Accelerating Universes with Scaling Dark Matter

Friedmann-Robertson-Walker universes with a presently large fraction of the energy density stored in an X-component with w_X<-1/3, are considered. We find all the critical points of the system for constant equations of state in that range. We consider further several background quantities that can distinguish the models with different w_X values. Using a simple toy model with a varying equation of state, we show that even a large variation of w_X at small redshifts is very difficult to observe with d_L(z) measurements up to z~1. Therefore, it will require accurate measurements in the range 1<z<2 and independent accurate knowledge of Ω_m,0 (and/or Ω_X,0) in order to resolve a variable w_X from a constant w_X.