From the Spacetime Unity Postulate to the Cosmological Constant: An Open, Flat Reading

We consider a postulate we call Spacetime Unity: that space and time are a single reality, so that space, like time, flows and is added for every observer without an external cause. The postulate acts on the separation between an observer and the initial cosmic point and has two sub-postulates: P1, an expansive part of raw magnitude 1 that acts along the observer's single dimension; and P2, an equal and opposite contractive part that acts through the three dimensions of the initial point, each dimension carrying one third. Applied directly as local rates, P1 and P2 give a constant-rate power law, H(z) = H₀(1+z)^(3/2), which is decisively excluded by DESI DR2 baryon acoustic oscillation (BAO) data, as is exact linear coasting. Read instead as energy densities in the Einstein equations under the unimodular constraint, the postulate reproduces the ΛCDM expansion history exactly. Its geometric matter share is 1/4, a value obtained both by adding raw magnitudes along the observer's line and by treating them as velocity components whose squares add, as the quadratic structure of the Friedmann equation requires. As the present matter share, 1/4 is excluded at about 6σ. We propose that the two lateral dimensions of P2, which carry no magnitude along the line, displace the point at which P2 acts, so that the separation over which it acts is the hypotenuse (√11/3)ct and the geometric share holds at the cosmic time (√11/3)t₀, about 1.45 billion years in the future. In flat ΛCDM this gives, independently of H₀, Ωm0 = sech²[(3/√11) ln(2+√3)] ≈ 0.3095, 1.4σ above DESI DR2 BAO and 0.8σ below Planck. The rule is anchored to the present observer as the reference. We show that the data require a single reference observer, since taking every observer as reference leads to the steady-state cosmology, which is excluded. We state explicitly that the displacement reading was formulated after 1/4 was found to be inconsistent with present-epoch data, that it rests on stated geometric choices, and that the framework does not explain why the reference is the present observer, nor address the fine-tuning problem.