Narrow Results

This paper is devoted to formulating exact solutions of axially symmetric spacetime through gravitational decoupling technique. For this purpose, we first evaluate an exact solution in the framework of cosmic strings by assuming some additional constraints on the metric coefficients and extend it to obtain two concrete anisotropic cosmological models. We investigate energy conditions as well as the speed of sound constraint to ensure the physical viability of the developed solutions. It is concluded that both anisotropic models meet all the energy bounds as well as stability criterion. The expanding behavior of the universe is also confirmed through different cosmological parameters.

Anisotropic cosmological models are constructed in f(R, T) gravity theory to investigate the dynamics of universe concerning the late time cosmic acceleration. Using a more general and simple approach, the effect of the coupling constant and anisotropy on the cosmic dynamics have been investigated. In this study, it is found that cosmic anisotropy substantially affects cosmic dynamics.

This study deals with spatially homogeneous and anisotropic locally rotationally symmetric (LRS) Bianchi type-I universe with cosmic string source in the framework of $f(R)$ theory of gravity [S. Capozziello, S. Carloni and A. Troisi, Quintessence without scalar fields, Recent Res. Dev. Astron. Astrophys.1 (2003), 625; S. Nojiri and S. D. Odintsov, Modified gravity with negative and positive powers of curvature: Unification of inflation and cosmic acceleration, Phys. Rev. D68 (2003) 123512]. Solving the field equations using (i) relation between metric potentials, (ii) power law relation between $F(R)$ and average scale factor $a(t)$ and (iii) equations of state for string models we have presented Takabayasi [T. Takabayasi, Quantum Mechanics Determinism, Causality, and Particles (Springer, Berlin, 1976)], Nambu [P. S. Letelier, String cosmologies, Phys. Rev. D28 (1983) 2414–2419] and Reddy [D. R. K. Reddy, A string cosmological model in a scalar–Tensor theory of gravitation, Astrophys. Space Sci.286 (2003) 359–363] string cosmological models. The dynamical parameters of our models are determined and their physical behavior is discussed. The most interesting result about the models is that the anisotropic effects are wiped out at late times.

This paper deals with the construction of locally rotationally symmetric (LRS) Bianchi type-II (B-II) cosmological models obtained by solving Einstein field equations coupled with an attractive massive scalar field (MSF) when the source of gravitation is the mixture of cosmic string cloud and anisotropic dark energy (DE) fluid which are minimally interacting. We have obtained exact cosmological models by using (i) shear scalar is proportional to the scalar expansion of the space–time and (ii) a power-law relation between the average scale factor of the universe and the scalar field. Our models represent string cosmological model and DE model in the presence of MSF. Using our model, we determine cosmological parameters such as energy densities, deceleration parameter, statefinders and equation of state parameter. We, also, present the tension density and energy density of the string. We discuss the physical aspects of these cosmological parameters. It is observed that our models represent accelerated expansion phenomenon of our universe as confirmed by Supernova Ia experiment.

We examine the shadow cast by a Kerr black hole pierced by a cosmic string. The observable images depend not only on the black hole spin parameter and the angle of inclination, but also on the deficit angle yielded by the cosmic string. The dependence of the observable characteristics of the shadow on the deficit angle is explored. The imprints in the black hole shadow left by the presence of a cosmic string can serve in principle as a method for observational detection of such strings.

The work in this paper is devoted to the study of Kaluza–Klein (KK) cosmological model with strange quark matter attached to string cloud in the framework of metric version of $f(R)$ gravity. By utilizing hybrid expansion law for average scale factor of the model i.e. $a(t)=e^{\alpha t}{t}^{\beta }$, here $\alpha$ and $\beta$ are positive constants, we derive the solution of the field equations of KK model in the $f(R)$ gravity. Various dynamical parameters of the model are obtained and their behavior is analyzed through graphical representation. We observe that the analysis of deceleration parameter exhibits a smooth transition of the universe from early decelerated phase to current accelerated epoch. Also, we discussed well-known state-finder parameters $\{r,s\}$ and $\{r,q\}$ planes which are analyzed by graphical representation.

The existence of time machines, understood as space–time constructions exhibiting physically realised closed timelike curves (CTC's), would raise fundamental problems with causality and challenge our current understanding of classical and quantum theories of gravity. In this paper, we investigate three proposals for time machines which share some common features: cosmic strings in relative motion, where the conical space–time appears to allow CTC's; colliding gravitational shock waves, which in Aichelburg–Sexl coordinates imply discontinuous geodesics; and the superluminal propagation of light in gravitational radiation metrics in a modified electrodynamics featuring violations of the strong equivalence principle. While we show that ultimately none of these constructions creates a working time machine, their study illustrates the subtle levels at which causal self-consistency imposes itself, and we consider what intuition can be drawn from these examples for future theories.

Some black hole-cosmic string models such as Reissner–Nordström, RN–de Sitter, Kerr–Newman and multi-black holes with cosmic string are given. Energy and angular momentum of a timelike particle in circular orbits in multi-black hole space–time are calculated. The geodesic equations for the timelike particles for the far region of the multi-black hole sources are calculated and small oscillations around the circular orbit obtained. It is seen that the particle's orbit precesses like the Lens–Thirring effect.

We study steady vortex sheet solutions of the Navier–Stokes in the limit of vanishing viscosity at fixed energy flow. We refer to this as the turbulent limit. These steady flows correspond to a minimum of the Euler Hamiltonian as a functional of the tangent discontinuity of the local velocity parametrized as $\mathrm{\Delta }{\overrightarrow{v}}_t=\overrightarrow{\nabla }\mathrm{\Gamma }$. This observation means that the steady flow represents the low-temperature limit of the Gibbs distribution for vortex sheet dynamics with the normal displacement $\delta r_{\perp }$ of the vortex sheet as a Hamiltonian coordinate and $\mathrm{\Gamma }$ as a conjugate momentum. An infinite number of Euler conservation laws lead to a degenerate vacuum of this system, which explains the complexity of turbulence statistics and provides the relevant degrees of freedom (random surfaces). The simplest example of a steady solution of the Navier–Stokes equation in the turbulent limit is a spherical vortex sheet whose flow outside is equivalent to a potential flow past a sphere, while the velocity is constant inside the sphere. Potential flow past other bodies provide other steady solutions. The new ingredient we add is a calculable gap in tangent velocity, leading to anomalous dissipation. This family of steady solutions provides an example of the Euler instanton advocated in our recent work, which is supposed to be responsible for the dissipation of the Navier–Stokes equation in the turbulent limit. We further conclude that one can obtain turbulent statistics from the Gibbs statistics of vortex sheets by adding Lagrange multipliers for the conserved volume inside closed surfaces, the rate of energy pumping, and energy dissipation. The effective temperature in our Gibbs distribution goes to zero as ${\mathrm{\text{Re}}}^{-\frac{1}{3}}$ with Reynolds number $\mathrm{\text{Re}}\sim {\nu }^{-\frac{6}{5}}$ in the turbulent limit. The Gibbs statistics in this limit reduces to the solvable string theory in two dimensions (so-called $c=1$ critical matrix model). This opens the way for nonperturbative calculations in the Vortex Sheet Turbulence, some of which we report here.

We describe a new code to search for signatures of cosmic strings in cosmic microwave anisotropy maps. The code implements the Canny algorithm, an edge detection algorithm designed to search for the lines of large gradients in maps. Such a gradient signature which is coherent in position-space is produced by cosmic strings via the Kaiser–Stebbins effect. We test the power of our new code to set limits on the tension of the cosmic strings by analyzing simulated data, with and without cosmic strings. We compare maps with a pure Gaussian scale-invariant power spectrum with maps which have a contribution of a distribution of cosmic strings obeying a scaling solution. The maps have angular scale and angular resolution comparable to what current and future ground-based small-scale cosmic microwave anisotropy experiments will achieve. We present tests of the codes, indicate the limits on the string tension which could be set with the current code, and describe various ways to refine the analysis. Our results indicate that when applied to the data of ongoing cosmic microwave experiments such as the South Pole Telescope project, the sensitivity of our method to the presence of cosmic strings will be more than an order of magnitude better than the limits from existing analyses.

At its very beginning, the universe is believed to have grown exponentially in size via the mechanism of inflation. The almost scale-invariant density perturbation spectrum predicted by inflation is strongly supported by cosmological observations, in particular the cosmic microwave background (MB) radiation. However, the universe's precise inflationary scenario remains a profound problem for cosmology and for fundamental physics. String theory, the most-studied theory as the final physical theory of nature, should provide an answer to this question. Some of the proposals on how inflation is realized in string theory are reviewed. Since everything is made of strings, some string loops of cosmological sizes are likely to survive in the hot big bang that followed inflation. They appear as cosmic strings, which can have intricate properties. Because of the warped geometry in flux compactification of the extra spatial dimensions in string theory, some of the cosmic strings may have tensions substantially below the Planck or string scale. Such strings cluster in a manner similar to dark matter leading to hugely enhanced densities. As a result, numerous fossil remnants of the low tension cosmic strings may exist within the galaxy. They can be revealed through the optical lensing of background stars in the near future and studied in detail through gravitational wave emission. We anticipate that these cosmic strings will permit us to address central questions about the properties of string theory as well as the birth of our universe.

We study, in this paper, curvature inheritance symmetry (CI), , where α is a scalar function, for string cloud and string fluid in the context of general relativity. Also, we have obtained some result when a proper CI (i.e., α ≠ 0) is also a conformal Killing vector.

We study null bulk geodesic motion in the brane world cosmology in the RS2 scenario and in the static universe in the bulk of the charged topological AdS black hole. We obtain equations describing the null bulk geodesic motion as observed in one lower dimension. We find that the null geodesic motion in the bulk of the brane world cosmology in the RS2 scenario is observed to be under the additional influence of extra non-gravitational force by the observer on the three-brane, if the brane universe does not possess the Z₂ symmetry. As for the null geodesic motion in the static universe in the bulk of the charged AdS black hole, the extra force is realized even when the brane universe has the Z₂ symmetry.

In the standard model, stabilization of a classically unstable cosmic string may occur through the quantum fluctuations of a heavy fermion doublet. We review numerical results from a semiclassical expansion in a reduced version of the standard model. In this expansion, the leading quantum corrections emerge at one loop level for many internal degrees of freedom. The resulting vacuum polarization energy and the binding energies of occupied fermion energy levels are of the same order, and must therefore be treated on equal footing. Populating these bound states lowers the total energy compared to the same number of free fermions and assigns a charge to the string. Charged strings are already stabilized for a fermion mass only somewhat larger than the top quark mass. Though obtained in a reduced version, these results suggest that neither extraordinarily large fermion masses nor unrealistic couplings are required to bind a cosmic string in the standard model. Furthermore, we also review results for a quantum stabilization mechanism that prevents closed Nielsen–Olesen-type strings from collapsing.

Exact cylindrically symmetric solutions of Einstein–Maxwell equations are presented. They are regular on the axis of symmetry, asymptotically flat and free from curvature singularities. The solutions can be interpreted as nonrotating cosmic strings interacting with gravitational and electromagnetic waves.

This paper deals with a study of the cylindrically symmetric accretion of dark energy with equation of state p = wρ onto wiggly straight cosmic strings, interpreting that the accreted energy only contributes the extent of the wiggliness, but consistently leaves invariant the unperturbed line density μ₀. We have obtained that when w > -1, the perturbed linear energy density in the string core gradually increases tending to a finite maximum value as time increases, for all considered dark energy models. Where the dominant energy condition is violated, all such models predict a steady decreasing of the perturbed linear energy density μ of the cosmic strings as phantom energy is being accreted. The final state of the string after such an accretion process is a wiggleless defect. It is argued however, that if accreation of phantom energy would proceed by successive quantum steps, then the defect would continue losing perturbed linear energy density beyond μ₀, until a minimum nonzero value which can be smaller than that corresponding to the unperturbed string.

We study the deformation of a long cosmic string by a nearby rotating black hole. We examine whether the deformation of a cosmic string, induced by the gravitational field of a Kerr black hole, may lead to the formation of a string loop. The segment of the string which enters the ergo-sphere of a rotating black hole gets deformed and, if it is sufficiently twisted, it can self-intersect, chopping off a loop. We find that the formation of a loop, via such a mechanism, is a rare event. It will only arise in a small region of the collision phase space, which depends on the string velocity, the impact parameter and the black hole angular momentum. We conclude that, generically, a long cosmic string is simply scattered, or captured, by a nearby rotating black hole.

Cosmic strings, as remnants of the symmetry breaking phase in the Early universe, may be susceptible to nonlocal physics. Here, we show that the presence of a Poincaré-invariant nonlocality—parametrized by a factor $exp(-\square {\ell }^2)$—regularizes the gravitational field and thereby changes the properties of spacetime: it is now simply connected and the angle deficit around the cosmic string becomes a function of the radial distance. Similar changes occur for the nonlocal gravitomagnetic field of a rotating cosmic string, and we translate these mathematical facts into the language of nonlocal gravitoelectromagnetism and thereby provide a physical interpretation. We hope that these insights might provide a helpful perspective in the search for traces of nonlocal physics in our universe.