  We discuss how a cosmological magnetic field could affect the expansion of
the universe, through its interaction with the spacetime geometry. The tension
of the field lines means that the magneto-curvature coupling tends to
accelerate positively curved regions and decelerate those with negative
curvature. Depending on the equation of state of the matter, the effect varies
from mimicking that of a cosmological constant to resembling a time-decaying
quintessence. Most interestingly, the coupling between magnetism and geometry
implies that even weak fields have a significant impact if the curvature
contribution is strong. This leads to kinematical complications that can
inhibit the onset of the accelerated phase in spatially open inflationary
models. We employ a simple cosmological model to illustrate these effects and
examine the conditions necessary for them to have a appreciable impact.
