1. The Hubble Parameter
The Hubble parameter is the normalized rate of expansion:2. The Matter Density Parameter.
Note that the Hubble parameter is not a constant! The Hubble constant is the Hubble parameter measured today -- we denote its value by H0. Current estimates are in the range of H0 = 65-75 km/s/Mpc -- we will discuss these efforts in more detail later.
Also note you will often see the parameter h, particularly in distance-dependant quantities (for example, 75h-1 Mpc). This is usually defined by h=H0/100.
Look at the Friedmann equation:
Rewriting this using the Hubble parameter, and for now set Lambda=0:
The Universe is flat if k=0, or if it has a critical density of
Remember, this is not the density that the universe actually is, its the density the universe would have to have if it was spatially flat by matter alone.
We define the matter density parameter to describe the actual density of the universe relative to this critical density:
So if Om=1, and there was no dark energy/lambda, the universe would be spatially flat. Best measurements for Om are about 0.25 - 0.35
3. The "dark energy"