Friedmann-Robertson-Walker universes with a presently large fraction of the energy density stored in an X-component with wX<-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 wX values. Using a simple toy model with a varying equation of state, we show that even a large variation of wX at small redshifts is very difficult to observe with dL(z) measurements up to z~1. Therefore, it will require accurate measurements in the range 1<z<2 and independent accurate knowledge of Ωm,0 (and/or ΩX,0) in order to resolve a variable wX from a constant wX.
International Journal of Modern Physics D
- Pub Date:
- General Relativity and Quantum Cosmology;
- submitted to IJMPD (uses Latex, 12 pages, 6 Figures) Minor corrections, Figures 4, 6 revised. Conclusions unchanged