gravitational field is weak. The effects of the gravitational field are essential for lengths of compact dimensions larger than the curvature radius. In particular, instead of the exponential suppression of the current density in LM bulk one can have power law decay in LdS and LAdS spacetimes.

hep-th↗

arXiv2023

Fermionic condensate and the mean energy-momentum tensor in the Fulling-Rindler vacuum

We investigate the properties of the fermionic Fulling-Rindler vacuum for a massive Dirac field in a general number of spatial dimensions. As important local characteristics, the fermionic condensate and the expectation value of the energy-momentum tensor are evaluated. The renormalization is reduced to the subtraction of the corresponding expectation values for the Minkowski vacuum. It is shown that the fermion condensate vanishes for a massless field and is negative for nonzero mass. Unlike the case of scalar fields, the fermionic vacuum stresses are isotropic for general case of massive fields. The energy density and the pressures are negative. For a massless field the corresponding spectral distributions exhibit thermal properties with the standard Unruh temperature. However, the density-of-states factor is not Planckian for general number of spatial dimensions. Another interesting feature is that the thermal distribution is of the Bose-Einstein type in even number of spatial dimensions. This feature has been observed previously in the response of a particle detector uniformly accelerating through the Minkowski vacuum. In an even number of space dimensions the fermion condensate and the mean energy-momentum tensor coincide for the fields realizing two inequivalent irreducible representations of the Clifford algebra. In the massless case, we consider also the vacuum energy-momentum tensor for Dirac fields in the conformal vacuum of the Milne universe, in static open universe and in the hyperbolic vacuum of de Sitter spacetime.

hep-th↗

arXiv2023

Surface polariton excitation and energy losses by a charged particle in cylindrical waveguides

We investigate the emission of surface polaritons (SPs) by a charged particle moving inside a dielectric cylinder parallel to its axis. It is assumed that the cylinder is immersed into a homogeneous medium with negative dielectric permittivity in the spectral range under consideration (active medium). The expressions for scalar and vector potentials and for electromagnetic field strengths are provided inside and outside the cylinder. Those fields are expressed in terms of the SP eigenmodes of the waveguide and we give detailed analysis for their properties. The SP energy fluxes through the plane perpendicular to the cylinder axis are evaluated in the interior and exterior media. The energy flux is directed towards the charge motion inside the cylinder and towards the opposite direction in the exterior region. The relativistic effects may essentially increase the radiated energy. Important features of relativistic effects include the possibility of essential increase of the radiated energy, the narrowing the confinement region of the SP fields near the cylinder surface in the exterior region, enlarging the frequency range for radiated SPs, and the decrease of the cutoff factor for radiation at small wavelengths compared with the waveguide radius. The general results are specified for the Drude dispersion in the exterior medium. By using the Green tensor we also evaluate the total energy losses of the charged particle for general case of the interior and exterior dielectric functions. The corresponding results are compared with those previously discussed in the literature. The numerical data are presented in terms of scale invariant quantities that allows to clarify the features of the SP radiation for different values of the waveguide radius.

physics.optics↗

arXiv2023

Electromagnetic vacuum stresses and energy fluxes induced by a cosmic string in de Sitter spacetime

For the electromagnetic field in (D+1)-dimensional locally de Sitter (dS) spacetime, we analyze the effects of a generalized cosmic string type defect on the vacuum expectation value of the energy-momentum tensor. For the Bunch-Davies vacuum state, the topological contributions are explicitly extracted in both the diagonal and off-diagonal components. The latter describes the presence of radially directed energy flux in the vacuum state. It vanishes for $D=3$ because of the conformal invariance of the electromagnetic field and is directed towards the cosmic string for $D\geq 4$ . The topological contributions in the vacuum stresses are anisotropic and, unlike to the geometry of a cosmic string in the Minkowski spacetime, for $D>3$ the stresses along the directions parallel to the string core differ from the energy density. Depending on the planar angle deficit and the distance from the cosmic string, the corresponding expectation values can be either positive or negative. Near the cosmic string the effect of the gravitational field on the diagonal components of the topological part is weak. The spacetime curvature essentially modifies the behavior of the topological terms at proper distances from the cosmic string larger than the dS curvature radius. In that region, the topological contributions in the diagonal components of the energy-momentum tensor decay in inverse proportion to the fourth power of the proper distance and the energy flux density behaves as inverse-fifth power for all values of the spatial dimension $D$. The exception is the energy density in the special case $D=4$.

hep-th↗

arXiv2023

Coherent radiation from a chain of charged particles on a circular orbit around a dielectric ball

We investigate the electromagnetic radiation from a chain of relativistic charged particles uniformly rotating along equatorial orbit around a dielectric ball. For weak absorption of radiation in the ball material, at certain rotation frequency the radiation intensity becomes significantly higher than the corresponding value for a chain of charges rotating in free space or in a transparent medium with the same dielectric constant as that for the ball material. In the case of an equidistant distribution of charged particles along the orbit we determine the values of parameters of the problem for which the charges in the chain emit coherently. It is shown that the radiation intensity on a given harmonic increases in proportion to the square of the number of emitting charges. We also show that the relative shifts in the particles locations up to 10\% do not destroy the coherence properties of the radiation. It is demonstrated that the coherence effects may also dominate in the radiation intensity for chains with non-equidistant distributions of particles. The numerical results obtained for different dielectric balls have revealed the emitted radiation to be in the GHz/THz frequency ranges. The high-power radiation of the chain propagates in the angular range with respect to the rotation plane determined by the Cherenkov condition for the velocity of the chain projection onto the ball surface.

physics.acc-ph↗

arXiv2022

Finite temperature charge and current densities around a cosmic string in AdS spacetime with compact dimension

For a massive scalar field with a general curvature coupling parameter, we investigate the finite temperature contributions to the Hadamard function and to the charge and current densities in the geometry of a magnetic flux carrying generalized cosmic string embedded in $(D+1)$-dimensional locally AdS spacetime with a compactified spatial dimension. For $D=4$, the geometry on the AdS boundary, in the context of the AdS/CFT duality, corresponds to a cosmic string as a linear defect, compactified along its axis. In contrast to the case of the Minkowski bulk, the upper bound on the chemical potential does not depend on the field mass and is completely determined by the length of compact dimension and by the enclosed magnetic flux. The only nonzero components correspond to the charge density, to the azimuthal current and to the current along the compact dimension. They are periodic functions of magnetic fluxes with the period equal to the flux quantum. The charge density is an odd function and the currents are even functions of the chemical potential. At high temperatures the influence of the gravitational field and topology on the charge density is subdominant and the leading term in the corresponding expansion coincides with that for the charge density in the Minkowski spacetime. The current densities are topology-induced quantities and their behavior at high temperatures is completely different with the linear dependence on the temperature. At small temperatures and for chemical potentials smaller than the critical value, the thermal expectation values are exponentially suppressed for both massive and massless fields.

hep-th↗

arXiv2022

Cosmological evolution with negative energy densities

For general number of spatial dimensions we investigate the cosmological dynamics driven by a cosmological constant and by a source with barotropic equation of state. It is assumed that for both those sources the energy density can be either positive or negative. Exact solutions of the cosmological equations are provided for flat models. For models with curved space and with zero cosmological constant the general solutions are expressed in terms of the hypergeometric function. The qualitative evolution is described for all values of the equation of state parameter. We specify the values of that parameter and the combinations of the signs for the cosmological constant and matter energy density for which the cosmological dynamics is nonsingular. An example is considered with positive cosmological constant and negative matter energy density induced by the polarization of the hyperbolic vacuum.

gr-qc↗

arXiv2022

Vacuum densities and the Casimir forces for branes orthogonal to the AdS boundary

For a massive scalar field with general curvature coupling we evaluate the Wightman function in the geometry of two parallel branes perpendicular to the AdS boundary. On the separate branes, the field operator is constrained by Robin boundary conditions, in general, with different coefficients. In the region between the branes their contribution to the Wightman function is explicitly separated. By using this decomposition, the brane-induced effects on the vacuum expectation values (VEVs) for the field squared and energy-momentum tensor are investigated. The behavior of those expectation values is studied in various asymptotic regions of the parameters. The vacuum energy-momentum tensor in addition to the diagonal components has a nonzero off-diagonal stress. Depending on the boundary conditions and also on the distance from the branes, the vacuum energy density can be either positive or negative. The Casimir forces acting on the branes have two components. The first one corresponds to the standard normal force and the second one is parallel to the branes and presents the vacuum shear force. Unlike to the problem of parallel plates in the Minkowski bulk, the normal Casimir forces acting on separate branes differ if the boundary conditions on the branes are different. They can be either repulsive or attractive. In a similar way, depending on the coefficients in the boundary conditions, the shear force is directed toward or from the AdS boundary. The separate components may also change their signs as functions of the interbrane separation. At large proper separations between the branes, compared to the AdS curvature radius, both the components of the Casimir forces exhibit a power-law decay. For a massive scalar field this behavior is in contrast to that for the Minkowski bulk, where the decrease is exponential.

hep-th↗

arXiv2022

Vacuum currents in partially compactified Rindler spacetime with an application to cylindrical black holes

The vacuum expectation value of the current density for a charged scalar field is investigated in Rindler spacetime with a part of spatial dimensions compactified to a torus. It is assumed that the field is prepared in the Fulling-Rindler vacuum state. For general values of the phases in the periodicity conditions and the lengths of compact dimensions, the expressions are provided for the Hadamard function and vacuum currents. The current density along compact dimensions is a periodic function of the magnetic flux enclosed by those dimensions and vanishes on the Rindler horizon. The obtained results are compared with the corresponding currents in the Minkowski vacuum. The near-horizon and large-distance asymptotics are discussed for the vacuum currents around cylindrical black holes. In the near-horizon approximation the lengths of compact dimensions are determined by the horizon radius. At large distances from the horizon the geometry is approximated by a locally anti-de Sitter spacetime with toroidally compact dimensions and the lengths of compact dimensions are determined by negative cosmological constant.

hep-th↗

arXiv2022

Cosmic string and brane induced effects on the fermionic vacuum in AdS spacetime

We investigate the combined effects of a magnetic flux-carrying cosmic string and a brane on the fermionic condensate (FC) and on the vacuum expectation value (VEV) of the energy-momentum tensor for a massive charged fermionic field in background of 5-dimensional anti-de Sitter (AdS) spacetime. The brane is parallel to the AdS boundary and it divides the space into two regions with distinct properties of the fermionic vacuum. For two types of boundary conditions on the field operator and for the fields realizing two inequivalent representations of the Clifford algebra, the brane-induced contributions in VEVs are explicitly separated. The VEVs are even periodic functions of the magnetic flux, confined in the core, with the period of flux quantum. Near the horizon the FC and the vacuum energy-momentum tensor are dominated by the brane-free contribution, whereas the brane-induced part dominates in the region near the brane. Both the contributions vanish on the AdS boundary. At large distances from the cosmic string, the topological contributions in the VEVs, as functions of the proper distance, exhibit an inverse power-law decrease in the region between the brane and AdS horizon and an exponential decrease in the region between the brane and AdS boundary. We show that the FC and the vacuum energy density can be either positive or negative, depending on the distance from the brane. Applications are discussed in fermionic models invariant under the charge conjugation and parity transformation and also in $Z_{2}$% -symmetric braneworld models. By the limiting transition we derive the expressions of the FC and the vacuum energy-momentum tensor for a cosmic string on 5-dimensional Minkowski bulk in the presence of a boundary perpendicular to the string.

hep-th↗

arXiv2022

Topological effects in fermion condensate induced by cosmic string and compactification on AdS bulk

We investigate the fermion condensate (FC) for a massive spinor field on background of the 5-dimensional locally anti-de Sitter (AdS) spacetime with a compact dimension and in the presence of a cosmic string carrying a magnetic flux. The FC is decomposed into two contributions. The first one corresponds to the geometry without compactification and the second one is induced by the compactification. Depending on the values of the parameters, the total FC can be either positive or negative. As a limiting case, the expression for the FC on locally Minkowski spacetime is derived. It vanishes for a massless fermion field and the nonzero FC on the AdS bulk in the massless case is an effect induced by gravitation. This shows that the gravitational field may essentially influence the parameters space for phase transitions. For a massive field the FC diverges on the string as the inverse cube of the proper distance from the string. In the case of a massless field, depending on the magnetic flux along the string and planar angle deficit, the limiting value of the FC on the string can be either finite or infinite. At large distances, the decay of the FC as a function of the distance from the string is power law for both cases of massive and massless fields. For a cosmic string on the Minkowski bulk and for a massive field the decay is exponential. The topological part in the FC vanishes on the AdS boundary. We show that the FCs coincide for the fields realizing two inequivalent irreducible representations of the Clifford algebra. In the special case of the zero planar angle deficit, the results presented in this paper describe Aharonov-Bohm-type effects induced by magnetic fluxes in curved spacetime.

hep-th↗

arXiv2021

Mean field squared and energy-momentum tensor for the hyperbolic vacuum in dS spacetime

We evaluate the Hadamard function and the vacuum expectation values (VEVs) of the field squared and energy-momentum tensor for a massless conformally coupled scalar field in $(D+1)$-dimensional de Sitter (dS) spacetime foliated by spatial sections of negative constant curvature. It is assumed that the field is prepared in the hyperbolic vacuum state. An integral representation for the difference of the Hadamard functions corresponding to the hyperbolic and Bunch-Davies vacua is provided that is well adapted for the evaluation of the expectation values in the coincidence limit. It is shown that the Bunch-Davies state is interpreted as thermal with respect to the hyperbolic vacuum. An expression for the corresponding density of states is provided. The relations obtained for the difference in the VEVs for the Bunch-Davies and hyperbolic vacua are compared with the corresponding relations for the Fulling-Rindler and Minkowski vacua in flat spacetime. The similarity between those relations is explained by the conformal connection of dS spacetime with hyperbolic foliation and Rindler spacetime. As a limiting case, the VEVs for the conformal vacuum in the Milne universe are discussed.

hep-th↗

arXiv2021

Fermionic condensate in de Sitter spacetime

Fermionic condensate is investigated in $(D+1)$-dimensional de Sitter spacetime by using the cutoff function regularization. In order to fix the renormalization ambiguity for massive fields an additional condition is imposed, requiring the condensate to vanish in the infinite mass limit. For large values of the field mass the condensate decays exponentially in odd dimensional spacetimes and follows a power law decay in even dimensional spacetimes. For a massless field the fermionic condensate vanishes for odd values of the spatial dimension $D$ and is nonzero for even $D$. Depending on the spatial dimension the fermionic condensate can be either positive or negative. The change in the sign of the condensate may lead to instabilities in interacting field theories.

hep-th↗

arXiv2021

Fermionic vacuum polarization around a cosmic string in compactified AdS spacetime

We investigate topological effects of a cosmic string and compactification of a spatial dimension on the vacuum expectation value (VEV) of the energy-momentum tensor for a fermionic field in (4+1)-dimensional locally AdS spacetime. The contribution induced by the compactification is explicitly extracted by using the Abel-Plana summation formula. The mean energy-momentum tensor is diagonal and the vacuum stresses along the direction perpendicular to the AdS boundary and along the cosmic string are equal to the energy density. All the components are even periodic functions of the magnetic fluxes inside the string core and enclosed by compact dimension, with the period equal to the flux quantum. The vacuum energy density can be either positive or negative, depending on the values of the parameters and the distance from the string. The topological contributions in the VEV of the energy-momentum tensor vanish on the AdS boundary. Near the string the effects of compactification and gravitational field are weak and the leading term in the asymptotic expansion coincides with the corresponding VEV in (4+1)-dimensional Minkowski spacetime. At large distances, the decay of the cosmic string induced contribution in the vacuum energy-momentum tensor, as a function of the proper distance from the string, follows a power law. For a cosmic string in the Minkowski bulk and for massive fields the corresponding fall off is exponential. Within the framework of the AdS/CFT correspondence, the geometry for conformal field theory on the AdS boundary corresponds to the standard cosmic string in (3+1)-dimensional Minkowski spacetime compactified along its axis.

hep-th↗

arXiv2021

Casimir densities induced by a sphere in the hyperbolic vacuum of de Sitter spacetime

Complete set of modes and the Hadamard function are constructed for a scalar field inside and outside a sphere in (D+1)-dimensional de Sitter spacetime foliated by negative constant curvature spaces. We assume that the field obeys Robin boundary condition on the sphere. The contributions in the Hadamard function induced by the sphere are explicitly separated and the vacuum expectation values (VEVs) of the field squared and energy-momentum tensor are investigated for the hyperbolic vacuum. In the flat spacetime limit the latter is reduced to the conformal vacuum in the Milne universe and is different from the maximally symmetric Bunch-Davies vacuum state. The vacuum energy-momentum tensor has a nonzero off-diagonal component that describes the energy flux in the radial direction. The latter is a purely sphere-induced effect and is absent in the boundary-free geometry. Depending on the constant in Robin boundary condition and also on the radial coordinate, the energy flux can be directed either from the sphere or towards the sphere. At early stages of the cosmological expansion the effects of the spacetime curvature on the sphere-induced VEVs are weak and the leading terms in the corresponding expansions coincide with those for a sphere in the Milne universe. The influence of the gravitational field is essential at late stages of the expansion. Depending on the field mass and the curvature coupling parameter, the decay of the sphere-induced VEVs, as functions of the time coordinate, is monotonic or damping oscillatory. At large distances from the sphere the fall-off of the sphere-induced VEVs, as functions of the geodesic distance, is exponential for both massless and massive fields.

hep-th↗

arXiv2021

The Holographic cosmology with axion field

In present article we consider an axion F(R) gravity model and described with the help of holographic principle the cosmological models of viscous dark fluid coupled with axion matter in a spatially flat Friedmann-Robertson-Walker (FRW) universe. This description based on generalized infrared-cutoff holographic dark energy, proposed by Nojiri and Odintsov. We explored the Little Rip, the Pseudo Rip, and the power-law bounce cosmological models in terms of the parameters of the inhomogeneous equation of the state of viscous dark fluid and calculated the infrared cutoffs analytically. We represented the energy conservation equation for the dark fluid from a holographic point of view and showed a correspondence between the cosmology of a viscous fluid and holographic cosmology. We analyzed the autonomous dynamic system. In the absence of interaction between fluids, solutions are obtained corresponding to two cases. In the first case, dark energy is missing and the extension describes the component of dark matter. The second case corresponds to cosmological models with an extension due to dark energy. The solutions obtained are investigated for stability. For a cosmological model with the interaction of a special type, the stability of solutions of the dynamic system is also investigated.

gr-qc↗

arXiv2021

Casimir effect for fermion condensate in conical rings

The fermion condensate (FC) is investigated for a (2+1)-dimensional massive fermionic field confined on a truncated cone with an arbitrary planar angle deficit and threaded by a magnetic flux. Different combinations of the boundary conditions are imposed on the edges of the cone. They include the bag boundary condition as a special case. By using the generalized Abel-Plana-type summation formula for the series over the eigenvalues of the radial quantum number, the edge-induced contributions in the FC are explicitly extracted. The FC is an even periodic function of the magnetic flux with the period equal to the flux quantum. Depending on the boundary conditions, the condensate can be either positive or negative. For a massless field the FC in the boundary-free conical geometry vanishes and the nonzero contributions are purely edge-induced effects. This provides a mechanism for time-reversal symmetry breaking in the absence of magnetic fields. Combining the results for the fields corresponding to two inequivalent irreducible representations of the Clifford algebra, the FC is investigated in the parity and time-reversal symmetric fermionic models and applications are discussed for graphitic cones.

hep-th↗