A thawing tachyon as physical dark energy after DESI DR2: imaginary mass, ghost-free dynamics and a non-crossing prediction

We test the rolling tachyon of string theory as the physical origin of the evolving dark energy suggested by DESI DR2. The tachyon obeys the null energy condition identically (w ≥ −1), has a Hamiltonian bounded below, and thaws from a Hubble-frozen state into the quadrant w₀ > −1, wₐ < 0 preferred by DESI. Thawing tachyons follow wₐ ≈ −(1.2–1.5)(1+w₀) and reduce distance residuals relative to the best-fitting CPL histories by 22–31% compared with ΛCDM. The model predicts that no reconstruction of w(z) will show a significant excursion below w = −1.