Linné on line 
Physics and the Cosmos Macrocosmos Big Bang Is the Universe open or closed?
Is the Universe open or closed?
Currently the Universe is expanding. Will this expansion continue forever or will it stop and possibly reverse? This depends on the total amount of energy (matter) in the Universe.
The amount
of matter in the Universe is usually given as a fraction
of the critical mass, which is the mass that is needed
in order that the rate of expansion eventually will
reach a certain limit. This quantity is called In the latter two cases the Universe will experience the so called heat death. This means that all the matter and radiation will be spread infinitely thin and homogeneously throughout the Universe. |
 "The size of the Universe" as a function of time for three different scenarios for the expansion of the Universe;  < 1 =
1 > 1 |
In other words the central question is: How much matter is there in the Universe?
If there was only the visible luminous matter the density would be less than a percent of the critical density. However, there are strong indications that there is more mass than than we can see. This is called the "dark mass" ("dark matter") and there are several ideas for what it consists of. The most popular one is that it consists of particles that we have not yet discovered, remnants from the Big Bang.
Many models for the evolution of the
Universe are based on 
=1.
There is no proof that this is the case, but alternative
models have even larger problems to describe the Universe
as we observe it today.