We perform a comprehensive thermodynamic analysis of three sign-switching dark energy models in a flat FLRW cosmology: graduated dark energy (gDE), sign-switching cosmological constant (Λs), and smoothed sign-switching cosmological constant (Λt). We systematically derive key cosmological thermodynamic quantities—horizon temperature, horizon entropy, internal entropy, total entropy, and their first and second derivatives—using the Generalised Second Law (GSL) as the fundamental evaluation criterion. We first confirm the compliance of the ΛCDM model with the GSL, establishing a baseline for comparison. We find that despite their unconventional negative-to-positive energy density transitions, both Λs and Λt remain thermodynamically consistent. In contrast, gDE exhibits significant issues: divergences in its equation-of-state lead to infinite horizon temperature and entropy derivatives; and asymptotically, the horizon temperature diverges while entropy approaches zero, causing entropy reduction and violating the GSL. We highlight a key insight: models with divergences in the product of the dark energy equation-of-state parameter and its energy density (w x Ω x ) inevitably produce thermodynamic inconsistencies in standard cosmology. This thermodynamic approach provides a complementary criterion alongside observational constraints for evaluating the physical viability of cosmological models.
David Tamayo (Sat,) studied this question.