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

TL;DR

This paper investigates whether an open-AdS universe with a negative cosmological constant can avoid recollapse by using a self-tuning mechanism within Fab-Four theory, leading to a curvature-dominated linear expansion.

Contribution

It demonstrates that a scalar sector can self-tune to counteract negative b4, preventing recollapse and extending self-tuning to AdS universes in string theory contexts.

Findings

The universe evolves toward a linear-expansion regime proportional to time.

The scalar sector compensates the negative b4 while leaving curvature unscreened.

Auxiliary analysis supports the dominance of curvature over scalar-b4 contributions.

Abstract

We study whether an open FLRW Universe with a negative cosmological constant can evade the eventual recollapse characteristic of AdS-type Universe. Within a power-law realization of Fab-Four theory, we solve the background equations numerically and analyze the asymptotic dynamics. We find that the scalar sector provides a self-tuning-like compensation of the negative {\Lambda} while leaving the spatial-curvature term unscreened. Consequently, the expansion does not reverse. 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. The physically admissible trajectories approach a critical point at infinity where the compensating scalar-{\Lambda} sector becomes stiff-like (w_{{\phi}+{\Lambda}} {\to} 1), so that its 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.