The Dark Sector from Cosmic Acceleration Alone: A Two-Point Rindler Construction

We introduce two kinematic length scales — L₁ (r) = c² / ä (r) and L₂ = RH / √X — defined entirely frompresent-epoch quantities (the Hubble constant H₀ and the cosmic-acceleration parameter X = Ω_Λ − Ωₘ / 2, which equals the negative of the standard deceleration parameter, X = −q₀). L₁ (r) is the one-sided Rindler horizon distance corresponding to the Friedmann acceleration ä (r) at proper distance r. L₂ is the position-independent self-consistent separation at which two mutually receding comoving points sit on each other's Rindler horizons. Both scales are defined without invoking the cosmological event horizon REH or the particle horizon RPH. Evaluated at r = RH, the volume ratio (1/2) RH³: L₂³: L₁ (RH) ³ = (1/2): X^ (−3/2): X^ (−3) matches the dark-sector density ratio Ωb: Ωc: Ω_Λ in Planck 2018 to 0. 16 –2. 75 % accuracy. Combined with flatness and the definition of X, this volume–density correspondence closes a self-consistent equation that determines (X, Ωb, Ωc, Ω_Λ) from H₀ alone, with X = 0. 52864 reproducing Planck 2018 to 0. 16 % on X and Ω_Λ. Across seven cosmological datasets, the framework agrees with low-H₀ (CMB-anchored) measurements at the percent level and disagrees with high-H₀ (late-universe) measurements at the 4 – 9 % level. The arithmetic structure of the X-exponents (0, −3/2, −3) admits a half-integer quantum-number reading, which is noted as an open structural observation. Prior emergent-dark-sector work and prior deceleration-parameter–baryon relations are acknowledged. Keywords: Hubble flow; Rindler horizon; deceleration parameter; dark matter; dark energy; cosmological self-consistency; Hubble tension.