The Umami Chaplygin Gas as a Unified Dark Fluid: Observational Constraints, Perturbative and Thermodynamic Stability Analysis

We study a cosmological scenario in which the Universe contains standard baryonic matter, radiation, and a single unified dark fluid, the umami Chaplygin gas, which accounts for both dark matter and dark energy with no separate dark-matter component. We first analyse the thermodynamic behaviour of the fluid and show that, unlike earlier Chaplygin-type models, its thermodynamic stability is conditional rather than global: stability holds only above a critical density $ρ_1=\sqrt{|A/w|}$, the fluid reheats once this density is crossed, and a viable low-density regime requires $|w|<2$. Thermodynamic stability at the present epoch requires $|A|<|w|ρ_0^2$, a condition that is more easily met for $|w|>1$. We then constrain both the $|w|<1$ and $|w|>1$ cases with Type Ia supernovae, DESI DR2 baryon acoustic oscillations, cosmic-chronometer Hubble data and Planck 2018 distance priors through a Markov Chain Monte Carlo analysis. The $|w|<1$ case fits all four datasets well, with $w \simeq -0.73$, $Ω_m^{\rm eff} \simeq 0.32$ and $H_0 \simeq 66.8$ km s$^{-1}$ Mpc$^{-1}$. It is statistically comparable to flat $Λ$CDM, with $Δ{\rm AIC}=-4.8$ and $Δ{\rm BIC}=+0.6$. Its best-fit parameters violate the stability condition. The $|w|>1$ case with a Gaussian prior satisfies the condition but fits worse, with $Δ{\rm AIC}\simeq+66$ relative to $Λ$CDM. The best fit that obeys the condition exactly still has $Δ{\rm AIC}\simeq+50$. Finally, a linear perturbation analysis shows that the adiabatic sound speed turns negative at exactly the same critical density, triggering exponential small-scale growth at late times in the observationally preferred case. These results reveal a tension between a good fit to background data and the physical stability of the umami unified dark fluid.