Abstract
In order to explain the fact that the pressure in the Friedmann equations is negative, only the hypothetical presence of dark energy is used in present theories. But, the dimensions of the pressure $p$ are $f/r^2$ and thus $p$ can not account for the acceleration of the expansion of the universe. Indeed, the hypersurface of our universe is threedimensional and curved, so a force has an effect on the universe if it is applied on the universe's boundaries. As these boundaries (hypersurface) correspond to the threedimensional universe itself at time $t$, there must exist a positive force density $f/r^3$. The relation between the pressure $p$ (calculated within the Friedmann model) and the force density is a simple derivation with respect to $r$ the space variable. And the derivation of a negative pressure leads to a positive force density.