Primordial Magnetic Helicity from Stochastic Electric Currents

We study the possibility that primordial magnetic fields generated in the transition between inflation and reheating posses magnetic helicity, $H_M$. The fields are induced by stochastic currents of scalar charged particles created during the mentioned transition. We estimate the rms value of the induced magnetic helicity by computing different four-point SQED Feynman diagrams. For any considered volume, the magnetic flux across its boundaries is in principle non null, which means that the magnetic helicity in those regions is gauge dependent. We use the prescription given by Berger and Field and interpret our result as the difference between two magnetic configurations that coincide in the exterior volume. In this case the magnetic helicity gives only the number of magnetic links inside the considered volume. We calculate a concrete value of $H_M$ for large scales and analyze the distribution of magnetic defects as a function of the scale. Those defects correspond to regular as well as random fields in the considered volume. We find that the fractal dimension of the distribution of topological defects is $D = 1/2$. We also study if the regular fields induced on large scales are helical, finding that they are and that the associated number of magnetic defects is independent of the scale. In this case the fractal dimension is $D=0$. We finally estimate the intensity of fields induced at the horizon scale of reheating, and evolve them until the decoupling of matter and radiation under the hypothesis of inverse cascade of magnetic helicity. The resulting intensity is high enough and the coherence length long enough to have an impact on the subsequent process of structure formation.Comment: 32 pages, 4 figure

Cosmological Magnetic Fields from Gauge-Mediated Supersymmetry-Breaking Models

We study the generation of primordial magnetic fields, coherent over cosmologically interesting scales, by gravitational creation of charged scalar particles during the reheating period. We show that magnetic fields consistent with those detected by observation may obtained if the particle mean life \tau_s is in the range 10^{-14} sec \leq \tau_s \leq 10{-7} sec. We apply this mechanism to minimal gauge mediated supersymmetry-breaking models, in the case in which the lightest stau \tilde\tau_1 is the next-to-lightest supersymmetric particle. We show that, for a large range of phenomenologically acceptable values of the supersymmetry-breaking scale \sqrt{F}, the generated primordial magnetic field can be strong enough to seed the galactic dynamo.Comment: 12 pages, Latex. Final version accepted for publication in Phys. Lett.

Primordial Weibel instability

We study the onset of vector instabilities in a post-inflationary epoch of the Universe as a mechanism for primordial magnetic fields amplification between the end of inflation and the electroweak (EW) transition. We assume the presence of a charged spectator scalar field arbitrarily coupled to gravity. This field is in its vacuum state during inflation, but becomes highly excited after the transition to the radiation dominance due to the gravitational particle creation. At the beginning of radiation era the ensuing state admits a hydrodynamic description. In particular since its high temperature, the fluid may be regarded as a conformal one. The large quantum fluctuations induced during reheating now become statistical fluctuations whereby an excess charge and anisotropic pressure will be observed in any finite domain. Under these conditions a Weibel instability could be triggered thus opposing the dilution of a primordial magnetic field because of the expansion of the Universe. The magnitude of the effect is determined by the size of the domain, the coupling to curvature of the field and the relaxation time of the fluid. We find that for scales of the order or smaller than the particle horizon at the EW phase transition, the Weibel instability can overcome the cosmic expansion provided that the reheating temperature of the Universe and the coupling of the scalar field to gravity are small enough.Comment: 23 pages, 1 figure. Comments wellcom

Nonlinear fluctuations in relativistic causal fluids

In the Second Order Theories (SOT) of real relativistic fluids, the non-ideal properties of the flows are described by a new set of dynamical tensor variables. In this work we explore the non-linear dynamics of those variables in a conformal fluid. Among all possible SOTs, we choose to work with the Divergence Type Theories (DTT) formalism, which ensures that the second law of thermodynamics is fulfilled non-perturbatively. The tensor modes include two divergence-free modes which have no analog in theories based on covariant generalizations of the Navier-Stokes equation, and that are particularly relevant because they couple linearly to a gravitational field. To study the dynamics of this irreducible tensor sector, we observe that in causal theories such as DTTs, thermal fluctuations induce a stochastic stirring force, which excites the tensor modes while preserving energy momentum conservation. From fluctuation-dissipation considerations it follows that the random force is Gaussian with a white spectrum. The irreducible tensor modes in turn excite vector modes, which back-react on the tensor sector, thus producing a consistent non-linear, second order description of the divergence-free tensor dynamics. Using the Martin-Siggia-Rose (MSR) formalism plus the Two-Particle Irreducible Effective Action (2PIEA) formalism, we obtain the one-loop corrected equations for the relevant two-point correlation functions of the model: the retarded propagator and the Hadamard function. The overall result of the self-consistent dynamics of the irreducible tensor modes at this order is a depletion of the spectrum in the UV sector, which suggests that tensor modes could sustain an inverse entropy cascade.Fil: Mirón Granese, Nahuel Omar. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Kandus, Alejandra. Universidade Estadual de Santa Cruz; BrasilFil: Calzetta, Esteban Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; Argentin

Superadiabatic-type magnetic amplification in conventional cosmology

We consider the evolution of cosmological magnetic fields in FRW models and outline a geometrical mechanism for their superadiabatic amplification on large scales. The mechanism operates within standard electromagnetic theory and applies to FRW universes with open spatial sections. We discuss the general relativistic nature of the effect and show how it modifies the adiabatic magnetic evolution. Assuming a universe that is only marginally open today, we estimate the main features of the superadiabatically amplified residual field.Comment: Minor changes. Published versio

Self consistent estimates of magnetic fields from reheating

We investigate the generation of primordial magnetic fields from stochastic currents created by the cosmological transition from inflation to reheating. We consider N charged scalar fields coupled to the electromagnetic field in a curved background and derive self-consistent equations for the evolution of the two point functions of the fields, which in the large N limit give a decoupled set for the scalar and the electromagnetic functions. The main contribution to the electric current comes from the infrared portion of the spectrum of created particles, and in this limit the damping of the magnetic field is not due to normal conductivity but to London currents in the scalar field. For a given set of the physical parameters of the problem, we solved this equation numerically and found that, due to the fact that the London currents are oscillating, the field actually grows exponentially during the time interval in which our large-N limit equations are valid. Although for the chosen parameters the induced field is weak, the present uncertainties on their actual values leave open the possibility for higher intensities.Comment: 25 pages, three figures. One figure added, discussion on non adiabatic induction of the magnetic field improved. References added. Final version accepted for publication in Phys. Rev.

Primordial magnetic fields induced by cosmological particle creation

We study the primordial magnetic field generated by stochastic currents produced by scalar charged particles created at the beginning of the radiation dominated epoch. We find that for the mass range 10^(-6)GeV \leq m \leq 10^2 GeV, a field of sufficient intensity to seed different mechanisms of galactic magnetic field generation, while still consistent with observational and theoretical constraints, is created coherently over a galactic scale.Comment: accepted for publication in Phys. Rev.