We adopt as our basic working hypothesis the standard cosmological model of Friedmann Robertson Walker (FRW), where we assume homogeneity and isotropy and describe gravity by general relativity. We limit the discussion to the matter-dominated era in the ``dust'' approximation.

The Friedmann equation that governs the universal expansion
can be written in terms of the different contributions to
the energy density (*e.g.*,
[1]):

Here, *a*(*t*) is the expansion factor of the universe,
*H*(*t*)
/ *a* is the Hubble constant,
_{m}(*t*)
is mean the mass density,
is the cosmological constant,
and *k* is the curvature parameter.
Hereafter, the above symbols for the cosmological parameters
refer to their values at the present time, *t*_{0}.

We denote
_{tot}
_{m} +
_{}, which by Eq. 1 equals
1 - _{k}; its value
relative to unity determines whether the universe
is open (*k* = - 1), flat (*k* = 0), or closed (*k* = + 1).
Another quantity of interest is the deceleration parameter,
*q*_{0} -
*a* /
^{2},
which by Eq. 1
is related to the other parameters via
*q*_{0} =
_{m}/2 -
_{}.

The FRW model also predicts a relation between the dimensionless product
*H*_{0}*t*_{0} and the parameters
_{m} and
_{}.
For
_{} = 0, this product ranges
between 1 and 2/3 for
_{m} in the range 0 to
1 respectively, and it is computable for any values of
_{m} and
_{}
(Section 2.5).

The global measures commonly involve combinations of the cosmological
parameters. Constraints in the
_{m} -
_{} plane
are displayed in Figure 1.