Avoiding recollapse in an open-AdS universe via a self-tuning-like mechanism

Abstract

We study whether an open FLRW universe with a negative cosmological constant can evade the eventual recollapse characteristic of an AdS-type universe. Within a power-law realization of Fab-Four theory, we solve the background equations numerically and analyze the asymptotic dynamics. For the representative branch and parameter choice studied here, we find that the scalar sector provides a self-tuning-like compensation for the negative {\Lambda}, while the curvature term remains unscreened. As a result, the universe can continue expanding instead of recollapsing. Instead, the universe evolves toward a curvature-dominated linear-expansion regime, a {\propto} t. To probe the underlying compensation mechanism, we further analyze an auxiliary zero-curvature subsystem using Poincar\'e compactification. In the {\Lambda}<0 domain, there exist background trajectories that approach a critical point at infinity. Near this point, the compensating scalar-{\Lambda} sector becomes stiff-like, w_{{\phi}+{\Lambda}} {\to} 1, so that the system effective energy density redshifts faster than curvature (w_k = -1/3). Although this auxiliary analysis does not cover the full curved cosmology, it is consistent with and qualitatively supports the numerical finding that the net {\phi}+{\Lambda} contribution becomes subdominant to curvature, thereby preventing recollapse despite {\Lambda}<0. This extends the application of the self-tuning mechanism to the AdS region and offers a possibility for the AdS Universe predicted by string theory to become a reality.