Hypothesis of Longitudinal-Dimension Space (Primal Substance Cyclic Universe Hypothesis · Volume III)

General relativity attributes gravity to the geometric curvature of three-dimensional spacetime. While this framework works well at ordinary scales, it cannot unify gravity with other fundamental interactions, nor can it coherently account for observational puzzles including galactic gravitational anomalies and the accelerated expansion of the universe. This paper puts forward the hypothesis of longitudinal-dimension space built upon the axioms of the Primal Substance substrate and the Motile Force: the universe hosts two co-originated and coexisting spacetime systems: transverse three-dimensional space and longitudinal coupling space. The transverse three-dimensional space is bounded rigidly by position and temporal sequence and tends to disperse alongside entropy growth. The longitudinal-dimension space embeds fully within three-dimensional space, free from three-dimensional coordinate confinement. Within it, the Primal Substance flows freely and tends to converge as entropy declines. The inherent distinction between these two spaces forms the root cause of gravitational effects. Galactic-scale gravitational reinforcement and the expansive trend of the universe on cosmic scales can both be interpreted by the coupling of the dual spaces. At the Planck microscopic scale, the barrier separating transverse and longitudinal spaces fades, offering a fundamental mechanism to explain vacuum fluctuation and wave-particle duality. This paper identifies four sets of potential critical conditions under which three-dimensional configurations may penetrate into longitudinal-dimension space. It only states that such cross-dimensional penetration is possible under these extreme conditions, without claiming it is inevitable.