Gravitational forces are the only forces presently known which are long range (in contrast to the nuclear forces, for example) and which cannot be compensated (there are no negative masses). It is therefore to be expected that, for large quantities of matter distributed over wide regions of space, they will be the decisive forces, and hence the gravitational forces will determine the evolution and dynamics of the universe.

Physical laws get their importance from the fact that a single law describes many very different situations. Technically this comes out by writing the laws as differential equations (usually of second order), which admit a multitude of initial or boundary conditions. The law itself has often been found by extracting some common rules from the observed variety of effects. All these features are also present in the theory of gravitation.

In cosmology, however, we encounter a very different situation. There is only one realization of a cosmos, that which we are living in. And if there was an extra physical law for this cosmos, we could not find and prove it the usual way. That is to say, if we find a surprising new phenomenon, we cannot easily decide which of its properties are a new law, and which are due to initial conditions. Sometimes it is claimed that, in a proper theory, initial conditions should be excluded, the cosmos must not depend on them (this was one of the assumptions of the ‘inflation’ theory). Or one claims that the new law can be obtained from other principles, such us the anthropic principle.