Magnetohydrodynamic waves through a plasma with a general equation of
state P proportional to rho(gamma) in the presence of a homogeneous am
bient background magnetic field are studied in a general Friedmann-Rob
ertson-Walker background universe with the metric components being an
arbitrary power function of time (similar to t(n)). We find that for a
suitable choice of gamma and n our results reduce to those found by H
olcomb and Tajima [Phys. Rev. D 40, 3809 (1989)] and later by Holcomb
[Astrophys. J. 362, 381 (1990)]. Our general approach to this problem
makes it possible to realize the existence of an extra term in the equ
ation for the velocity of the fluid, which until now had remained unno
ticed. This extra term has no Newtonian analogue. Unlike the usual res
ults in magnetohydrodynamics, transverse waves traveling perpendicular
to the background magnetic held are found to exist.