Cosmological Parameters
Before going further, it is useful to
define some of the
cosmological parameters in the Friedmann equation (and elsewhere).
1. The Hubble Parameter
The Hubble
parameter is the normalized rate of expansion:
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.
2. The Matter Density Parameter.
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
We define the matter
density parameter
as
Best measurements for Om are about 0.2
- 0.4
3. The "dark energy"
density parameter
We can express a similar density
parameter for lambda again by using the Friedmann equation and setting
rhom=0. We then get
Best measurements for OL are about 0.6-0.75
4. "Total Omega"
What
is Omega if the Universe is flat? What is Omega if the Universe is
accelerating?
5. The deceleration parameter
Not used as often any more, describes
the de/acceleration rate of the Universe's expansion:
