Eventually, however, the expansion will slow, stop, and then the Universe will begin to contract. The contraction will continue until all of the mass of the Universe is contained in a singularity, a process known as the big crunch. The singularity then undergoes a big bang, and the process begins afresh. Although we shall discuss reasons why this is probably not the case, it does explain what happened before the big bang. The question of whether the Universe will collapse in a big crunch or continue expanding forever hinges on knowing the density of the Universe.
Density is defined as mass divided by volume.
One can measure the density of the universe by observing the local group of galaxies and assuming that the Universe is all the same. One can also calculate the density required such that the Universe will eventually stop expanding. That density is called the critical density, and the ratio of the observed density to the critical density is given by the Greek letter omega . If omega is less than one the Universe will continue expanding until it is so large that it dies a cold death. If omega equals one the Universe will eventually stop expanding but will not collapse.