**D. Matter Dominated Models**

The present epoch is
best described by a matter dominated universe, so it is perhaps
best to explore this model first. Again, matter domination
corresponds to a non-relativistic, homogeneous, isotropic `dust'
filled universe with zero pressure. By setting *p* = 0 in (15)
and incorporating the term
into a total density,
_{T} =
+
/ (8
*G*), the Friedmann equations
for a matter dominated universe emerge,

Again, the value of *k* describes the geometry of the
space. Equation (29) is actually obtained by
combining (14) and (15) and is often called the
acceleration equation.

The idea of the *total* density,
_{T}, might be a bit
confusing since it has been stated that the model is matter
dominated. Although this is true, there can still be a small
contribution in the form of radiation and other forms of energy,
such as dark matter. The point is that any of these should be much
less than _{M}
for the model to be accurate. It will also be
seen that _{M}
can also be broken into different
contributions as tacitly stated in the previous remark about dark
matter. For the remainder of this section we take
to mean
_{T} to keep
the notation as simple as possible.