Phantom Crossing as Template Artifact: Cosine-Modulated Λ and the DESI w(z) Signal

DESI DR2 reports 2.8–4.2σ evidence that the dark energy equation of state w evolves with redshift, including an apparent "phantom crossing" through w = −1. We show that this crossing can arise as a fitting artifact. If the effective dark energy density carries a cosine modulation—as would occur in any cosmology where Λ occupies a standing-wave eigenmode on a bounded spatial domain—the Friedmann-inverted equation of state satisfies w_eff(z) > −1 at all redshifts. Yet fitting the resulting distances with the CPL parameterization w₀ + wₐ(1 − a) recovers w₀ > −1 and wₐ < 0, mimicking phantom crossing without exotic physics. Tested against DESI DR2 BAO data, Pantheon+ supernovae, and a Planck-calibrated H₀rₐ prior, the modulated model achieves ΔAIC = −2.1 over ΛCDM at the same parameter count (k = 2). The modulation zero-crossing at z_cross = 0.663 is registered prior to Euclid DR1 as a falsifiable prediction.