The angular diameter distance peaks at a model-fixed turnover: flat Planck ΛCDM puts z_max near 1.59. A model-independent reconstruction from 140 compact quasar cores measures z_max = 1.70 +/- 0.20 and ranks ΛCDM behind alternative expansion histories. The mismatch sits in the unmeasured interpolation zone between the supernova regime and the CMB anchor, the same band where the quasar Hubble diagram and DESI both pull away from constant Λ.
With a constant Λ, the parameters fixed at recombination determine every distance at every epoch; d_A(z) and its peak have zero remaining freedom. The model must interpolate across eight billion unmeasured years with a single rigid curve, and a turnover landing off the prediction can only be blamed on the rulers.
SCT replaces the hot-dense-center with a superluminal collision, and the constant Λ with the dynamical ratio Λ_eff(x,t) = C × Λ_parent(x,t) / λ_local(x,t) (P17), sourced by progressive mesh dissipation (P14, P15, P16). Because Λ_eff falls toward zero at high redshift faster than a constant and grows through the structure era (P18), the comoving distance integral and with it the d_A(z) curve bend away from the ΛCDM shape exactly in the interpolation zone. The turnover migrates accordingly: a late-strengthening dark energy term pushes z_max above the constant-Λ value, the direction the quasar-core measurement reports.
The shape change is small and specific. SCT's modification to H(z) at z below 2 shifts the angular diameter distance at the 0.04 percent level at recombination, enough to move theta_* by 1.5σ relative to Planck, and grows toward the percent level through the matter era. The d_A peak is the most sensitive single landmark for this shape because it differentiates the curve: a measurement of where d_A turns over tests d(d_A)/dz directly, which is where a time-varying Λ_eff and a constant Λ differ most cleanly.
This is the same dynamical-ratio physics that produces the DESI w₀w_a preference, the quasar Hubble-diagram deviation above z of 1.5, and the Hubble tension at z of 0. One environment-and-epoch-dependent Λ_eff, read across four distance probes. There is no need to invoke quintessence fields or ruler conspiracies.
The registered kill: Euclid and DESI BAO plus full-shape reconstruction measuring d_A(z) through the turnover and finding the curve indistinguishable from constant-Λ ΛCDM at the 0.1 percent level for z below 2 falsifies the dynamical-Λ_eff shape modification. A precision z_max localized at the ΛCDM value with few-percent errors has the same effect; SCT requires the turnover displaced in the late-strengthening direction.