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Complicated cosmic string loops will fragment until they reach simple, non-intersecting ("stable") configurations. Through extensive numerical study, these attractor loop shapes are characterized including their length, velocity, kink, and cusp distributions. An initial loop containing $M$ harmonic modes will, on average, split into 3M stable loops. These stable loops are approximately described by the degenerate kinky loop, which is planar and rectangular, independently of the number of modes on the initial loop. This is confirmed by an analytic construction of a stable family of perturbed degenerate kinky loops. The average stable loop is also found to have a 40% chance of containing a cusp. This new analytic scheme explicitly solves the string constraint equations.
This code is a quick and exact calculator of B-mode angular spectrum due to Faraday rotation by stochastic magnetic fields. Faraday rotation induced B-modes can provide a distinctive signature of primordial magnetic fields because of their characteristic frequency dependence and because they are only weakly damped on small scales, allowing them to dominate B-modes from other sources. By numerically solving the full CMB radiative transport equations, we study the B-mode power spectrum induced by stochastic magnetic fields that have significant power on scales smaller than the thickness of the last scattering surface. Constraints on the magnetic field energy density and inertial scale are derived from WMAP 7-year data, and are stronger than the big bang nucleosynthesis (BBN) bound for a range of parameters. Observations of the CMB polarization at smaller angular scales are crucial to provide tighter constraints or a detection.
This code is based on the cosmic string model described in this paper by Pogosian and Vachaspati, as well as on the CMBFAST code created by Uros Seljak and Matias Zaldarriaga. It contains an integrator for the vector contribution to the CMB temperature and polarization. The code is reconfigured to make it easier to use with or without active sources. To produce inflationary CMB spectra one simply sets the string tension to zero (gmu=0.0d0). For a non-zero value of tension only the string contribution is calculated.
An option is added to randomize the directions of velocities of consolidated segments as they evolve in time. In the original segment model, which is still the default version (irandomv=0), each segment is given a random velocity initially, but then continues to move in a straight line for the rest of its life. The new option (irandomv=1) allows to additionally randomize velocities of each segment at roughly each Hubble time. However, the merits of this new option are still under investigation. The default version (irandomv=0) is strongly recommended, since it actually gives reasonable unequal time correlators. For each Fourier mode, k, the string stress-energy components are now evaluated on a time grid sufficiently fine for that k.