IDE Models Permit Weak Dark-Sector Coupling but ΛCDM Remains Consistent at 1σ with Current H(z)+BAO+QSO Data

The analysis combines 31 cosmic-chronometer H(z) points, 17 BAO measurements, and 1598 quasar angular-size data to fit energy-transfer rates in three IDE parameterizations. All three yield a mild posterior tilt toward dark-energy-to-dark-matter flow, yet the no-interaction value lies well inside the 68 % credible interval, and information criteria penalize the extra parameter enough to keep ΛCDM favored. This directly tests the coincidence problem by allowing the dark-sector density ratio to evolve, yet the data still accommodate the observed near-equality today without new physics.

The result aligns with earlier Planck and DES analyses that likewise found interaction strengths below a few percent of the Hubble rate. What the preprint under-emphasizes is that current quasar samples remain systematics-limited at z>1.5; any claimed alleviation of coincidence therefore rests on the assumption that angular-size distances trace the same expansion history as BAO, an assumption that future line-intensity mapping could falsify independently.

Next-generation surveys will decide the issue. DESI Year-5 and Euclid will shrink the uncertainty on the interaction parameter by roughly a factor of three, pushing the 1σ threshold below the level required for dynamical resolution of the coincidence problem. If the central value stays near zero, the motivation for IDE will largely evaporate; a >3σ detection would reopen the question of whether dark energy is a dynamical field coupled to dark matter.