Once and Future Universe?

Will the universe go on expanding forever? The answer to this question depends on knowing the average density of the matter it contains. As the universe expands, gravity will continuously act to slow down and decelerate the expansion. If the gravitational strength generated by this matter is too weak, then the universe will go on expanding and the galaxies will continue to move away from each other. In this case cosmologists say the universe is unbounded.

If the average density of matter is high enough, then gravity will halt the expansion and at some point in the future the universe will reach a maximum size. Gravity will then start pulling all the galaxies back towards each other and the universe will contract.

Critical Density

Between an unbounded universe and a bounded one is the case where the average density of matter is just high enough so that the galaxies come to halt in an infinite time when they are infinitely far apart and cosmologists call this value the critical density, P_c.
Cosmologists express these three scenarios by denoting the amount of deceleration by a density parameter, Omega0.

Omega₀ is defined as density of matter in the universe / P_c

For 0 < Omega₀ < 1 the universe is unbounded and will expand for ever.

How dense is the Universe?

Measuring P_c

Strictly speaking, because of the equivalence of mass and energy, cosmologists talk about the 'mass-energy' density of the universe. But, can we know what the average density of the universe is?

Astronomers estimate the masses of galaxies and galaxy clusters in a large volume of space and obtain the average density by dividing the total mass by the total volume. This gives an average density of about 4 x 10^-28kg m^-3 suggesting that the universe will expand forever. However estimates like this do not take into account any hidden material or 'dark matter' that can not be detected by usual observational methods. Some cosmological models require P_c to equal unity which, if true, means that there is a considerable amount of missing mass which hasn't been discovered.

EDWIN HUBBLE
Starbase's page with links for Edwin Hubble
http://www.ph.surrey.ac.uk/astrophysics/hubble.html

Further information links
Dark Matter page - also with direct links to many fields of cosmology
http://www.netlabs.net/hp/tremor/dmatter.html

Information on dark matter
http://www.interstellar.addr.com/darkmatr.htm

More on dark matter
http://map.gsfc.nasa.gov/html/comp.html

A very up-to-date page about Open/closed Universe models
http://www.bowdoin.edu/dept/physics/astro.1998/astro04/openmdl.html