Polytropic Gas Cosmology and Late-time Acceleration

Abstract The unequivocal acceleration of the Universe's expansion has ignited a deep inquiry into the mysterious domain of dark energy within the well-established cosmological framework. Despite the emergence of alternative explanations to dark energy, a multitude of theoretical models has arisen, seeking to unravel the enigmatic origins and unique characteristics of this cosmic entity. This research work is an investigation into the realm of cosmic expansion employing the Polytropic Gas (PG) paradigm, harmonizing the enigmatic constituents, Dark Matter (DM) and Dark Energy (DE), under the umbrella of a singular fluid of enigmatic origin. We have considered the conventional principles of general relativity, adopting a homogeneous and isotropic framework for our model building within the Friedmann-Lemaître-Robertson-Walker (FLRW) spacetime. To attain this endeavor, a novel rendition of the Original Polytropic Gas (OPG) formulation is reconstructed, signifying its broadened applicability beyond the OPG, the $\Lambda$CDM model, and two distinct Chaplygin gas models. The model’s auxiliary parameters are meticulously fitted, elucidating the fundamental cosmological attributes inherent to the variable Polytropic gas proposition. To constrain our cosmological model, we performed a Markov chain Monte Carlo analysis using recent Supernova data, namely Pantheon+ combined with 36 points from the Hubble datasets. The AIC and BIC criteria show that our model is slightly favored by the observational data compared to the $\Lambda$CDM model. We also checked our model against Hubble and Supernova datasets. Finally, we have performed a diagnostic analysis and showed that our model fits perfectly inside the quintessence fields. In summation, the variable Polytropic Gas Model (PGM) emerges as a compelling candidate, effectively encompassing the enigmatic interplay between cosmic acceleration, dark energy, and the intricacies of thermodynamics.