Cosmological magnetogenesis from extra-dimensional Gauss-Bonnet gravity

Abstract

Generation of primordial magnetic fields during inflation typically requires the breaking of conformal invariance of the Electromagnetic action. In this paper this has been achieved naturally in a higher dimensional cosmological model with a Gauss-Bonnet term in the action. The evolution of the scale factor of the extra dimension (whose dynamics is influenced by the Gauss-Bonnet term) acts as the cause for the breaking of conformal invariance. Different cases have been investigated, each of which is characterized by the number of higher dimensions, the value of the Gauss-Bonnet parameter, and the cosmological constant. Many of the scenarios considered are highly constrained by the requirements that the cosmic evolution is stable, that the normal dimensions expand and that there is no back reaction due to growing electric fields. However there do exist scenarios which satisfy the above requirements and are well suited for magnetogenesis. In particular, a scenario where the number of extra dimensions D = 4 and the cosmological constant is non-zero, turns out to be best suited for generating primordial magnetic fields. It is shown that for these values of parameters, a scale invariant magnetic field of the order of 10 −10 − 10 −9 G can be produced. Even in these most favorable scenarios, the higher dimensional space expands during inflation at the same rate as the normal dimension. Hence if a mechanism could freeze the evolution of the higher dimension, this seems to be a viable mechanism to produce acceptable primordial magnetic fields.

Imprints of Schwinger effect on primordial spectra

“…However, due to the conformal invariance, the magnetic field also drops as a −4 . Lots of efforts have been made to generate the primordial magnetic field during inflation by breaking the conformal invariance [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. By far, the best model we know of is still not sufficient to generate the required amount of primordial magnetic field to explain the large scale magnetic field today.…”

Magnetogenesis during inflation and preheating in the Starobinsky model

“…by coupling it to a scalar or a pseudo-scalar field or to a curvature invariant. Although many ways to break the conformal invariance of the electromagnetic action during inflation were suggested in the literature [31][32][33][34][35], we adopt in our study the kinetic coupling model f 2 F F firstly introduced by Ratra [31], where f is a function of the inflaton field φ and F is the electromagnetic field tensor. Depending on the form of the coupling f , this gives rise to different magnetic field power spectra [31,[36][37][38][39][40][41].…”

Helical cosmological magnetic fields from extra-dimensions

“…A natural scenario for breaking conformal invariance and generating primordial magnetic fields arises in the context of higher-dimensional theories [17,20,22,23]. We have earlier explored this possibility, where extradimensional model with a Gauss-Bonnet term provides a coupling function for breaking of conformal invariance in the reduced four dimensional action [35]. We now extend this work by adding a parity breaking piece to the action, which allows for the generation of helical primordial magnetic fields.…”

“…βD/2α = 2), which as we saw leads to a scale invariant spectrum for the magnetic field. Because of the exponential inflation of scale factor we take [35],…”