A new cosmological model offers a possible explanation for why the expansion of the Universe is speeding up while remaining statistically competitive with the standard picture of cosmology. In a study published in Astrophysics and Space Science, Amit Samaddar, Meghanil Sinha and S. Surendra Singh examine whether a generalized Chaplygin gas can produce the observed history of cosmic expansion when combined with a modified theory of gravity in which spacetime curvature interacts directly with matter. Their calculations describe a Universe that evolves naturally from an early, matter-dominated phase into the accelerated expansion seen today. The model predicts a present-day Hubble constant of approximately 67.6–67.8 kilometres per second per megaparsec, a current deceleration parameter near –0.50 and an estimated cosmic age of 13.1–13.6 billion years. Those values are broadly consistent with several major astronomical observations, including measurements of baryon acoustic oscillations, Type Ia supernovae and the cosmic microwave background.
The work addresses one of modern cosmology’s most persistent puzzles: the origin of dark energy. In the conventional ΛCDM model, cosmic acceleration is attributed to a cosmological constant, represented by Λ, which behaves like an energy density intrinsic to empty space. Although ΛCDM fits a wide range of observations remarkably well, its physical origin remains unexplained, and tensions have emerged between different measurements of the current expansion rate. The new study explores a different possibility in which the accelerating component is represented by a generalized Chaplygin gas, an exotic fluid originally proposed in a different physical context. The model is especially attractive because the same effective substance can imitate matter during the young Universe and dark energy at late times, potentially reducing the need to assign those roles to entirely separate cosmic ingredients.
The generalized Chaplygin gas is defined through an unusual relationship between pressure and density. In its commonly used form, its pressure is written as (p=-A/\rho^\alpha), where (A) is a positive constant, (\rho) is the energy density and (\alpha) controls how rapidly the fluid changes as the Universe expands. At high density, the pressure contribution becomes relatively small, so the fluid behaves approximately like pressureless matter. As the density falls during cosmic expansion, the negative pressure becomes increasingly important. Negative pressure has a gravitationally repulsive effect on cosmic scales: in the Friedmann equations, sufficiently negative pressure can drive the scale factor’s second time derivative positive, meaning that the expansion accelerates rather than slows. The generalized form gives researchers more flexibility than the original Chaplygin gas, allowing its expansion history to be confronted with modern data.
Samaddar and colleagues combine this fluid with (f(R,L_m)) gravity, a framework in which the gravitational action depends on both the Ricci scalar (R), which summarizes spacetime curvature, and the matter Lagrangian (L_m), which represents the physical contents of the Universe. In ordinary general relativity, matter and geometry are linked through the Einstein–Hilbert action, while the matter sector is generally treated as separately coupled to the metric. In curvature–matter-coupled theories, that separation is altered: the way matter contributes to the gravitational field can itself depend on curvature. Such a coupling can modify the effective Friedmann equations and the evolution of cosmic fluids, potentially generating acceleration without inserting a conventional cosmological constant.
A central result of the analysis is that the form of the coupling matters. The researchers investigate a linear gravitational Lagrangian, (f(R,L_m)=R/2+\gamma L_m), with (\gamma) describing the strength of the matter contribution. According to their calculations, a linear matter coupling is necessary to preserve the conventional Friedmann scaling while also reproducing the characteristic expansion behaviour of the generalized Chaplygin gas. This requirement is significant because a modified-gravity model must do more than create late-time acceleration: it must also recover the successful early-Universe behaviour that underpins structure formation, the cosmic microwave background and the standard distance–redshift relation. If the coupling altered the scaling of matter too strongly, the theory could conflict with observations long before acceleration began.
To test the model, the team compares it with several independent cosmological data sets. The analysis includes 31 cosmic-chronometer measurements, which estimate the Hubble expansion rate at different redshifts by using the ages of passively evolving galaxies. It also incorporates the second data release from the Dark Energy Spectroscopic Instrument, or DESI, whose baryon acoustic oscillation measurements trace a characteristic scale imprinted by sound waves in the early Universe. That scale acts as a standard ruler for reconstructing how distances and expansion rates have changed over cosmic time. The researchers additionally use three compilations of Type Ia supernova observations: Pantheon+, DES-SN5Y and Union 3. These stellar explosions provide luminosity distances and have played a decisive role in revealing that cosmic expansion is accelerating.
Across the combined data choices, the inferred Hubble constant remains stable at roughly 67.6–67.8 kilometres per second per megaparsec. The value is close to estimates derived from the cosmic microwave background under ΛCDM, rather than the higher values obtained from some local distance-ladder measurements. This does not by itself resolve the so-called Hubble tension, because the result depends on the assumptions and data included in the fit, but it indicates that the proposed model does not require an extreme expansion rate to match observations. The model also produces a smooth transition from deceleration to acceleration. In its early phase, the effective cosmic fluid behaves in a matter-like way, allowing gravitational clumping and the growth of galaxies. At later times, its pressure becomes negative enough for the expansion to accelerate, with the present deceleration parameter reaching approximately (q_0=-0.50).
The model’s effective dark-energy equation of state provides another important clue. The researchers find a present value near (\omega_0=-0.83), remaining above –1 throughout cosmic history. A value of (\omega=-1) corresponds to a cosmological constant, while values between –1 and –1/3 are commonly described as quintessence-like and can generate accelerated expansion. Values below –1 would indicate a so-called phantom regime, which can be associated with theoretical instabilities or an eventual “big rip” in some scenarios. By remaining quintessence-like, the Chaplygin-gas model avoids crossing that boundary in the analysis. Its negative pressure is therefore strong enough to accelerate the Universe, but not so extreme that the model enters the phantom domain.
The authors also apply statefinder diagnostics, a set of higher-order geometric quantities designed to distinguish competing explanations for cosmic expansion. Whereas the Hubble parameter and deceleration parameter describe the expansion rate and its first change, statefinder variables incorporate higher derivatives of the scale factor. In a diagram built from these quantities, different cosmological models trace different trajectories. The analysis places the present Universe in a region associated with Chaplygin-gas behaviour, while the model’s future trajectory approaches the attractor expected for ΛCDM. This suggests that the two frameworks may become increasingly difficult to distinguish using only the broad expansion history, even though their underlying physics is different. The predicted age of 13.1–13.6 billion years likewise agrees with estimates from cosmic microwave background analyses and independent studies of old stars and stellar populations.
Statistical comparison is crucial because a model can fit data while still being disfavoured if it introduces unnecessary complexity. Using information criteria, including measures related to the Akaike and Bayesian approaches, the researchers report that the generalized Chaplygin gas in linear (f(R,L_m)) gravity remains statistically competitive with ΛCDM. The result does not establish that the new model is the correct description of reality, nor does it eliminate the cosmological constant. Instead, it identifies a physically viable alternative whose parameters are compatible with current observations. Future measurements from DESI and other large-scale structure surveys, together with improved supernova samples, gravitational-lensing observations and refined cosmic-chronometer data, could test whether the expansion history departs subtly from the ΛCDM prediction. The study’s main implication is that cosmic acceleration may be reproduced by a unified effective fluid and a carefully constrained interaction between matter and curvature, keeping open a dramatic possibility: the dark sector could reflect modified gravitational dynamics rather than a perfectly constant energy hidden in empty space.
