# Primordial magnetic helicity evolution with homogeneous magnetic field from inflation

Axel Brandenburg, Ruth Durrer, Yiwen Huang, Tina Kahniashvili, Sayan Mandal, Shinji Mukohyama

Axel Brandenburg, Ruth Durrer, Yiwen Huang, Tina Kahniashvili, Sayan Mandal, Shinji Mukohyama

**13/5/20**Published in : arXiv:2005.06449

Motivated by a scenario of magnetogenesis in which a homogeneous magnetic field is generated during inflation, we study the magnetohydrodynamic evolution of the primordial plasma motions for two kinds of initial conditions -- (i) a spatially homogeneous imposed field with an unlimited correlation length, and (ii) a zero flux scale-invariant statistically homogeneous magnetic field. In both cases, we apply, for a short initial time interval, monochromatic forcing at a certain wave number so that the correlation length is finite, but much smaller than the typical length scale of turbulence. In particular, we investigate the decay of non-helical and helical hydromagnetic turbulence. We show that, in the presence of an imposed magnetic field, the decay of helical and nonhelical small-scale fields can occur rapidly. This is a special property of a system with a perfectly homogeneous magnetic field, which is sometimes considered as a local approximation to a slowly varying background field. This is in a sharp contrast to the case of a statistically homogeneous magnetic field, where we recover familiar decay properties: a much slower decay of magnetic energy and a faster growth of the correlation length, especially in the case with magnetic helicity. The result suggests that a homogeneous magnetic field, if generated during inflation, should persist under the influence of small-scale fields and could be the origin of the large-scale magnetic field in the universe.