Dilaton and axion bremsstrahlung from collisions of cosmic (super)strings (original) (raw)

Dilaton production in string cosmology

Physical Review D, 1994

We consider the coupled evolution of density, (scalar) metric and dilaton perturbations in the transition from a "stringy" phase of growing curvature and gravitational coupling to the standard radiation-dominated cosmology. We show that dilaton production, with a spectrum tilted towards large frequencies, emerges as a general property of this scenario. We discuss the frame-independence of the dilaton spectrum and of the inflationary properties of the metric background by using, as model of source, a pressureless gas of weakly interacting strings, which is shown to provide an approximate but consistent solution to the full system of background equations and string equations of motion. We combine various cosmological bounds on a growing dilaton spectrum with the bound on the dilaton mass obtained from tests of the equivalence principle, and we find allowed windows compatible with a universe presently dominated by a relic background of dilatonic dark matter.

Dark matter axions in models of string cosmology

Physics Letters B, 1998

Axions are produced during a period of dilaton-driven inflation by amplification of quantum fluctuations. We show that for some range of string cosmology parameters and some range of axion masses, primordial axions may constitute a large fraction of the present energy density in the universe in the form of cold dark matter. Due to the periodic nature of the axion potential energy density fluctuations are strongly suppressed. The spectrum of primordial axions is not thermal, allowing a small fraction of the axions to remain relativistic until quite late.

Local axion cosmic strings from superstrings

Physics Letters B, 2005

Axionic cosmic string solutions are investigated in a superstring motivated model with a pseudo-anomalous U (1) gauge symmetry. The inclusion of a gauge field and spatially varying dilaton allow local defect solutions with finite energy per unit length to be found. Fermion zero modes (whose presence is implied by supersymmetry) are also analysed. The corresponding fermion currents suggest strong cosmological bounds on the model. It is shown that the unusual form of the axion strings weakens these bounds. Other cosmological constraints on the underlying theory are also discussed. * Stephen.Davis@epfl.ch † Pierre.Binetruy@th.u-psud.fr ‡ A.C. Davis@damtp.cam.ac.uk

Cerenkov radiation from collisions of straight cosmic (super)strings

2006

We consider Cerenkov radiation which must arise when randomly oriented straight cosmic (super)strings move with relativistic velocities without intercommutation. String interactions via dilaton, two-form and gravity (gravity being the dominant force in the ultra-relativistic regime) leads to formation of superluminal sources which generate Cerenkov radiation of dilatons and axions. Though the effect is of the second order in the couplings of strings to these fields, its total efficiency is increased by high dependence of the radiation rate on the Lorentz-factor of the collision.

Observable signals in a string inspired axion-dilaton background and Randall-Sundrum scenario

Physical Review D, 2005

Rotation angle of the plane of polarization of the distant galactic radio waves has been estimated in a string inspired axion-dilaton background. It is found that the axion,dual to the field strength of the second rank antisymmetric massless Kalb-Ramond field in the string spectrum, produces a wavelength independent optical rotation which is much larger than that produced by the dilaton. Detection of such rotation has been reported in some recent cosmological experiments. The observed value has been compared with our estimated theoretical value following various cosmological constraints. The effects of warped extra dimensions in a braneworld scenario on such an optical rotation have been investigated.

Parametric resonances in axionic cosmic strings

Journal of Cosmology and Astroparticle Physics

In this letter we uncover a new parametric resonance of axionic cosmic strings. This process is triggered by the presence on the string of internal mode excitations that resonantly amplify the amplitude of transverse displacements of the string. We study this process by running numerical simulations that demonstrate the existence of this phenomenon in a (3+1) dimensional lattice field theory and compare the results with the analytic expectations for the effective Lagrangian of the amplitude of these modes and their interactions. Finally, we also analyze the massless and massive radiation produced by these excited strings and comment on its relevance for the interpretation of the results of current numerical simulations of axionic cosmic string networks.

String theoretic QCD axions in the light of PLANCK and BICEP2

Journal of High Energy Physics, 2014

The QCD axion solving the strong CP problem may originate from antisymmetric tensor gauge fields in compactified string theory, with a decay constant around the GUT scale. Such possibility appears to be ruled out now by the detection of tensor modes by BICEP2 and the PLANCK constraints on isocurvature density perturbations. A more interesting and still viable possibility is that the string theoretic QCD axion is charged under an anomalous U(1) A gauge symmetry. In such case, the axion decay constant can be much lower than the GUT scale if moduli are stabilized near the point of vanishing Fayet-Illiopoulos term, and U(1) A-charged matter fields get a vacuum value v ∼ (m SUSY M n P l) 1/(n+1) (n ≥ 0) induced by a tachyonic SUSY breaking mass m SUSY. We examine the symmetry breaking pattern of such models during the inflationary epoch with H I ≃ 10 14 GeV, and identify the range of the QCD axion decay constant, as well as the corresponding relic axion abundance, consistent with known cosmological constraints. In addition to the case that the PQ symmetry is restored during inflation, i.e. v(t I) = 0, there are other viable scenarios, including that the PQ symmetry is broken during inflation with v(t I) ∼ (4πH I M n P l) 1/(n+1) ∼ 10 16-10 17 GeV due to the Hubble-induced D-term D A ∼ 8π 2 H 2 I , while v(t 0) ∼ (m SUSY M n P l) 1/(n+1) ∼ 10 9-5×10 13 GeV in the present universe, where v(t 0) above 10 12 GeV requires a fine-tuning of the axion misalignment angle. We also discuss the implications of our results for the size of SUSY breaking soft masses.

Seeds of large-scale anisotropy in string cosmology

Physical Review D, 1999

Pre-big-bang cosmology predicts tiny first-order dilaton and metric perturbations at very large scales. Here we discuss the possibility that other, more copiously generated, perturbations may act, at second order, as scalar seeds of large-scale structure and CMB anisotropies. We study, in particular, the cases of electromagnetic and axionic seeds. We compute the stochastic fluctuations of their energy-momentum tensor and determine the resulting contributions to the multipole expansion of the temperature anisotropy. In the axion case it is possible to obtain a flat or slightly tilted blue spectrum that fits present data consistently, both for massless and for massive ͑but very light͒ axions. ͓S0556-2821͑99͒03102-1͔

Particle production in string cosmology models

Physical Review D, 1998

We compute spectra of particles produced during a dilaton-driven kinetic inflation phase within string cosmology models. The resulting spectra depend on the parameters of the model and on the type of particle and are quite varied, some increasing and some decreasing with frequency. We use an approximation scheme in which all spectra can be expressed in a nice symmetric form, perhaps hinting at a deeper symmetry of the underlying physics. Our results may serve as a starting point for detailed studies of relic abundances, dark matter candidates, and possible sources of large scale anisotropy.

Cosmic string collision in cosmological backgrounds

Physical Review D, 2010

The collisions of cosmic strings loops and the dynamics of junctions formations in expanding backgrounds are studied. The key parameter controlling the dynamics of junctions formation, the cosmic strings zipping and unzipping is the relative size of the loops compared to the Hubble expansion rate at the time of collision. We study analytically and numerically these processes for large super-horizon size loops, for small sub-horizon size loops as well as for loops with the radii comparable to the Hubble expansion rate at the time of collision.