Accelerating internal dimensions and nonzero positive cosmological constant

Abstract

We present a new scenario for the moduli stabilization with a very small but nonzero positive cosmological constant $\lambda$. In this scenario the complex structure moduli are still stabilized by the three-form fluxes as in the usual flux compactifications, but the K$\ddot{\rm a}$hler modulus is not fixed by the KKLT scenario. In our case the scale factor (or the K$\ddot{\rm a}$hler modulus) of the internal dimensions is basically allowed to change with time. But at the supergravity level it is fixed by a set of dynamical (plus constraint) equations defined on the 4D spacetime, not by the nonperturbative corrections of KKLT. Also at the supergravity level it is shown that $\lambda$ is fine-tuned to zero, $\lambda =0$, by the same set of 4D equations. This result changes once we admit $\alpha^{\prime}$-corrections of the string theory. The fine-tuning $\lambda=0$ changes into $\lambda = \frac{2}{3} Q$, where $Q$ is a constant representing quantum corrections of the brane and 6D action defined on the internal dimensions and its value is determined by the $\alpha^{\prime}$-corrections. It is also shown that this nonzero $\lambda$ must be positive and at the same time the internal dimensions must evolve with time almost at the same rate as the external dimensions in the case of nonzero $\lambda$.