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We investigate a new class of LRS Bianchi type-II cosmological models by revisiting the paper of Mishra et al (Int. J. Theor. Phys. 52, 2546 (2013)) by considering a new deceleration parameter (DP) depending on the time in string cosmology for the modified gravity theory suggested by Sáez–Ballester (Phys. Lett. 113, 467 (1986)). We have considered the energy–momentum tensor proposed by Letelier (Phys. Rev. 28, 2414 (1983)) for bulk viscous and perfect fluid under some assumptions. To make our models consistent with recent astronomical observations, we have used the scale factor (Sharma et al, Astron Astrophys. 19, 55 (2018), Garg et al, Int. J. Geo. Meth. Mod. Phys.16, 1950007 (2019)) a (t) = exp [ 1 β 2 β t + k ] , where β and k are positive constants and it provides a time-varying DP. By using the recent constraints ( H 0 = 73.8 and q 0 = - 0.54 ) from SN Ia data in combination with BAO and CMB observations (Giostri et al, JCAP3, 27 (2012), arXiv:1203.3213v2[astro-ph.CO]), we affirm β = 0.0062 and k = 0.000016 . For these constraints, we have substantiated a new class of cosmological transit models for which the expansion takes place from the early decelerated phase to the current accelerated phase. Also, we have studied some physical, kinematic and geometric behaviour of the models, and have found them consistent with observations and well-established theoretical results. We have also compared our present results with those of Mishra et al (Int. J. Theor. Phys. 52, 2546 (2013)) and observed that the results in this paper are much better, stable under perturbation and in good agreement with cosmological reflections. [ABSTRACT FROM AUTHOR]

We discuss the relation between string theory/supergravity and the observational data in cosmology, as well as at LHC, past and future. We pay particular attention to the possibility of the future detection of primordial gravitational waves and how this might affect our understanding of fundamental physics. [ABSTRACT FROM AUTHOR]

I review the present status of the problem of initial conditions for inflation and describe several ways to solve this problem for many popular inflationary models, including the recent generation of the models with plateau potentials favored by cosmological observations. [ABSTRACT FROM AUTHOR]

A generic feature of string compactifications is the presence of many scalar fields, called moduli. Moduli are usually displaced from their post-inflationary minimum during inflation. Their relaxation to the minimum could lead to the production of oscillons: localised, long-lived, non-linear excitations of the scalar fields. Here we discuss under which conditions oscillons can be produced in string cosmology and illustrate their production and potential phenomenology with two explicit examples: the case of an initially displaced volume modulus in the KKLT scenario and the case of a displaced blow-up Kähler modulus in the Large Volume Scenario (LVS). One, in principle, observable consequence of oscillon dynamics is the production of gravitational waves which, contrary to those produced from preheating after high scale inflation, could have lower frequencies, closer to the currently observable range. We also show that, for the considered parameter ranges, oscillating fibre and volume moduli do not develop any significant non-perturbative dynamics. Furthermore, we find that the vacua in the LVS and the KKLT scenario are stable against local overshootings of the field into the decompatification region, which provides an additional check on the longevity of these metastable configurations. [ABSTRACT FROM AUTHOR]

We discuss a spatially homogeneous and anisotropic string cosmological models in the Brans-Dicke theory of gravitation. For a spatially homogeneous metric, it is assumed that the expansion scalar θ is proportional to the shear scalar σ. This condition leads to A = kB , where k and m are constants. With these assumptions and also assuming a variable scale factor a = a( t), we find solutions of the Brans-Dicke field equations. Various phenomena like the Big Bang, expanding universe, and shift from anisotropy to isotropy are observed in the model. It can also be seen that in early stage of the evolution of the universe, strings dominate over particles, whereas the universe is dominated by massive strings at the late time. Some physical and geometrical behaviors of the models are also discussed and observed to be in good agreement with the recent observations of SNe la supernovae. [ABSTRACT FROM AUTHOR]

The spatially homogeneous and anisotropic Bianchi type V cosmological solutions of massive string have been investigated in the presence of a magnetic field within the framework of f( R, T) gravity theory. With the help of the hybrid expansion law for the average scale factor a cosmological model is obtained. It is shown that the universe exhibits transition from deceleration to acceleration. We also find that Bianchi type V model is anisotropic at the early stage of evolution of the universe and become isotropic for large time. We also check the stability of solutions through cosmological perturbation. The physical and kinematical behaviors of the model are also studied. [ABSTRACT FROM AUTHOR]

The spatially homogeneous and totally anisotropic Bianchi type-II cosmological solutions of massive strings have been investigated in the presence of the magnetic field in the framework of scale-covariant theory of gravitation formulated by Canuto et al. (Phys. Rev. Lett. 39, 429, ). With the help of special law of variation for Hubbles parameter proposed by Berman (Nuovo Cimento 74, 182, ) string cosmological model is obtained in this theory. We use the power law relation between scalar field ϕ and scale factor R to find the solutions. Some physical and kinematical properties of the model are also discussed. [ABSTRACT FROM AUTHOR]

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, ) 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. [ABSTRACT FROM AUTHOR]

In this paper, we study anisotropic Bianchi-V Universe with magnetic field and bulk viscous fluid in string cosmology. Exact solutions of the field equations are obtained by using the equation of state (EoS) for a cloud of strings, and a relationship between bulk viscous coefficient and scalar expansion. The bulk viscous coefficient is assumed to be inversely proportional to the expansion scalar. It is interesting to examine the effects of magnetized bulk viscous string model in early and late stages of evolution of the Universe. This paper presents different string models like geometrical (Nambu string), Takabayasi (p-string) and Reddy string models by taking certain physical conditions. We discuss the nature of classical potential for viscous fluid with and without magnetic field. The presence of bulk viscosity stops the Universe from becoming empty in its future evolution. It is observed that the Universe expands with decelerated rate in the presence of viscous fluid with magnetic field whereas, it expands with marginal inflation in the presence of viscous fluid without magnetic field. The other physical and geometrical aspects of each string model are discussed in detail. [ABSTRACT FROM AUTHOR]

A spatially homogeneous and anisotropic LRS Bianchi type-II space-time is considered in the frame work of second self-creation theory of gravitation proposed by Barber (Gen. Relativ. Gravit. 14:117, ) in the presence of bulk viscous fluid containing one dimensional cosmic strings. A determinate solution of the field equations is presented using special variation for Hubble's parameter given by Berman (Nuovo Cimento B 74:182, ) and some physically plausible conditions. The solution represents a bulk viscous string model in the second self-creation cosmology. We have also discussed some physical and kinematical properties of the model. [ABSTRACT FROM AUTHOR]

In this paper, we investigate spatially homogenous and anisotropic Kantowski-Sachs cosmological model in Brans-Dicke (Phys. Rev. 124:925, ) scalar-tensor theory of gravitation in the presence of bulk viscous fluid containing one dimensional cosmic strings. To find a determinate solution of the highly non-linear field equations we use the special law of variation for Hubble's parameter proposed by Berman (Nuovo Cimento B 74:182, ). We also use (i) the barotropic equation of state for the pressure and density. (ii) the trace free energy momentum tensor of the fluid and (iii) shear scalar is proportional to scalar expansion. Some physical and kinematical properties of the model obtained are, also, discussed. [ABSTRACT FROM AUTHOR]

In this contribution, we study the Dirac oscillator under the influence of noninertial effects of a rotating frame in the cosmic string spacetime. We show that both noninertial effects and the topology of the cosmic string spacetime restrict the physical region of the spacetime where the quantum particle can be placed, and discuss two different cases of bound states solutions of the Dirac equation by analysing the behaviour of the Dirac oscillator frequency. [ABSTRACT FROM AUTHOR]

In the present study, anisotropic locally rotationally symmetric (LRS) Bianchi type II models representing massive strings have been presented with variable mean deceleration parameter. The field equations in scalar-tensor theory with energy momentum tensor proposed by Letelier (Phys. Rev. D 28: 2414, ), have been solved for the bulk viscous fluid and perfect fluid under the following physically relevant assumptions: (a) scale factor varies with time as $a(t) = (\sinh(\alpha t))^{\frac{1}{n}}$, (b) Expansion ( θ) in the model is proportional to shear ( σ), (c) p= γρ, where γ (0≤ γ≤1) is a constant, (d) ξ( t)= ξ ρ. It has been observed that the presented universe has a phase transition from the early decelerating phase to the accelerating phase at present epoch which is in good agreement with the recent astronomical observations. Moreover, some physical and geometric properties of the models have been discussed in detail. [ABSTRACT FROM AUTHOR]

LRS Bianchi type-I bulk viscous string cosmological models are obtained in scalar tensor theory of gravitation proposed by Saez and Ballester (Phys. Lett. A 113:467, ). It is shown that cosmic string does not survive for ρ+ λ=0 whereas it survives for the equations of state ρ=(1+ ω) λ (Takabayasi string) and ρ= λ (Geometric string). Some physical and geometrical properties of the exhibited model are discussed. [ABSTRACT FROM AUTHOR]

In this paper, the effect of electromagnetic field in the string Bianchi type-VI Universe is investigated. Einstein's field equations have been solved exactly with suitable physical assumptions for two types of strings: (i) massive strings and (ii) Nambu strings. It is found that when the Universe is dominated by massive strings, the existence of electromagnetic field is necessary as it accelerates the expansion of the Universe. But when our Universe is dominated by Nambu strings, the electromagnetic field does not have significant effect on the evolution of the Universe. We have also shown that the early massive string-dominated Universe got converted to Nambu string-dominated Universe later. Our models are derived from an early deceleration phase to an accelerating phase which is consistent with the recent observations of supernovae type-Ia. The physical and geometrical behaviour of these models are also discussed. [ABSTRACT FROM AUTHOR]

The study started by Cabo and Brandenberger (J. Cosmol. Astropart. Phys. 2:15, ) about the Dilaton mean field stabilization thanks to the effective potential generated by the existence of massive fermions, is here extended. Three loop corrections are evaluated in addition to the previously calculated two loop terms. The results indicate that the Dilaton vacuum field tends to be fixed at a high value close to the Planck scale, in accordance with the need for predicting Einstein gravity from string theory. The mass of the Dilaton is evaluated to be also a high value close to the Planck mass, which implies the absence of Dilaton scalar signals in modern cosmological observations. These properties arise when the fermion mass is chosen to be either at a lower bound corresponding to the top quark mass, or alternatively, at a very much higher value assumed to be in the grand unification energy range. One of the three 3-loop terms is exactly evaluated in terms of Master integrals. The other two graphs are however evaluated in their leading logarithm correction in the perturbative expansion. The calculation of the non leading logarithmic contribution and the inclusion of higher loops terms could made more precise the numerical estimates of the vacuum field value and masses, but seemingly are expected not to change the qualitative behavior obtained. The validity of the here employed Yukawa model approximation is argued for small value of the fermion masses with respect to the Planck one. A correction to the two loop calculation done in the previous work is here underlined. [ABSTRACT FROM AUTHOR]

We review recent achievements and open questions in string field theory. We comment on the history of the construction of the covariant gauge-invariant string field formulation and briefly consider possible applications of string field theory in D-brane physics and cosmology. [ABSTRACT FROM AUTHOR]

Bulk viscous fluid distribution with massive strings in LRS Bianchi type-1 space time is studied. The exact solutions of the field equations are obtained by using the equation of state ρ=− λ and ρ= λ. We observed that the bulk viscous fluid does not survive for ρ=− λ whereas it survives for ρ= λ. Some physical and geometrical properties of the models are discussed. [ABSTRACT FROM AUTHOR]

String cloud cosmological models are studied using spatially homogeneous and anisotropic Bianchi type VI0 metric in Saez-Ballester Scalar-Tensor theory of gravitation. The field equations are solved for massive string cloud with particles attached to them. A more general linear equation of state of the cosmic string tension density with the proper energy density of the universe is considered instead of taking any particular relationships like pure geometric string or the case of the p-string. The pure geometric string and p-string solutions can be easily inferred from the models. For all viable models the possible limiting values of the linear connection between the proper energy density and string tension density have been calculated. The physical and kinematical properties of the models have been discussed in detail. [ABSTRACT FROM AUTHOR]

We review various general issues involved in the study of the motion of strings in time-dependent backgrounds. We discuss the interplay between the General Relativity and string points of view. Results and open problems are highlighted. The motion in one particular background is studied in some detail. [ABSTRACT FROM AUTHOR]

We briefly review two aspects of string cosmology: (1) the presence of chaos in the generic cosmological solutions of the tree-level low-energy effective actions coming out of string theory, and (2) the remarkable link between the latter chaos and the Weyl groups of some hyperbolic Kac-Moody algebras. [ABSTRACT FROM AUTHOR]

We study the problem of the behaviour of cosmological gravitational waves under conformal transformations. In spite of the apparent triviality of this question, the informations we can obtain from gravitational waves in the so-called Einstein's and Jordan's frame are not the same, mainly with respect to the choice of the initial conditions and of graviton production. The only exception seems to occur in string cosmology due to the duality properties. [ABSTRACT FROM AUTHOR]

A new class of string cosmological models withand without magnetic field in the context of aspace-time with G3 symmetry has beenpresented. In order to study the effects of magneticfield, the standard energy-momentum tensor is modified byincorporating additional term due to magnetic field. Thephysical and kinematical behaviours of the stringcosmological models have been discussed. [ABSTRACT FROM AUTHOR]

Recently the low-energy effective string theory has been used by Gasperini and Veneziano to elaborate a very interesting scenario for the early history of the universe (“birth of the universe as quantum scattering”). Here we investigate the gauge fixing and the problem of the definition of a global time parameter for the quantum form of the model, and obtain the positive norm Hilbert space of states. [ABSTRACT FROM AUTHOR]

The role of anisotropic components on the dark energy and the dynamics of the universe is investigated. An anisotropic dark energy fluid with different pressures along different spatial directions is assumed to incorporate the effect of anisotropy. One dimensional cosmic strings aligned along x-direction supplement some kind of anisotropy. Anisotropy in the dark energy pressure is found to evolve with cosmic expansion at least at late times. At an early phase, the anisotropic effect due to the cosmic strings substantially affect the dynamics of the accelerating universe. [ABSTRACT FROM AUTHOR]

A correction regarding the address marks on authors' names for the article "Exact solution in a string cosmological model" is presented.