arXiv subjects

I. Yu. Rybak

Publications and source records attributed to I. Yu. Rybak.

14 recordsLinked to original sources

arXiv2024

Stochastic Gravitational Wave Background from Chiral Superconducting Cosmic Strings

We investigate the emission of vector radiation by superconducting cosmic string loops, deriving general relations to characterize the vector radiation emission efficiency, and study its impact on the evolution of loops. Building on these results, we compute the stochastic gravitational wave background generated by a chiral superconducting cosmic string network. Our analysis reveals that strong coupling between superconducting cosmic strings and the vector field may lead to a substantial suppression of the gravitational wave signal, while moderate coupling may still produce a detectable signal. We demonstrate that, in this intermediate limit, the presence of superconductivity in cosmic strings may help reconcile their gravitational wave spectrum with pulsar timing array data for large enough values of current.

astro-ph.CO↗

arXiv2024

Cosmic Microwave Background Signatures from Current-carrying Cosmic Strings

We continue our studies of the evolution and cosmological consequences of current-carrying cosmic string networks, described by a charge-velocity-dependent one scale (CVOS) model. We present a detailed calculation of the effects of these networks on the cosmic microwave background (CMB), in the context of this model, and specifically discuss how such current-carrying strings may be distinguished from their uncharged (Nambu-Goto) counterparts by current or forthcoming CMB data. We find that, under the CVOS hypothesis, the constraints on current-carrying strings should not differ much from those of their structureless counterparts in that the impact on the CMB can at most be reduced by a factor of ~25%. Nevertheless, the presence of a current and charge affects the distribution of power among scalar, vector and tensor modes, and also its distribution between small and large scales. It should therefore be possible for future high-sensitivity CMB experiments to distinguish between the two types of strings.

astro-ph.CO↗

arXiv2023

Decay of Nambu-Goto Cosmic String Loops via Coupling to a Massive Kalb-Ramond Field

We study the dynamics of Nambu-Goto cosmic string loops coupled to a massive Kalb-Ramond field. This coupling provides a framework for describing the interaction between the cosmic string and the gauge field. Within this setup, we compute the energy flux associated with the radiation of massive Kalb-Ramond modes from oscillating string loops. For loops featuring kinks, we find that the decay time scales with the square of the loop length. In contrast, loops with cusps exhibit a decay time proportional to 3/2 times the loop length. The results are in good agreement with field-theoretic simulations of Nambu-Goto-like loops in the Abelian-Higgs model, supporting the validity of the calculation.

hep-th↗

arXiv2023

Cosmological evolution of Witten superconducting string networks

We consider the evolution of current-carrying cosmic string networks described by the charge-velocity-dependent one scale (CVOS) model beyond the linear equation of state regime, specifically focusing on the Witten superconducting model. We find that, generically, for almost chiral currents, the network evolution reduces dynamically to that of the linear case, which has been discussed in our previous work. However, the Witten model introduces a maximum critical current which constrains the network scaling behaviour during the radiation era when currents can grow and approach this limit. Unlike the linear model, only if the energy density in the critical current is comparable to the bare string tension will there be substantial backreaction on the network evolution, thus changing the observational predictions of superconducting strings from those expected from a Nambu-Goto network. During the matter era, if there are no external sources, then dynamical effects dilute these network currents and they disappear at late times.

astro-ph.CO↗

arXiv2022

Emission of gravitational waves by superconducting cosmic strings

We study the gravitational radiation emission efficiency $Γ$ of superconducting cosmic strings. We demonstrate, by using a solvable model of transonic strings, that the presence of a current leads to a suppression of the gravitational emission of cusps, kinks and different types of loops. We also show that, when a current is present, the spectrum of emission of loops with cusps is exponentially suppressed as the harmonic mode increases, thus being significantly different from the power law spectrum of currentless loops. Furthermore, we establish a phenomenological relationship between $Γ$ and the value of the current on cosmic strings. We conjecture that this relation should be valid for an arbitrary type of current-carrying string. We use this result to study the potential impact of current on the stochastic gravitational wave background generated by cosmic strings with additional degrees of freedom and show that both the amplitude and shape of the spectrum may be significantly affected.

gr-qc↗

arXiv2021

Charge-velocity-dependent one-scale linear model

We apply a recently developed formalism to study the evolution of a current-carrying string network under the simple but generic assumption of a linear equation of state. We demonstrate that the existence of a scaling solution with non-trivial current depends on the expansion rate of the universe, the initial root mean square current on the string, and the available energy loss mechanisms. We find that the fast expansion rate after radiation-matter equality will tend to rapidly dilute any pre-existing current and the network will evolve towards the standard Nambu-Goto scaling solution (provided there are no external current-generating mechanisms). During the radiation era, current growth is possible provided the initial conditions for the network generate a relatively large current and/or there is significant early string damping. The network can then achieve scaling with a stable non-trivial current, assuming large currents will be regulated by some leakage mechanism. The potential existence of current-carrying string networks in the radiation era, unlike the standard Nambu-Goto networks expected in the matter era, could have interesting phenomenological consequences.

astro-ph.CO↗

arXiv2021

CMB anisotropies generated by cosmic string loops

We investigate the contribution of cosmic string loops to the Cosmic Microwave Background (CMB) anisotropies. This is done by extending the Unconnected Segment Model (USM) to include the contribution of the cosmic string loops created throughout the cosmological evolution of a cosmic string network to the stress-energy tensor. We then implement this extended USM in the publicly available CMBACT code and obtain the linear CDM power spectrum and the CMB angular power spectra generated by cosmic string loops. We find that the shape of the angular power spectra generated by loops is, in general, similar to that of long strings. However, there is generally an enhancement of the anisotropies on small angular scales. Vector modes produced by loops dominate over those produced by long strings for large multipole moments $\ell$. The contribution of loops to the CMB anisotropies generated by cosmic string networks may reach a level of $10\%$ for large loops but decreases as the size of loops decreases. This contribution may then be significant and, thus, this extension provides a more accurate prediction of the CMB anisotropies generated by cosmic string networks.

astro-ph.CO↗

arXiv2020

Generalised velocity-dependent one-scale model for current-carrying strings

We develop an analytic model to quantitatively describe the evolution of superconducting cosmic string networks. Specifically, we extend the velocity-dependent one-scale (VOS) model to incorporate arbitrary currents and charges on cosmic string worldsheets under two main assumptions, the validity of which we also discuss. We derive equations that describe the string network evolution in terms of four macroscopic parameters: the mean string separation (or alternatively the string correlation length) and the root mean square (RMS) velocity which are the cornerstones of the VOS model, together with parameters describing the averaged timelike and spacelike current contributions. We show that our extended description reproduces the particular cases of wiggly and chiral cosmic strings, previously studied in the literature. This VOS model enables investigation of the evolution and possible observational signatures of superconducting cosmic string networks for more general equations of state, and these opportunities will be exploited in a companion paper.

astro-ph.CO↗

arXiv2020

Semianalytic calculation of cosmic microwave background anisotropies from wiggly and superconducting cosmic strings

We study how the presence of world-sheet currents affects the evolution of cosmic string networks, and their impact on predictions for the cosmic microwave background (CMB) anisotropies generated by these networks. We provide a general description of string networks with currents and explicitly investigate in detail two physically motivated examples: wiggly and superconducting cosmic string networks. By using a modified version of the CMBact code, we show quantitatively how the relevant network parameters in both of these cases influence the predicted CMB signal. Our analysis suggests that previous studies have overestimated the amplitude of the anisotropies for wiggly strings. For superconducting strings the amplitude of the anisotropies depends on parameters which presently are not well known - but which can be measured in future high-resolution numerical simulations.

astro-ph.CO↗

arXiv2020

Revisit of Y-junctions for strings with currents: transonic elastic case

We studied the formation of Y-junctions for transonic elastic strings. In particular, using the gen-eral solution for these strings, which is described by left- and right-moving modes, we obtained thedynamics of kinks and Y-junctions. Considering the linearized ansatz for straight strings, we con-structed the parameter region space for which the formation of Y-junctions due to strings collisionsis allowed.

astro-ph.CO↗

arXiv2019

Dynamics of junctions and the multi-tension velocity-dependent one-scale model

The dynamics of string junctions and their influence on the evolution of cosmic superstring networks are studied in full detail. We review kinematic constraints for colliding strings in a Friedmann-Lemaître-Robertson-Walker background and obtain the average distribution of possible string configurations after string collisions. The study of small-scale structure enables us to investigate the average growth/reduction rate of string junctions for a given cosmic string network. Incorporating the averaged junction dynamics into the velocity-dependent one-scale model for multi-tension string networks, we improve the semi-analytic description and quantitative understanding of cosmic superstring network evolution.

astro-ph.CO↗

arXiv2018

Collisions of cosmic strings with chiral currents

We present an analytic study of cosmic superconducting chiral string collisions in Minkowski space, applying the kinematic constraints that arise from the relevant generalization of the Nambu-Goto action. In particular, we revisit the solution for chiral superconducting cosmic strings and demonstrate that Y junction production for such strings is possible. We consider the collision of chiral current-carrying straight strings and obtain the region in angle-velocity space that allows the production of string junctions. This study contributes to the understanding of the complex evolution of chiral superconducting string networks.

astro-ph.CO↗

arXiv2016

Stretching and Kibble scaling regimes for Hubble-damped defect networks

The cosmological evolution of topological defect networks can broadly be divided into two stages. At early times they are friction-dominated due to particle scattering and therefore non-relativistic, and may either be conformally stretched or evolve in the Kibble regime. At late times they are relativistic and evolve in the well known linear scaling regime. In this work we show that a sufficiently large Hubble damping (that is a sufficiently fast expansion rate) leads to a linear scaling regime where the network is non-relativistic. This is therefore another realization of a Kibble scaling regime, and also has a conformal stretching regime counterpart which we characterize for the first time. We describe these regimes using analytic arguments in the context of the velocity-dependent one-scale model, and we confirm them using high-resolution $4096^3$ field theory simulations of domain wall networks. We also use these simulations to improve the calibration of this analytic model for the case of domain walls.

hep-ph↗

arXiv2016

Extending the velocity-dependent one-scale model for domain walls

We report on an extensive study of the evolution of domain wall networks in Friedmann-Lema\^ıtre-Robertson-Walker universes by means of the largest currently available field-theory simulations. These simulations were done in $4096^3$ boxes and for a range of different fixed expansion rates, as well as for the transition between the radiation and matter eras. A detailed comparison with the velocity-dependent one-scale (VOS) model shows that this cannot accurately reproduce the results of the entire range of simulated regimes if one assumes that the phenomenological energy loss and momentum parameters are constants. We therefore discuss how a more accurate modeling of these parameters can be done, specifically by introducing an additional mechanism of energy loss (scalar radiation, which is particularly relevant for regimes with relatively little damping) and a modified momentum parameter which is a function of velocity (in analogy to what was previously done for cosmic strings). We finally show that this extended model, appropriately calibrated, provides an accurate fit to our simulations.

hep-ph↗