D. A. Tamayo Ramirez

Abstract
In this paper, 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 (Λ_s), and smoothed sign-switching cosmological constant (Λ_t). We systematically derive key cosmological thermodynamic quantities horizon temperature, horizon entropy, internal entropy, total entropy, and their first and second derivatives using the Generalized 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 Λ_s and Λ_t 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_xΩ_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.

Keywords
Dark energy thermodynamics / graduated dark energy / sign-switching cosmological constant / generalized second law / horizon entropy

International Journal of Modern Physics D
2026 February

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