Low frequency gravity wave spectra generated by cosmic strings

Gravity wave spectra generated by populations of cosmic string loops are computed, showing that loop evolution and radiation properties have important effects at observable wavelengths. The spectra are compared with limits derived from various observations; bounds from millisecond pulsar timing are becoming severe for several string scenarios, but implied constraints on string parameters depend sensitively on details of the loop population and dynamics.     

Gravitational-wave stochastic background from cosmic strings

We consider the stochastic background of gravitational waves produced by a network of cosmic strings and assess their accessibility to current and planned gravitational wave detectors, as well as to big bang nucleosynthesis (BBN), cosmic microwave background (CMB), and pulsar timing constraints. We find that current data from interferometric gravitational wave detectors, such as Laser Interferometer Gravitational Wave Observatory (LIGO), are sensitive to areas of parameter space of cosmic string models complementary to those accessible to pulsar, BBN, and CMB bounds. Future more sensitive LIGO runs and interferometers such as Advanced LIGO and Laser Interferometer Space Antenna (LISA) will be able to explore substantial parts of the parameter space.     

Cosmic sparks from superconducting strings

We investigate cosmic sparks from cusps on superconducting cosmic strings in light of the recently discovered millisecond radio burst by Lorimer et al.. We find that the observed duration, fluence, spectrum, and event rate can be reasonably explained by grand unification scale superconducting cosmic strings that carry currents approximately 10{5} GeV. The superconducting string model predicts an event rate that falls off only as S{-1/2}, where S is the energy flux, and hence predicts a population of very bright bursts. Other surveys, with different observational parameters, are shown to impose tight constraints on the superconducting string model.     

Anomalies and effective lagrangians of superconducting cosmic strings

We study the effective lagrangians of superconducting cosmic strings and the underlying anomaly structure leading to various effects. We show that fermionic superconducting cosmic (Nielsen-Olesen) strings arise as a consequence of a mixed anomaly of the form E · B′ where E is the electromagnetic field strength and B′ the core flux of the string. We show how this controls superconductivity, the index theorems, topological mass terms on domain walls, and string-axion interactions. We contrast this to global strinfs with Chern-Simons currents.     

The Role of Magnetic Field in Anisotropic String Cosmological Model

In this paper we consider a spatially homogenous and anisotropic Bianchi type-V space-time model to investigate the effects of a magnetic field in string cosmology. We assume that the string’s direction and magnetic field are along x-axis. The field equations are solved by using the equation of state for a cloud of strings and variable magnetic permeability. We derive exact solutions for three types of strings: (i) Nambu strings, (ii) string model where the sum of energy density and string tension density is zero and (iii) Takabayasi strings. We examine the behaviour of scale factors and other physical parameters with and without magnetic field and it is found that the magnetic field effects the dynamics of the universe at early time. During late time the universe becomes isotropic even in the presence of magnetic field. The universe expands with decelerated rate during early stages of the evolution of the universe but it goes to marginal inflation at late times.     

Constraints on the fundamental string coupling from B-mode experiments

We study signatures of cosmic superstring networks containing strings of multiple tensions and Y junctions, on the cosmic microwave background (CMB) temperature and polarization spectra. Focusing on the crucial role of the string coupling constant g(s), we show that the number density and energy density of the scaling network are dominated by different types of string in the g(s) ~ 1 and g(s) ? 1 limits. This can lead to an observable shift in the position of the B-mode peak--a distinct signal leading to a direct constraint on g(s). We forecast the joint bounds on g(s) and the fundamental string tension μ(F) from upcoming and future CMB polarization experiments, as well as the signal to noise in detecting the difference between B-mode signals in the limiting cases of large and small g(s). We show that such a detectable shift is within reach of planned experiments.     

Fitting cosmic microwave background data with cosmic strings and inflation

We perform a multiparameter likelihood analysis to compare measurements of the cosmic microwave background (CMB) power spectra with predictions from models involving cosmic strings. Adding strings to the standard case of a primordial spectrum with power-law tilt ns, we find a 2sigma detection of strings: f10=0.11+/-0.05, where f10 is the fractional contribution made by strings in the temperature power spectrum (at l=10). CMB data give moderate preference to the model ns=1 with cosmic strings over the standard zero-strings model with variable tilt. When additional non-CMB data are incorporated, the two models become on a par. With variable ns and these extra data, we find that f10<0.11, which corresponds to Gmicro<0.7x10(-6) (where micro is the string tension and G is the gravitational constant).     

Statistical properties of strings

We investigate numerically the configurational statistics of strings. The algorithm models an ensemble of global U(1) cosmic strings, or equivalently vortices in superfluid ⁴He. We use a new method which avoids the specification of boundary conditions on the lattice. We therefore do not have the artificial distinction between short and long string loops or a “second phase” in the string network statistics associated with strings winding around a toroidal lattice. Our lattice is also tetrahedral, which avoids ambiguities associated with the cubic lattices of previous work. We find that the percentage of infinite string is somewhat lower than on cubic lattices, 63% instead of 80%. We also investigate the Hagedorn transition, at which infinite string percolate, controlling the string density by rendering one of the equilibrium states more probable. We measure the percolation threshold, the critical exponent associated with the divergence of a suitably defined susceptibility of the string loops, and that associated with the divergence of the correlation length.     

Cosmic (Super)String Constraints from 21 cm Radiation

We calculate the contribution of cosmic strings arising from a phase transition in the early Universe, or cosmic superstrings arising from brane inflation, to the cosmic 21 cm power spectrum at redshifts z > or =30. Future experiments can exploit this effect to constrain the cosmic string tension G mu and probe virtually the entire brane inflation model space allowed by current observations. Although current experiments with a collecting area of approximately 1 km2 will not provide any useful constraints, future experiments with a collecting area of 10(4)-10(6) km2 covering the cleanest 10% of the sky can, in principle, constrain cosmic strings with tension G mu > or = 10(-10)-10(-12) (superstring/phase transition mass scale >10(13) GeV).     

Test particle trajectories near cosmic strings

We present a detailed analysis of the motion of test particle in the gravitational field of cosmic strings in different situations using the Hamilton-Jacobi (H-J) formalism. We have discussed the trajectories near static cosmic string, cosmic string in Brans-Dicke theory and cosmic string in dilaton gravity.     

Polynomial solutions for coupledU(1)-gauge einstein equations

We point out numerical solutions in polynomial form to the field equations derived by Garfinkle and Laguna forU(1)-gauge cosmic strings. With these solutions we evaluate the second-order term for the deficit angle produced by the string.     

String cosmology in Bianchi type-VI0 dusty Universe with electromagnetic field

In this paper, the effect of electromagnetic field in the string Bianchi type-VI₀ 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.     

On the evolution of global strings in the early universe

We find that a bent global string straightens itself out by dissipation into Nambu-Goldstone (NG) bosons in the order of one oscillation time and that the spectrum of radiated NG bosons is 1/k. In the early universe, the assumption that there is one global string per causal horizon is justified after an initial period during which the effect of the primordial plasma on the motion of the strings is non-negligible. We find that the upper bound that axion radiation by strings places on the axion decay constant is comparable to, but not more stringent than the one derived from coherent oscillations of the axion field, i.e. ∫_a ≲ 10¹²GeV. We also discuss the model-dependence of this bound.     

Probabilistic estimates of the number of cosmic strings

The dependences of the mean expected number of cosmic strings on their redshift up to the surface of last scattering have been derived. The calculations are based on the geometric probability of a straight string segment crossing a given field and on information about the absence of strings when they are searched for via their gravitational lensing effects in optical catalogs. It is shown that there are no strings for redshifts 0 < z < 1.954, but the expected number of strings for 0 < z < 5 can be no more than 2.1 × 10³ at the 95% confidence level. The expected number of strings for redshifts up to z = 1100 can be no more than 2.4 × 10⁴ at the 95% confidence level. The latter estimate is sensitive to a priori information about the absence of cosmic strings in the redshift range 0 < z < 1.954 in a field of 4.48 square degrees in optical catalogs; it is smaller than the estimate without allowance for this information by 6%.     

Superconducting strings

It is known that certain spontaneously broken gauge theories give rise to stable strings or vortex lines. In this paper it is shown that under certain conditions such strings behave like superconducting wires whose passage through astrophysical magnetic fields would generate a variety of striking and perhaps observable effects. The superconducting charge carriers may be either bosons (if a charged Higgs field has an expectation value in the core of the string) or fermions (if charged fermions are trapped in zero modes along the string, as is known to occur in certain circumstances). They might be observable as synchrotron sources or as sources of high-energy cosmic rays. If the charge carriers are ordinary quarks and leptons, the strings have important baryon number violating interactions with magnetic fields; such a string, traversing a galactic magnetic field of 10^?6 G, creates baryons (or antibaryons) at a rate of order 10¹² particles/cm of string per second.     

Strong Field Gravitational Lensing in a Magnetic Charged Reissner-Nordstr?m Black Hole Pierced by a Cosmic String

The principal focus of this paper is to study the strong field gravitational lensing in a magnetic charged Reissner-Nordstr?m black hole based on the method of cosmic string. We obtain the new coefficients including the tension of the cosmic strings, the strong field deflection limit coefficients, the deflection angle and the magnification, and obtain the relationship between the cosmic string parameter and the new coefficients. The result shows that the cosmic strings have some important effect on the gravitational lensing in a black hole when they pierce it.     

Strings and statistics

We discuss the possibility of unusual statistics in four dimensional string theory through the use of Aharonov-Bohm like phases. Such phases are closely related to anomaly cancellation in four dimensions and thus can be exactly calculated in terms of the low energy fermion spectrum of the theory. We illustrate this connection by calculating the statistical phases exactly in sample models of heterotic string compactification. We discuss some of the difficulties in applying these ideas to fundamental strings, and mention some possible applications to cosmic string scenarios.     

Spiky strings on NS5-branes

We study rigidly rotating strings in the near-horizon geometry of a stack of Neveu-Schwarz (NS) 5-branes. We solve the Nambu-Goto action of the fundamental string in the presence of a NS-NS two form (Bμν) and find out limiting cases corresponding to magnon and spike like solutions.     

Nonsupersymmetric string solitons

We begin a search for nonsupersymmetric/supersymmetric dual string pairs by constructing candidate critical nonsupersymmetric strings as solitons in supersymmetric string theories. Using orbifold techniques, one can construct cosmic string solutions which lie in supersymmetric vacua but which do not fall in supermultiplets. We discuss two three-dimensional examples in detail. The effective worldsheet actions for the soliton strings have (0,2) and (1,1) supersymmetry and the correct numbers of massless bosons and fermions to be critical heterotic and type II strings, respectively.