Primordial magnetic helicity evolution with a homogeneous magnetic field
  from inflation
  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 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 nonhelical and helical hydromagnetic turbulence. We show that, in the
presence of a homogeneous 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. It can never change
and acts as an imposed magnetic 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 felicity. 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.
