In standard FLRW cosmology with constant Λ, the angular-diameter distance D_A(z) should rise to a maximum near z ≈ 1.5 and then decrease, so that very distant objects appear larger again at higher redshifts. Observational tests using compact radio structures and galaxies often fail to show this predicted turnover, instead showing D_A continuing to plateau or decrease as if space were nearer to static or Euclidean (Jackson 2008; Lopez-Corredoira 2010).
The standard model uses a single FLRW expansion history with a fixed cosmological constant. Under that assumption, D_A(z) takes a specific shape with a sharp turnover at a specific redshift. Recovering the predicted turnover requires that the intrinsic size evolution of the observed standard rods is precisely controlled. Any deviation from the predicted shape demands either source-evolution corrections or new physics.
SCT replaces the hot-dense-center with a superluminal collision and the thermalized debris field that became our visible universe. From this single change, Λ stops being a fundamental constant and becomes a dynamical ratio Λ_eff(x,t) = κ · U_local(x,t) / U_parent(x,t) (P17). The integrated expansion history that determines D_A(z) is then computed against a varying-Λ_eff background rather than a fixed-Λ background, and the resulting D_A(z) curve is broader and smoother than the classical FLRW prediction.
The two M5 contributions that drive the Hubble tension also drive the D_A(z) shape. The KBC-supervoid local enhancement of Λ_eff (P19) and the temporal evolution of Λ_eff between z=1100 and now from progressive mesh dissipation (P14, P15, P16, P18) both modify the integral that converts comoving distance to apparent angular size. The classical sharp turnover at z ≈ 1.5 gets smeared out into a much broader plateau, and the redshift of maximum D_A shifts modestly. The lack of a clean observable turnover in compact-source datasets is the predicted SCT signature of a dynamical Λ_eff field, not a failure of expansion-cosmology generally.
This is the same mechanism that resolves the Hubble tension, the S₈ tension, the late-time exponential trend question, and the coincidence problem. One field-equation extension addresses all of them simultaneously, because they are all surface manifestations of the same underlying replacement of the hot-dense-center origin (which forced Λ to be a constant) with the superluminal-collision origin (which makes Λ_eff a derived field). The angular-diameter-distance shape is just one more observational window onto that single underlying mechanism.
If a precision JWST + Roman + ELT survey of compact radio cores and standard rods across the redshift range z = 0.5 to 5 measures D_A(z) with the sharp ΛCDM-predicted turnover at z ≈ 1.5 (after rigorous source-evolution and selection-bias corrections), the dynamical-Λ_eff explanation for the smeared turnover is refuted. If the broader D_A(z) curve predicted by SCT is confirmed at high precision, the M5 mechanism is observationally vindicated against constant-Λ expectations.