  Following the wellknown spacetime decomposition technique as applied to (d+1)
dimensions we write down the equations of magnetohydrodynamics (MHD) in a
spatially at generalised FRW universe. Assuming an equation of state for the
background cosmic fluid we find solutions in turn for acous- tic waves and also
for Alfven waves in a warm (cold) magnetised plasma. Interestingly the
different plasma modes closely resemble the at space coun- terparts except that
here the field variables all redshift with their time due to the expansion of
the background. It is observed that in the ultrarelativistic limit the field
parameters all scale as the free photon. The situation changes in the
prerelativistic limit where the frequencies change in a bizarre fashion
depending on initial conditions. It is observed that for a fixed magnetic field
in a particular medium the Alfven wave velocity decreases with the number of
dimensions, being the maximum in the usual 4D. Further for a fixed dimension
the velocity attenuation is more significant in dust compared to the radiation
era. We also find that in an expanding background the Alfven wave propaga- tion
is possible only in the high frequency range, determined by the strength of the
external magnetic field, the mass density of the medium and also the dimensions
of the spacetime. Further it is found that with expansion the cosmic magnetic
field decays more sharply in higher dimensional cosmology, which is in line
with observational demand.
