Electromagnetic plasma waves in Big Bounce and Big Crunch cosmologies

We study the exact dynamics of electromagnetic waves in cold electron-positron plasma in the simplest background of Big Bounce and Big Crunch cosmologies. We show that these waves are described by a Mathieu equation in a Big Bounce cosmology, which opens the possibility of electromagnetic wave amplification due to the cosmological constant in this Universe. On the contrary, in a Big Crunch cosmology, electromagnetic plasma waves are described by a modified Mathieu equation. In both cases, for each Universe, its expansion or contraction imprints equivalent characteristics in the temporal evolution of the waves.