Primordial black holes from monopoles connected by strings

Primordial black holes (PBHs) are known to be produced from collapsing cosmic defects such as domain walls and strings. In this paper we show how PBHs are produced in monopole-string networks.     

Five dimensional string cosmological models in a scalar-tensor theory of gravitation

Five dimensional Kaluza-Klein space-time is considered in the presence of cosmic string source in the frame work of scalar-tensor theory of gravitation proposed by Saez and Ballester (Phys. Lett. A 113, 467 (1985)). Exact cosmological models, which represent Nambu, Takabayasi and Reddy strings are presented. Some physical and kinematical properties of the models are also discussed.     

Non-existence of kinks in a modified gravity

Spherically symmetric kink space-time is considered in the framework of f(R,T) gravity proposed by Harko et al. (Phys. Rev. D 84:024020, 2011) in the presence of a cloud of massive strings with perfect fluid. Solving the field equations of this modified theory of gravity, we observe that cosmic strings and perfect fluid do not survive in this theory of gravitation and in this particular space-time. Hence a vacuum kink model, which is asymptotically flat, is presented.     

LRS Bianchi type-II Universe with cosmic strings and bulk viscosity in a scalar tensor theory of gravitation

A spatially homogeneous and anisotropic Bianchi type-II cosmological model is obtained in a scalar tensor theory of gravitation proposed by Saez and Ballester (Phys. Lett. A 130:467, 1986) when the source for energy momentum tensor is a bulk viscous fluid containing one-dimensional cosmic strings. Some physical and kinematical properties of the model are also discussed.     

Axially symmetric cosmological models in a scalar tensor theory based on Lyra manifold

Axially symmetric cosmological models are obtained in a scalar tensor theory proposed by Sen (Z. Phys. 149:311, 1957) based on Lyra manifold with time dependent β in the presence of string source, perfect fluid distribution, dust distribution and thick domain walls. Some physical and geometrical properties of these models are discussed.     

Anisotropic bulk viscous cosmological models in a modified gravity

A spatially homogeneous Bianchi type-VI₀ space-time is considered in the frame work of f(R,T) gravity proposed by Harko et al. (Phys. Rev. D 84:024020, 2011) when the source for energy momentum tensor is a bulk viscous fluid containing one dimensional cosmic strings. Exact solutions of the field equations are obtained both in the absence and in the presence of cosmic strings under some specific plausible physical conditions. Some physical and kinematical properties of the model are, also, studied.     

Non-existence of cosmic strings in bimetric theory of gravitation

Bianchi type cosmological models are considered in Bimetric theory of gravitation proposed by Rosen (1973) in the context of cosmic strings. It is interesting to note that cosmic strings do not occur in Bianchi type cosmologies. This revised version was published online in July 2006 with corrections to the Cover Date.     

Bianchi type-V bulk viscous string cosmological model in Saez-Ballester scalar-tensor theory of gravitation

In this paper, a spatially homogeneous and anisotropic Bianchi type-V cosmological model is considered in a scalar-tensor theory of gravitation proposed by Saez and Ballester (in Phys. Lett. A 113:467, 1986) when the source for energy momentum tensor is a bulk viscous fluid containing one dimensional cosmic strings. The field equations being highly non-linear, we obtain a determinate solution using the plausible physical conditions (i) the scalar of expansion of the space-time is proportional to shear scalar (ii) the baratropic equation of state for pressure and density and (iii) the bulk viscous pressure is proportional to the energy density. It is interesting to observe that cosmic strings do not survive in this model. Some physical and kinematical properties of the model are also discussed.     

Anisotropic universe with cosmic strings and bulk viscosity in a scalar–tensor theory of gravitation

A spatially homogeneous and anisotropic Bianchi type-I cosmological model is obtained in a scalar–tensor theory of gravitation proposed by Saez and Ballester (Phys. Lett. A 113:467, 1986) when the source for energy momentum tensor is a bulk viscous fluid containing one dimensional cosmic strings. Some physical and kinematical properties of the model are discussed. It is observed that the bulk viscosity has a greater role in getting an accelerated expansion of the universe in this theory.     

Kantowski–Sachs bulk viscous cosmological model in a scalar–tensor theory of gravitation

Field equations are obtained in the scalar–tensor theory of gravitation proposed by Saez and Ballester (Phys. Lett. A 113:467, 1986) with the aid of spatially homogenous and anisotropic Kantowski–Sachs space–time in the presence of bulk viscous fluid containing one dimensional cosmic strings. A determinate solution of the field equations is obtained, using some plausible physical conditions, which represents a Kantowski–Sach’s bulk viscous Cosmological model in the new scalar–tensor theory. Physical and kinematical properties of the model are also discussed.     

Bianchi type-V bulk viscous string cosmological model in scale-covariant theory of gravitation

A spatially homogeneous and anisotropic Bianchi type-V space–time is considered in the frame work of a scale covariant theory of gravitation proposed by Canuto et al. (Phys. Rev. Lett. 39:429, 1977) when the matter sources is a bulk viscous fluid containing one dimensional cosmic strings. Using some physically plausible conditions, we have obtained a determinate solution of the field equations of the theory which represents a Bianchi type-V bulk viscous string cosmological model in this theory. Some physical and kinematical properties of the model are also discussed.     

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.     

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.     

Axially Symmetric Cosmic Strings in a Scalar-Tensor Theory

Axially symmetric cosmological models with cosmic string source are obtained in a scalar-tensor theory of gravitation proposed by Saez and Ballester (Phys. Lett. A113, 467, 1985). The models obtained give us axially symmetric geometric (Nambu) string, p-string and Reddy string (Astrophys. Space Sci. 286, 2003b) in Saez-Ballester theory. Some physical properties of the models are also discussed.     

Bianchi type-V bulk viscous string cosmological model in f(R,T) gravity

In this paper, we investigate a spatially homogeneous and anisotropic Bianchi type-V cosmological model in a scalar-tensor theory of gravitation proposed by Harko et al. (Phys. Rev. D 84:024020, 2011) when the source for energy momentum tensor is a bulk viscous fluid containing one dimensional cosmic strings. To obtain a determinate solution, a special law of variation proposed by Berman (Nuovo Cimento B 74:182, 1983) is used. We have also used the barotropic equation of state for the pressure and density and bulk viscous pressure is assumed to be proportional to energy density. It is interesting to note that the strings in this model do not survive. Also the model does not remain anisotropic throughout the evolution of the universe. Some physical and kinematical properties of the model are also discussed.     

Some string cosmological models in Bianchi Type I space-time

We obtain some cosmological models that are exact solutions of Einstein's field equations. The metric utilized is Marder's metric which is Bianchi Type I and the curvature source is a cloud of strings which are one dimensional objects. Bianchi type cosmological models play an important role in the study of the universe on a scale which anisotropy is not ignored. In this paper we have investigated the effect of cosmic strings on the cosmic microwave background anisotropy. Various physical and geometrical properties of the model are also discussed. The solutions have reported that the cosmic microwave background anisotropy may due to the cosmic strings.     

A five dimensional Kaluza-Klein bulk viscous string cosmological model in Brans-Dicke scalar-tensor theory of gravitation

In this paper, we have investigated a five dimensional Kaluza-Klein space-time in the frame work of Brans-Dicke (Phys. Rev. 124:925, 1961) scalar-tensor theory of gravitation when the source of energy momentum tensor is a bulk viscous fluid containing one dimensional cosmic strings. We have obtained a determinate solution of the field equations using the special law of variation for Hubble’s parameter proposed by Bermann (Nuovo Cimento B 74:182, 1983) We have also used a barotropic equation of state for the pressure and density. Some physical properties of the model are also discussed.     

String cosmological models in five dimensional bimetric theory of gravitation

Five-dimensional spherically symmetric space-time is considered in bimetric theory of gravitation formulated by Rosen (Gen. Rel. Grav. 4, 435, 1973) in the presence of cosmic string dust cloud. Exact cosmological models which represent geometric (Nambu) string, p-string (Takabayasi string) and Reddy string (Astrophys. Space Sci. 301, 2006) are obtained in the static and non-static cases. Some physical properties of the models are also discussed.     

Higher Dimensional String Cosmologies in Scale-Covariant theory of Gravitation

Field equations are obtained with the aid of higher dimensional Bianchi type-I cosmological model in scale covariant theory of gravitation in the context of cosmic strings. We present here isotropic and anisotropic solutions of the field equations and some physical implications of these solutions are briefly discussed.     

LRS Bianchi type-II universe with cosmic strings and bulk viscosity in a modified theory of gravity

A locally rotationally symmetric (LRS) Bianchi type-II space-time is considered in the frame work of a modified theory of gravitation proposed by Harko et al. (Phys. Rev. D 84:024020, 2011) when the source for energy momentum tensor is a bulk viscous fluid containing one dimensional cosmic strings. A barotropic equation of state is assumed to get a determinate solution of the field equations. Also, the bulk viscous pressure is assumed to be proportional to the energy density. The physical behavior of the model is also discussed.