  The cosmological observations provide a strong evidence that there is a
positive cosmological constant in our universe and thus the spacetime is
asymptotical de Sitter space. The conjecture of gravity as the weakest force in
the asymptotical dS space leads to a lower bound on the U(1) gauge coupling
$g$, or equivalently, the positive cosmological constant gets an upper bound
$\rho_V \leq g^2 M_p^4$ in order that the U(1) gauge theory can survive in four
dimensions. This result has a simple explanation in string theory, i.e. the
string scale $\sqrt{\alpha '}$ should not be greater than the size of the
cosmic horizon. Our proposal in string theory can be generalized to U(N) gauge
theory and gives a guideline to the microscopic explanation of the de Sitter
entropy. The similar results are also obtained in the asymptotical anti-de
Sitter space.
