Gravitational radiation from cosmic string splitting on a Friedmann-Robertson-Walker background

We present exact solutions of Einstein's equations that may be interpreted as representing the splitting of a primordial cosmic string imbedded in a perfect fluid Friedmann-Robertson-Walker (FRW) cosmology. The splitting leads to the creation of a bubble whose boundary is given by a gravitational shock wave, expanding from the point of splitting, associated to the motion of the free ends of the string. Inside the bubble we have a perturbed FRW metric. This perturbation is largest near the string ends, producing a sort of wake along the path of the free ends, but decreases rapidly with time and the metric approaches the FRW regime locally everywhere inside the bubble. Similar results are shown to hold also for flat vacuum de Sitter space-times.Supported by a CONICET fellowship.Supported by a CONICOR fellowship.     

Compactification and cosmic string phenomenology

A U(1) model of gravitational, Higgs and, gauge fields is analysed. Two phases of the system are considered. In the case with broken symmetry we have the Nielsen-Olesen configuration. In the symmetric phase we find a space-time metric S² × E₁². This appears in full analogy to the Freund-Rubin compactification due to an antisymmeric tensor field. The connection of these structures with cosmic string phenomenology is analysed.     

Dark energy cosmological models with cosmic string

In this paper we have studied the anisotropic Kantowski-Sachs, locally rotationally symmetric (LRS) Bianchi type-I and LRS Bianchi type-III geometries filled with dark energy and one dimensional cosmic string in the Saez-Ballester theory of gravitation. To get physically valid solution we take hybrid expansion law of the average scale factor which is a product of power and exponential type of functions that results in time dependent deceleration parameter (\(q\)). The equation of state parameter of dark energy (\(\omega _{\mathit{de}}\)) has been discussed and we have observed that for the three models it crosses the phantom divide line (\(\omega _{\mathit{de}} = -1\)) and shows quintom like behavior. The density of dark energy (\(\rho _{\mathit{de}}\)) is an increasing function of redshift and remains positive throughout the evolution of the universe for the three models. Moreover in Kantowski-Sachs and LRS Bianchi type-I geometries the dark energy density dominates the string tension density (\(\lambda \)) and proper density (\(\rho \)) throughout the evolution of the universe. The physical and geometrical aspects of the statefinder parameters (\(r,s\)), squared speed of sound (\(v_{s}^{2} \)) and \(\omega _{\mathit{de}}\)–\(\omega ^{\prime }_{\mathit{de}}\) plane are also discussed.     

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.     

Perturbation spectrum in the inflationary stage when a cosmic string is present

We examine the spectrum of perturbations for a scalar field with an arbitrary curvature coupling parameter in the de Sitter stage of cosmological expansion when a cosmic string is present. These perturbations are caused by vacuum fluctuations in the field and serve as seed perturbations for the formation of galaxies in the postinflationary stage. A cosmic string disrupts the homogeneity of a de Sitter space, so that the spectrum of the perturbations depends on the distance from the string. This dependence is oscillatory in character with a period on the order of the perturbation wavelength.     

Bianchi type-I string cosmological model in the presence of a magnetic field: classical versus loop quantum cosmology approaches

A Bianchi type-I cosmological model in the presence of a magnetic flux along a cosmological string is considered. The first objective of this study is to investigate Einstein equations using a tractable assumption usually accepted in the literature. Quantum effects of the present cosmological model are examined in the framework of loop quantum cosmology. Finally we draw a parallel between the classical and quantum approaches.     

Bianchi type-III bulk viscous cosmic string model in a scalar-tensor theory of gravitation

A spatially homogeneous and anisotropic Bianchi type-III space-time is considered in the presence of bulk viscous fluid containing one dimensional cosmic strings in the frame work of a scalar-tensor theory of gravity proposed by Saez and Ballester (in Phys. Lett. A 113:467, 1986). We have obtained a determinate solution of the field equations of this theory, using (i) a barotropic equation of state for the pressure and density and (ii) the bulk viscous pressure is proportional to the energy density. Some physical properties of the model are also discussed.     

Gravitational mass of magnetostatic field

Maxwell's displacement current equation is interpreted in the light of recent work to show that static magnetic field in free space should have a colocated and contemporaneous mass that is neither embodied in, nor can be anticipated from, the mass-energy relation. Thus magnetostatic field in the universe represents an invisible mass. Some consequences are discussed.     

Gravitational collapse of cosmic gas clouds and formation of star clusters

In this paper the gravitational collapse of cosmic gas clouds and formation of star clusters has been considered. Hoyle's view of successive fragmentation has been taken as the basic mechanim in the present work. The initial masses of protostars have been estimated as the function of their distances from the centre of the cluster and the intensity of the magnetic field of the medium. It has been shown that the fragmentation process is greatly inhibited by the presence of a strong magnetic field. A model has been constructed showing how a protostar grows in mass by accretion from the surrounding medium, on the basis of the assumption that as the star moves at random in the cluster it picks up a fraction of the material through which it passes. It has been estimated that a protostar of initial mass of about 0.1M grows to one of 1.0M in a time period which ranges from a few multiples of 10⁵ to a few multiples of 10⁷ yr, depending on the parameters involved in the accretion process. The number of stars per unit mass range has also been estimated; it is found to be proportional tom ^–3.3,m being the mass of the star.     

The cluster abundance in cosmic string models for structure formation

We use the present observed number density of large X-ray clusters to constrain the amplitude of matter density perturbations induced by cosmic strings on the scale of 8  h ⁻¹ Mpc ( σ ₈), in both open cosmologies and flat models with a non-zero cosmological constant. We find a slightly lower value of σ ₈ than that obtained in the context of primordial Gaussian fluctuations generated during inflation. This lower normalization of σ ₈ results from the mild non-Gaussianity on cluster scales, where the one-point probability distribution function is well approximated by a χ ² distribution and thus has a longer tail than a Gaussian distribution. We also show that σ ₈ normalized using cluster abundance is consistent with the COBE normalization.     

LRS Bianchi type-II bulk viscous cosmic string model in a scale covariant theory of gravitation

A locally rotationally symmetric(LRS) Bianchi type-II space-time is considered in the frame work of a modified theory of gravitation proposed by Canuto et al. (Phys. Rev. Lett. 39:429, 1977) when the source for energy momentum tensor is a bulk viscous fluid containing one dimensional cosmic strings. A special law of variation for Hubble’s parameter proposed by Bermann (Nuovo Cimento B 74:182, 1983) is used to obtain determinate solution of the field equations. We have also used the barotropic equation of state and the bulk viscous pressure is assumed to be proportional to the energy density. The physical and kinematical properties of the model are also discussed.     

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.     

Wavelet domain Bayesian denoising of string signal in the cosmic microwave background

An algorithm is proposed for denoising the signal induced by cosmic strings in the cosmic microwave background. A Bayesian approach is taken, based on modelling the string signal in the wavelet domain with generalized Gaussian distributions. Good performance of the algorithm is demonstrated by simulated experiments at arcminute resolution under noise conditions including primary and secondary cosmic microwave background anisotropies, as well as instrumental noise.     

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.     

Non-linear amplification of a magnetic field driven by cosmic ray streaming

One-, two- and three-dimensional numerical results of the non-linear interaction between cosmic rays and a magnetic field are presented. These show that cosmic ray streaming drives large-amplitude Alfvénic waves. The cosmic ray streaming energy is very efficiently transferred to the perturbed magnetic field of the Alfvén waves, and the non-linear time-scale of the growth of the waves is found to be very rapid, of the order of the gyro-period of the cosmic ray. Thus, a magnetic field of interstellar values, assumed in models of supernova remnant blast wave acceleration, would not be appropriate in the region of the shock. The increased magnetic field reduces the cosmic ray acceleration time and so increases the maximum cosmic ray energy, which may provide a simple and elegant resolution to the highest energy Galactic cosmic ray problem, where the cosmic rays themselves provide the fields necessary for their acceleration.     

Gravitational field of domain wall in Lyra geometry

In this paper, we study the domain wall with time dependent displacement vectors based on Lyra geometry in normal gauge i.e. displacement vector φ _i ^* =[β(t),0,0,0]. The field theoretic energy momentum tensor is considered with zero pressure perpendicular to the wall. We find an exact solutions of Einstein’s equation for a scalar field φ with a potential V(φ) describing the gravitational field of a plane symmetric domain wall. We have seen that the hyper surfaces parallel to the wall (z=constant) are three dimensional de-sitter spaces. It is also shown that the gravitational field experienced by test particle is attractive.     

Possible application of circular polarization for remote sensing of cosmic bodies

Phase dependences of circular polarization were obtained with a precision Stokes polarimeter designed and constructed at the Main Astronomical Observatory of AS Ukraine. A study was made of dielectric and metallic powders with grains of diameter 10–100 m. Metallic powders were found to produce an essential circular polarization - up to 3%, just as dielectric powders did not show circular polarization values more than 0.05% Change of circular polarization with phase angle V is greatly depended on surface structure. Loose powders give phase curves with the same sign of circular polarization everywhere and with maximum at large phase angles V > 120 . Measurements of compacted powders show curves which change the sign repeatedly and have additional maxima, including a maximum at small phase angles V < 40 . A theory was created which considers a circular polarization as a result of multiple reflections of light from particulate surface. The theory provides reasonable good fit to the experimental data. It was concluded that measurements of circular polarization can be used to find metals in surface material of cosmic bodies (especially asteroids) and to determine characteristics of surface structure, in particular, to establish presence of regolith on metal-rich bodies.