Density of the Universe

Density of the Universe:
The description of the various geometries of the Universe (open, closed, flat) also relate to their futures.
There are two possible futures for our Universe, continual expansion (open and flat), turn-around and
collapse (closed). Note that flat is the specific case of expansion to zero velocity.

The key factor that determines which history is correct is the amount of mass/gravity for the Universe as
a whole. If there is sufficient mass, then the expansion of the Universe will be slowed to the point of
stopping, then retraction to collapse. If there is not a sufficient amount of mass, then the Universe will
expand forever without stopping. The flat Universe is one where there is exactly the balance of mass to
slow the expansion to zero, but not for collapse.

The parameter that is used to measure the mass of the Universe is the critical density, Omega. Omega is
usually expressed as the ratio of the mean density observed to that of the density in a flat Universe.

Given all the range of values for the mean density of the Universe, it is strangely close to the density of a
flat Universe. And our theories of the early Universe (see inflation) strongly suggest the value of Omega
should be exactly equal to one. If so our measurements of the density by galaxy counts or dynamics are
grossly in error and remains one of the key problems for modern astrophysics.
Cosmological Constants:
The size, age and fate of the Universe are determined by two constants:

The measurement of these constants consumes major amounts of telescope time over all wavelengths.
Both constants remain uncertain to about 30%; however, within this decade we can expect to measure
highly accurate values for both due to the Hubble Space Telescope and the Keck twins.