28 research outputs found
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, . 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 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 . 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 . 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: MirĂłn Granese, Nahuel Omar. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de FĂsica; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Kandus, Alejandra. Universidade Estadual de Santa Cruz; BrasilFil: Calzetta, Esteban Adolfo. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Ciudad Universitaria. Instituto de FĂsica de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de FĂsica 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.