Interacting Cosmological Fluids and the Coincidence Problem

Understand

We examine the evolution of a universe comprising two interacting fluids, which interact via a term proportional to the product of their densities.

• In the case of two matter fluids it is shown that the ratio of the densities tends to a constant after an initial cooling-off period.
• We then obtain a complete solution for the cosmological constant (w = -1) scenario.
• Finally, we investigate the general case in which the dark energy equation of state is p = w*rho, where w is a constant, and show that periodic solutions can occur if w < -1.