Global Cosmic String Research Articles

Axion string source modeling

In this paper, we investigate the effect of the string radius of curvature RGaussian on the massive and massless scalar radiation emitted from collisions of traveling waves propagating along an axion (global cosmic) string. We construct initial conditions for two colliding Gaussians and perform parameter scans over their amplitude A and standard deviation σd. We show that these collisions emit isotropic bursts of massless radiation and that the energy emitted via this channel obeys a power law ∼∝Aγ, where the coefficient γ depends on the regime of A/δ and δ is the string width. Massive radiation is exponentially suppressed ∼∝e−ζRGaussian in the quasilinear regime σd≫δ and exhibits power-law decay ∼∝RGaussian−γ′ in the nonlinear regime σd≲2δ, with different ζ and γ′ in different regimes of RGaussian. In this nonlinear regime, massive particle radiation can comprise up to 50% of the total energy emitted. Drawing on a known parallel between axion radiation from global strings and gravitational radiation from Abelian-Higgs strings, this suggests that massive particle radiation may become significant with respect to the massless (gravitational) channel for such nonlinear burst signals. For all configurations studied, we obtain a spectral index q≳1 for the axion radiation, where q→1 as A increases; i.e., a higher proportion of radiation is emitted in high-frequency modes. Published by the American Physical Society 2024

Cosmic string gravitational waves from global U(1)B−L symmetry breaking as a probe of the type I seesaw scale

In type I seesaw models, the right-handed neutrinos are typically super-heavy, consistent with the generation of baryon asymmetry via standard leptogenesis. Primordial gravitational waves of cosmological origin provides a new window to probe such high scale physics, which would otherwise be inaccessible. By considering a global U(1)B−L extension of the type I seesaw model, we explore the connection between the heaviest right-handed neutrino mass and primordial gravitational waves arising from the dynamics of global cosmic string network. As a concrete example, we study a global U(1)B−L extension of the Littlest Seesaw model, and show that the inevitable GW signals, if detectable, probe the parameter space that can accommodate neutrino oscillation data and successful leptogenesis, while respecting theoretical constraints like perturbativity of the theory. Including CMB constraints from polarization and dark radiation leaves a large region of parameter space of the model, including the best fit regions, which can be probed by GW detectors like LISA and ET in the near future. In general, the GW detectors can test high scale type I seesaw models with the heaviest right-handed neutrino mass above 2.5 × 1014 GeV, assuming the perturbativity, and 7 × 1013 GeV assuming that the coupling between the heaviest right-handed neutrino and the U(1)B−L breaking scalar is less than unity.

Gravitational waves from global cosmic strings and cosmic archaeology

Global cosmic strings are predicted in many motivated extensions to the Standard Model of particle physics, with close connections to axion dark matter physics. Recent studies suggest that, although subdominant relative to Goldstone emission, gravitational wave (GW) signals from global strings can be detectable with current and planned GW detectors such as LIGO, LISA, DECIGO/BBO, ET/CE and AEDGE/AION, as well as pulsar timing arrays such as PPTA, NANOGrav and SKA. This work is an extensive, updated study on GWs from a global cosmic string network, taking into account of the most recent developments related to the subject. The main analysis is based on the analytical Velocity-dependent One-Scale (VOS) model calibrated with recent simulation results, which provides a generic protocol for such calculations with details given. We also demonstrate how the GW signal can be influenced with variations to the baseline model: this includes considering the uncertainties of model parameters and the potential deviation from the conventional VOS model prediction (i.e. the scaling behavior) as suggested by some of the recent simulation results. Furthermore, we investigated in detail the effect of a non-standard cosmology (e.g. early matter domination or kination) or new particle species on the GW signals from global strings. We demonstrate that the frequency spectrum of GW background from global cosmic strings can be used to probe the cosmic history prior to the Big Bang nucleosynthesis (BBN) (i.e. the primordial dark age) up to a temperature of T ∼ 108 GeV.

Global cosmic string networks as a function of tension

We investigate the properties of global cosmic string networks as a function of the ratio of string tension to Goldstone-field coupling, and as a function of the Hubble damping strength. Our results show unambiguously that the string density is sensitive to this ratio. We also find that existing semi-analytical (one-scale) models must be missing some important aspect of the network dynamics. Our results point the way towards improving such models.

Gravitational waves from global cosmic strings in quintessential inflation

The combination of non-minimal couplings to gravity with the post-inflationary kinetic-dominated era typically appearing in quintessential inflation scenarios may lead to the spontaneous symmetry breaking of internal symmetries and its eventual restoration at the onset of radiation domination. On general grounds, the breaking of these symmetries leads to the generation of short-lived topological defects that tend to produce gravitational waves until the symmetry is restored. We study here the background of gravitational waves generated by a global cosmic string network following the dynamical symmetry breaking and restoration of a U(1) symmetry. The resulting power spectrum depends on the duration of the heating process and it is potentially detectable, providing a test on the existence of non-minimal couplings to gravity and the characteristic energy scale of post-inflationary physics.

Diffraction of light by topological defects in liquid crystals

Light scattering due to hedgehog-like and linear disclination topological defects in a nematic liquid crystal has been studied. Light propagation close to such defects appears to be a metric equivalent to the spatial element of the global monopole and cosmic string geometries. Using a metric approach, the amplitude of the scattering and the differential and total scattering cross section have been obtained for the hedgehog defect in terms of the characteristic parameters of a liquid crystal. In the case of disclination, a cylindrical partial wave method has been developed. Applying the results of earlier studies, we have also examined the influence of temperature on the localisation of the diffraction patterns.

Towards a hybrid compactification with a scalar–tensor global cosmic string

We derive a solution of the gravitational equations which leads to a braneworld scenario insix dimensions, using a global cosmic string solution in a low energy effective string theoryframework. The final spacetime is composed by one warped brane with topology and a power law warp factor, and one non-compact extra dimension transverseto the brane; by looking at the current experimental bounds, we find a range of parametersin which, if the on-brane dimension has an acceptable size, it does not solve the hierarchyproblem. In another example, this problem is smoothed by means of the Brans–Dickeparameter.

Topology in the little Higgs models

We investigate the implications of the nontrivial vacuum structure of little Higgs models. In particular, focusing on the littlest Higgs model, we demonstrate the existence of three types of topological defects. One is a global cosmic string that is truly topological. The second is more subtle; a semilocal cosmic string, which may be stable due to dynamical effects. The final defect is a $Z_2$ monopole solution with an unusual structure. We briefly discuss the possible cosmological consequences of such nonperturbative structures, although we note that these depend crucially on the fermionic content of the models.

Non-singulardeformations of singularcompactifications, the cosmological constant and the hierarchy problem

We consider deformations of the singular `global cosmic string'compactifications, known to naturally generate exponentially largescales. The deformations are obtained by allowing aconstant-curvature metric on the brane and correspond to a choice ofintegration constant. We show that there exists a unique value ofthe integration constant that gives rise to a non-singular solution.The metric on the brane is dS4 with an exponentially smallvalue ofthe expansion parameter. We derive an upper bound on the branecosmological constant. We find and investigate more generalsingular solutions - `dilatonic global string'compactifications - and show that they can have non-singulardeformations. We give an embedding of these solutions in type IIBsupergravity. There is only one class of supersymmetry-preservingsingular dilatonic solutions. We show that they do not havenon-singular deformations of the type considered here.

Static cosmic strings in Brans-Dicke theory

The gravitational fields of both local and global static cosmic strings in the context of Brans-Dicke theory are investigated. It is shown that, for a local cosmic string, Vilenkin's prescription for the energy-momentum tensor due to the string is inconsistent with the Brans-Dicke theory. For the global string, two kinds of solutions for the spacetime outside the string core have been presented, one of which is given in a closed form and may be nonsingular at a finite distance from the string core while the other one cannot be written in a closed form and moreover contains a singularity at a finite distance from the string core. The motions of photons and timelike particles in the field of the global string are also investigated in one of the cases. It has been shown that such a global cosmic string has only a repulsive gravitational effect on a freely moving particle.

Gravitational field of a global cosmic string with non-linear Lagrangian

In this paper the metric outside a straight global string is derived, with the high-derivative gravitational theory which has a higher correction as compared with Einstein gravitational theory. The effect of accreting matter and the formation of the large-scale structure of the Universe may be very important.

Excitation fields in a superconducting global string

A model of a straight superconducting global cosmic string is examined in a setting wherein the string supports a charge/current pulse described by a travelling wave along the string. Linearized field equations are obtained for fluctuations of the scalar and vector fields of the theory, and a set of approximate particular solutions are found for the case in which the linear charge density and the current of the string have equal magnitudes. Although the equations of motion seem to suggest that the scalar and vector excitation fields are massive inside the string core, the particular solutions show that they behave as effectively massless fields which propagate at the speed of light along the string along with the primary charge/current pulse. The effect of the mass parameter is to modulate the radial profile of the excitation fields. The vector excitation field generates radial and angular components for both the electric and magnetic fields, but the particular solutions do not describe the emission or absorption of electromagnetic radiation from the string.

Gravitational field of a superconducting global string

The Newtonian gravitational field of a superconducting global cosmic string is investigated using the weak-field approximation, and is found to receive contributions from the global string field, the condensate field, and the electromagnetic fields. For certain parameter ranges the gravitational field in the region exterior to the string may be everywhere repulsive, everywhere attractive, or repulsive in some regions and attractive in others. For a neutral superconducting global string there can exist a particular current for which the gravitational field of the superconducting global string approximately coincides with that of a Grand Unified Theory (GUT) scale Weyl string, and, as pointed out by Widom, Srivastava, and Redington, observed asymptotic galactic rotation velocities can be roughly explained without the introduction of any further dark-matter hypothesis.

Gravitational effects of traveling waves along global cosmic strings.

Making use of the relationship between the corresponding field configurations, we derive the metric around a straight global cosmic string with traveling waves in terms of the static metric (without traveling waves) in the weak-field limit. We discuss under which conditions the effect of the traveling waves may overcome the repulsive gravitational potential of the static straight global string. We also extend the calculation beyond the weak-field limit combining our result with the recent observation made by Garfinkle and Vachaspati that the exact solution must be of the generalized Kerr-Schild type.

Electromagnetic radiation from a global cosmic string in an external magnetic field

A global cosmic string may couple to electromagnetism as a consequence of anomalous triangle diagrams. In an external magnetic field, the gradients of the Nambu-Goldstone boson around the string give rise to an extended polarization current. When the string oscillates, it radiates electromagnetic waves. We estimate the strength of the effect for representative values of astrophysical magnetic fields. We also discuss implications for axion strings if a primordial cosmological magnetic field existed at early times.