Phase Stability of the COCM Oscillatory Signal: A Comparison Between DR1+CC and DR2+CC Fits

The COCM semi-fixed parametrisation fits H(z) with amplitude A₀, phase φ, and background parameters, with frequency ωz = 11.1991 fixed. Applied to DR1+CC (n = 24) and DR2+CC (n = 14), the best-fit phase shifts by Δφ ≈ 1.85 rad. Among the fitted parameters, φ has the highest sensitivity to sampling, because the oscillation half-period Δz ≈ 0.28 is comparable to the typical spacing between data points. This paper examines whether the observed shift is statistically expected or indicates a systematic effect. Using mock datasets drawn under ΛCDM at the same sizes as DR1 and DR2, we argue that phase shifts of this magnitude are consistent with sampling noise. The phase φ remains undetermined with current data; its stabilisation requires DESI DR3. The propagation of phase uncertainty to the low-redshift crossing predictions of the COCM is quantified.