Can we reconcile this overall view of the Universe with the known laws of physics ? It is here that we leave the domain of observed fact and enter the realm of speculation. If the observed Universe can indeed be understood in terms of the laws of physics, then our confidence in our Earth-based laws is increased. However, and this is perhaps the exciting prospect, if the Universe cannot be under¬stood in this way we may be led to seek out new laws of physics! First of all we must decide which force controls the Universe. Of all the known forces, the force of gravity is the only one that has a long enough range of influence to affect the most distant parts of the Universe. As far as we know, gravity is always an attractive force . Gravity dominates the motions of the planets and satellites of the Solar System, it most probably dominates the motions of stars in galaxies, and it is reasonable to speculate that it governs :he motions of the galaxies and the dynamics of the Universe.

The Universe is expanding, whereas the effect of the force of gravity is to try to hold it together. A basic question the cosmologist would like to answer is: is the force strong enough eventually to halt the expansion and to reverse it ? There is a useful analogy here in thinking of a rocket launched from the Earth. If the rocket has a great enough velocity it will leave the Earth’s environment. On the other hand, if the velocity is below the escape velocity the gravitational force exerted by the Earth will stop the rocket and bring it back down. Correspondingly, there are two possible fates for the Universe. It may expand forever so that there will come a time when even the largest telescopes would fail to see more than a few galaxies. Alternatively, the force of gravity will dominate, the expansion will be reversed and the Universe will be crushed into the same kind of singularity from which we believe it emerged. It is, in principle, a simple matter to decide which fate awaits us.

Consider a typical volume of space which is expanding at a known rate (inferred from observing the motions of the galaxies). The force exerted by gravity on that volume depends on the density of material in the volume: a high density is associated with a strong gravitational pull, and conversely a low density is associated with a weaker pull. Hence the FATE OF THE UNIVERSE can be decided simply by comparing the relative values of the expansion rate and the averaged density of matter in the Universe. Astronomers believe that the rate of expansion is about 75 kilometers per second for every megaparsec of distance from us. There is less certainty as regards the density of the Universe. The sum total of the observed luminous matter (mostly galaxies) gives a density equivalent to one hydrogen atom for every 10000 liters. However, there could be significant quantities of non-luminous material (black holes, snowballs or bricks, for example) whose existence could only be deduced indirectly, if at all. Thus the quoted density of observed matter is in fact a lower limit. How¬ever, a simple calculation tells us that it would require a density in excess of one hydrogen atom per 100 liters to provide a strong enough gravitational pull eventually to reverse the present observed expansion. If there is not much more matter in the Universe than is seen through the largest telescopes, and if the force of gravity behaves the way our Earth-based experiment* would have us believe, it seems that the ultimate fate of the Universe is to expand into nothingness. The Universe observed by our descendants long in the future will be a very sparsely populated place. To avoid this conclusion would require that the observed matter is only one per cent or so of all that constitutes the Universe.

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