Gradient expansion formalism for magnetogenesis in the kinetic coupling model

Oleksandr Sobol
2020
Article

In order to describe magnetogenesis during inflation in the kinetic coupling model, we utilize a gradient expansion which is based on the fact that only long-wavelength (superhorizon) modes undergo amplification. For this purpose, we introduce a set of functions (bilinear combinations of electromagnetic fields with an arbitrary number of spatial curls) satisfying an infinite chain of equations. Apart from the usual mode enhancement due to interaction with the inflaton, these equations also take into account the fact that the number of relevant modes constantly grows during inflation. Truncating this chain, we show that even with a relatively small number of equations, it is possible to describe the electric and magnetic energy densities with a few percent accuracy during the whole inflation stage. We arrive at this conclusion for different types of coupling functions (increasing, decreasing, and nonmonotonic) in the regime with strong backreaction and its absence.

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Oleksandr Sobol
2020
Article

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https://infoscience.epfl.ch/record/282286?ln=fr

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Influence of backreaction of electric fields and Schwinger effect on inflationary magnetogenesis

Oleksandr Sobol

We study the generation of electromagnetic fields during inflation when the conformal invariance of Maxwell's action is broken by the kinetic coupling f(2)(phi)F mu nu F mu nu of the electromagnetic field to the inflaton field phi We consider the case where the coupling function f(phi) decreases in time during inflation and, as a result, the electric component of the energy density dominates over the magnetic one. The system of equations which governs the joint evolution of the scale factor, inflaton field, and electric energy density is derived. The backreaction occurs when the electric energy density becomes as large as the product of the slow-roll parameter. and inflaton energy density,rho(E) similar to epsilon rho(inf). It affects the inflaton field evolution and leads to the scale-invariant electric power spectrum and the magnetic one which is blue with the spectral index n(B) = 2 for any decreasing coupling function. This gives an upper limit on the present-day value of observed magnetic fields below 10(-22) G. It is worth emphasizing that since the effective electric charge of particles e(eff) = e/f is suppressed by the coupling function, the Schwinger effect becomes important only at the late stages of inflation when the inflaton field is close to the minimum of its potential. The Schwinger effect abruptly decreases the value of the electric field, helping to finish the inflation stage and enter the stage of preheating. It effectively produces the charged particles, implementing the Schwinger reheating scenario even before the fast oscillations of the inflaton. The numerical analysis is carried out in the Starobinsky model of inflation for the powerlike f proportional to a(a) and Ratra-type f = expo(beta phi/M-p) coupling functions.

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Backreaction of electromagnetic fields and the Schwinger effect in pseudoscalar inflation magnetogenesis

Oleksandr Sobol

We study magnetogenesis in axionlike inflation driven by a pseudoscalar field phi coupled axially to the electromagnetic (EM) field (beta/M-p)phi F-mu nu(F) over tilde mu nu with dimensionless coupling constant beta. A set of equations for the inflaton field, scale factor, and expectation values of quadratic functions of the EM field is derived. These equations take into account the Schwinger effect and the backreaction of generated EM fields on the Universe expansion. It is found that the backreaction becomes important when the EM energy density reaches the value rho(EM) similar to (root 2 epsilon/beta)rho(inf) (epsilon is the slow-roll parameter and rho(inf) is the energy density of the inflaton) slowing down the inflaton rolling and terminating magnetogenesis. The Schwinger effect becomes relevant when the electric energy density exceeds the value rho(E) similar to alpha(-3)(EM)(rho(2)(tot)/M-p(4)), where rho(tot) = 3H(2)M(p)(2) is the total energy density and alpha(EM) is the EM coupling constant. For large beta, produced charged particles could constitute a significant part of the Universe energy density even before the preheating stage. Numerically studying magnetogenesis in the alpha-attractor model of inflation, we find that it is possible to generate helical magnetic fields with the maximal strength 10(-15) G, however, only with the correlation length of order 1 pc at present.

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Magnetogenesis In Natural Inflation Model

Oleksandr Sobol

We study the process of inflationary magnetogenesis in the natural single-field inflation model, whose parameters are chosen in accordance with the recent observations by the Planck collaboration [1]. The conformal invariance of the Maxwell action is broken by a kinetic coupling with the inflaton field by means of the coupling function as a power of the scale factor, I(phi) proportional to a(alpha), and alpha < 0 is used in order to avoid the strong coupling problem. For such.., the electric component of the energy density dominates over the magnetic one and, for alpha less than or similar to -2.2, it causes a strong back-reaction, which can spoil inflation and terminate the enhancement of the magnetic field. It is found that the magnetic fields generated without back-reaction problem cannot exceed similar to 10(-20) G at the present epoch, and their spectrum has a blue tilt.

BOGOLYUBOV INST THEORETICAL PHYSICS NATL ACAD SCI UKRAINE2018

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