The Astrophysical Journal · 1990 · 42 citations · 0 references
The '3 + 1' formalism is applied to investigate the fundamental modes of a plasma in a 'dust'-dominated Friedman universe. The wave equation is solved in this background to obtain the propagator for a free photon. This solution is applied, in a manner analogous to the familiar plane-wave Fourier decomposition of flat spacetime, to obtain frequencies and dispersion relations for modes in both unmagnetized and magnetized cold plasmas. The magnetohydrodynamic (MHD) equations are written for a warm plasma in this background; solutions are obtained for the acoustic modes as well as for Alfven waves in the limit of negligible pressure relative to the magnetic field. In the latter case, it is shown that there exist nonoscillatory solutions. These results are relevant to theories of galactic magnetic fields which require the amplification of a primordial 'seed' field.