Limits on Cosmic Chiral Vortons

We study chiral vorton production for Witten-type superconducting string models in the context of a recently developed analytic formalism. We delineate three distinct scenarios: First, a low energy regime (including the electroweak scale) where vortons can be a source of dark matter. Secondly, an intermediate energy regime where the vorton density is too high to be compatible with the standard cosmology (thereby excluding these models). Finally, a high energy regime (including the GUT scale) in which no vortons are expected to form. The vorton density is most sensitive to the order of the string-forming phase transition and relatively insensitive to the current-forming transition. For a second-order string transition, vorton production is cosmologically disastrous for the range 10^{-28}\lsim G\mu \lsim 10^{-10} (10^{5} GeV \lsim T_{c} \lsim 10^{14} GeV), while for the first-order case we can only exclude 10^{-20}\lsim G\mu \lsim 10^{-14} (10^{9} GeV \lsim T_{c} \lsim 10^{12} GeV). We provide a fitting formula which summarises our results.Comment: 9 LaTeX pages, 5 .eps files; submitted to Phys.Lett.

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-Lemaˆıtre- Robertson-Walker universes by means of the largest currently available field-theory simulations. These simulations were done in 40963 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

Radiation constraints from cosmic strings

We show that it is possible to evolve a network of global strings numerically including the effects of radiative backreaction, using the renormalised equations for the Kalb-Ramond action. We calculate radiative corrections to the equations of motion and deduce the effect on a network of global strings. We also discuss the implications of this work for the cosmological axion density.Comment: 4 Pages, UUencoded postscript file, to appear in 'Trends in Astro-Particle Physics - Nuclear Physics B, Proceedings Supplement

Spectrum of radiation from axion strings

In the wide variety of axion cosmologies in which axion strings form, their radiative decay is the dominant mechanism for the production of axions, imposing a tight constraint on the axion mass. Here, we focus on the mechanism by which axions are produced in this scenario and, in particular, the key issue of the axion spectrum emitted by an evolving network of strings.Comment: to be published in the proceedings of the 5th IFT Workshop on Axion

Accurate Calibration of the Velocity-dependent One-scale Model for Domain Walls

We study the asymptotic scaling properties of standard domain wall networks in several cosmological epochs. We carry out the largest field theory simulations achieved to date, with simulation boxes of size 20483, and confirm that a scale-invariant evolution of the network is indeed the attractor solution. The simulations are also used to obtain an accurate calibration for the velocity-dependent one-scale model for domain walls: we numerically determine the two free model parameters to have the values $c_w = 0.34\pm0.16$ and $k_w = 0.98\pm0.07$, which are higher precision than (but in agreement with) earlier estimates.Comment: 8 pages, version to appear in Phys. Lett. B. arXiv admin note: substantial text overlap with arXiv:1110.348

On the Behaviour and Stability of Superconducting Currents

We present analytic and numerical results for the evolution of currents on superconducting strings in the classical $U(1) \times U(1)$ model. We derive an energy functional for the currents and charges on these strings, establishing rigorously that minima should exist in this model for loops of finite size (vortons) if both charge and current are present on the worldsheet. We then study the stability of the currents on these strings, and we find an analytic criterion for the onset of instability (in the neutral limit). This limit specifies a lower maximal current than previous heuristic estimates. We conclude with a discussion of the evolution of loops towards their final vorton state in the model under consideration.Comment: 18 pages, 6 figures. To be submitted to Nuclear Physics

Waveforms for Gravitational Radiation from Cosmic String Loops

We obtain general formulae for the plus- and cross- polarized waveforms of gravitational radiation emitted by a cosmic string loop in transverse, traceless (synchronous, harmonic) gauge. These equations are then specialized to the case of piecewise linear loops, and it is shown that the general waveform for such a loop is a piecewise linear function. We give several simple examples of the waveforms from such loops. We also discuss the relation between the gravitational radiation by a smooth loop and by a piecewise linear approximation to it.Comment: 16 pages, 6 figures, Revte

Isocurvature modes and Baryon Acoustic Oscillations

The measurement of Baryonic Acoustic Oscillations from galaxy surveys is well known to be a robust and powerful tool to constrain dark energy. This method relies on the knowledge of the size of the acoustic horizon at radiation drag derived from Cosmic Microwave Background Anisotropy measurements. In this paper we quantify the effect of non-standard initial conditions in the form of an isocurvature component on the determination of dark energy parameters from future BAO surveys. In particular, if there is an isocurvature component (at a level still allowed by present data) but it is ignored in the CMB analysis, the sound horizon and cosmological parameters determination is biased, and, as a consequence, future surveys may incorrectly suggest deviations from a cosmological constant. In order to recover an unbiased determination of the sound horizon and dark energy parameters, a component of isocurvature perturbations must be included in the model when analyzing CMB data. Fortunately, doing so does not increase parameter errors significantly.Comment: 23 pages, 3 figure

Scattering off an SO(10) cosmic string

The scattering of fermions from the abelian string arising during the phase transition $SO(10) \rightarrow SU(5) \times Z_2$ induced by the Higgs in the 126 representation is studied. Elastic cross-sections and baryon number violating cross-sections due to the coupling to gauge fields in the core of the string are computed by both a first quantised method and a perturbative second quantised method. The elastic cross-sections are found to be Aharonov-Bohm type. However, there is a marked asymmetry between the scattering cross-sections for left and right handed fields. The catalysis cross-sections are small, depending on the grand unified scale. If cosmic strings were observed our results could help tie down the underlying gauge group.Comment: 20 page

The CMB Bispectrum

We use a separable mode expansion estimator with WMAP data to estimate the bispectrum for all the primary families of non-Gaussian models. We review the late-time mode expansion estimator methodology which can be applied to any non-separable primordial and CMB bispectrum model, and we demonstrate how the method can be used to reconstruct the CMB bispectrum from an observational map. We extend the previous validation of the general estimator using local map simulations. We apply the estimator to the coadded WMAP 5-year data, reconstructing the WMAP bispectrum using $l<500$ multipoles and $n=31$ orthonormal 3D eigenmodes. We constrain all popular nearly scale-invariant models, ensuring that the theoretical bispectrum is well-described by a convergent mode expansion. Constraints from the local model \fnl=54.4\pm 29.4 and the equilateral model \fnl=143.5\pm 151.2 (\Fnl = 25.1\pm 26.4) are consistent with previously published results. (Here, we use a nonlinearity parameter \Fnl normalised to the local case, to allow more direct comparison between different models.) Notable new constraints from our method include those for the constant model \Fnl = 35.1 \pm 27.4 , the flattened model \Fnl = 35.4\pm 29.2, and warm inflation \Fnl = 10.3\pm 27.2. We investigate feature models surveying a wide parameter range in both the scale and phase, and we find no significant evidence of non-Gaussianity in the models surveyed. We propose a measure \barFnl for the total integrated bispectrum and find that the measured value is consistent with the null hypothesis that CMB anisotropies obey Gaussian statistics. We argue that this general bispectrum survey with the WMAP data represents the best evidence for Gaussianity to date and we discuss future prospects, notably from the Planck satellite