The Umami Chaplygin Gas as a Unified Dark Fluid: Observational Constraints, Perturbative and Thermodynamic Stability Analysis
Abstract
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 ρ1=√(|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|ρ02, 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 ≃ −0.73, Ωmeff ≃ 0.32 and H0 ≃ 66.8 km s−1 Mpc−1. It is statistically comparable to flat ΛCDM, with ΔAIC=−4.8 and ΔBIC=+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 ΔAIC≃+66 relative to ΛCDM. The best fit that obeys the condition exactly still has ΔAIC≃+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.
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BibTeX
@misc{banerjee2026umami,
title = {{The Umami Chaplygin Gas as a Unified Dark Fluid: Observational Constraints, Perturbative and Thermodynamic Stability Analysis}},
author = {Trishit Banerjee and Atreyee Biswas},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.03697},
eprint = {2611.03697},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.03697},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The Umami Chaplygin Gas as a Unified Dark Fluid: Observational Constraints, Perturbative and Thermodynamic Stability Analysis
AU - Trishit Banerjee
AU - Atreyee Biswas
PY - 2026
DA - 2026/11/01
PB - arXiv
DO - 10.48550/arxiv.2611.03697
UR - https://arxiv.org/abs/2611.03697
AB - 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=√(|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 ≃ −0.73, Ω_m^eff ≃ 0.32 and H_0 ≃ 66.8 km s^−1 Mpc^−1. It is statistically comparable to flat ΛCDM, with ΔAIC=−4.8 and ΔBIC=+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 ΔAIC≃+66 relative to ΛCDM. The best fit that obeys the condition exactly still has ΔAIC≃+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.
ER -