2025/09/18 by Yi-Syuan 宜軒 Wu 吳, Wu, Yi-Syuan, Chuan-Jui Li +3
Physics and Astronomy · Computer Science · #Cosmology and Gravitation Theories #Galaxies: Formation, Evolution, Phenomena #Computational Physics and Python Applications
paper · pdf · doi:10.48550/arxiv.2509.14826
We present a scalar-field formulation of the generalized Chaplygin gas (GCG) and modified Chaplygin gas (MCG) models, in which the cosmic fluid dynamics are reproduced by canonical Lagrangians with analytically derived energy density ρ(ϕ), pressure p(ϕ), and scalar potential V(ϕ). This framework provides a unified description of dark matter and dark energy, transitioning naturally from a matter-dominated phase at early times to a negative-pressure dark-energy phase at late times. In this scalar-field formulation, the GCG and MCG models are naturally applicable to both theoretical analyses and numerical simulations. Extending this approach, we develop a systematic method to obtain a class of integrable scalar-field cosmological models. In this study, we use this method to construct a new scalar-field altered Chaplygin gas (ACG) model. To investigate the viability of Chaplygin-type models, we perform a likelihood analysis using the Pantheon+ Type Ia supernova compilation together with Cepheid-calibrated distances. We examine four models, ΛCDM, GCG, MCG, and ACG, obtaining posterior constraints on the Hubble constant H0, the present-day effective equation of state ω0, the transition redshift z^⋆, and the cosmic age t0. With the Cepheid calibration fixing the absolute distance scale, the inferred H0 remains nearly model-independent. The Chaplygin-type models predict an earlier onset of cosmic acceleration than ΛCDM and give a broader range for the inferred age of the Universe, reflecting their greater flexibility in late-time expansion histories. Among them, the ACG model provides tighter parameter constraints, while the GCG and MCG models produce broader posteriors due to parameter degeneracies.