A new cosmological study is putting one of the most deeply rooted assumptions in modern physics under fresh observational pressure: that Newton’s gravitational constant, (G), and Einstein’s cosmological constant, (\Lambda), remain unchanged throughout the history of the universe. In research published in Astrophysics and Space Science, S. Mandal, A. Singh, and R. Chaubey investigate a model in which both quantities evolve with cosmic time, asking whether a changing gravitational interaction and dynamic dark energy can reproduce the expansion history measured by today’s most powerful astronomical surveys. Their analysis combines the latest Dark Energy Spectroscopic Instrument Data Release 2 baryon acoustic oscillation measurements with observations from Cosmic Chronometers and the Pantheon+SH0ES supernova compilation. The result is a data-driven test of whether the standard (\Lambda)CDM picture is the final word—or merely the simplest approximation to a more flexible cosmic theory.
The idea that (G) might vary is not new. It has appeared in theories inspired by Dirac’s large-number hypothesis, scalar–tensor gravity, Brans–Dicke theory, and several approaches to modified gravity. In ordinary general relativity, (G) sets the strength of the coupling between matter and spacetime curvature. If it changes, the gravitational response of the universe changes as well, potentially affecting the expansion rate, the growth of cosmic structure, stellar evolution, planetary motion, and the behavior of compact objects. The cosmological term (\Lambda), meanwhile, is usually interpreted as a constant vacuum-energy density that drives the late-time acceleration of cosmic expansion. Allowing both quantities to evolve introduces a new degree of freedom into the background dynamics and may provide a way to describe observations that do not fit perfectly within a rigid constant-(G), constant-(\Lambda) framework.
Mandal and colleagues focus on an isotropic and homogeneous universe, described by the Friedmann–Lemaître–Robertson–Walker metric. This approximation treats the cosmos on very large scales as spatially uniform and directionally equivalent, making it possible to connect theoretical equations with measurements of the Hubble expansion. The authors adopt a power-law parametrization for the gravitational constant, allowing (G) to change in a controlled mathematical form rather than introducing an entirely arbitrary function. They then solve the corresponding cosmological equations to derive the model’s expansion rate. Because a time-dependent gravitational coupling modifies the Friedmann equations and their consistency conditions, the model must be examined as a coupled system in which the evolution of (G), (\Lambda), matter, and the scale factor are not independent ingredients.
The observational engine of the study is DESI’s second data release, which provides baryon acoustic oscillation measurements across a broad range of cosmic epochs. Baryon acoustic oscillations are relics of sound waves that travelled through the hot plasma of the early universe before atoms formed. Their characteristic scale became imprinted in the distribution of galaxies, quasars, and intergalactic matter. Today, that scale functions as a cosmic ruler. By measuring its apparent size across different redshifts, astronomers can infer combinations of the expansion history, including the Hubble parameter (H(z)) and distance–redshift relations. DESI’s measurements are especially valuable because they probe the period when dark energy began to dominate the universe, precisely the era in which departures from a constant cosmological term could become visible.
To strengthen the analysis, the researchers combine DESI BAO observations with two independent classes of data. Cosmic Chronometers estimate the expansion rate directly through the differential ageing of passively evolving galaxies. Rather than relying primarily on a distance ladder, this method uses galaxies as cosmic clocks: the change in their ages over redshift provides an estimate of (dz/dt), which can be converted into (H(z)). The Pantheon+SH0ES supernova dataset contributes luminosity-distance information from Type Ia supernovae, stellar explosions whose calibrated brightness allows them to be used as standardisable candles. Together, the three datasets test the model through complementary observables—cosmic distances, direct expansion rates, and supernova brightness—reducing the risk that a preferred result is driven by a single observational technique.
The team estimates the parameters of the varying-(G), varying-(\Lambda) framework using Markov chain Monte Carlo analysis. MCMC methods explore the multidimensional parameter space by generating chains of possible cosmological models, assigning greater statistical weight to those that provide better agreement with the data. The resulting likelihood analysis allows the authors to constrain the parameters controlling the gravitational variation and the cosmic expansion while accounting for observational uncertainties and parameter correlations. The study also compares parameter estimates obtained from different dataset combinations, an important test of robustness. If independent observations point toward compatible regions of parameter space, confidence in the model’s viability increases; if they disagree, the apparent preference for evolving physics may instead reflect statistical fluctuations or hidden systematic effects.
Beyond fitting the expansion data, the researchers reconstruct the evolution of several quantities that describe the universe’s physical behavior. The Hubble parameter (H(z)) tracks the expansion rate at different times, while the deceleration parameter (q(z)) indicates whether expansion is slowing or accelerating. A transition from positive (q) in the matter-dominated past to negative (q) in the recent universe signals the onset of accelerated expansion. The authors also examine an effective equation-of-state parameter, (w_{\rm eff}), which summarizes the pressure-to-density behavior of the total cosmic contents. In standard terminology, values near (-1) resemble a cosmological constant, while departures from that value can indicate evolving dark energy or an effective modification of gravity. In a model where (G) and (\Lambda) vary, these reconstructed parameters provide an intuitive view of how the new physics reshapes cosmic history.
A central motivation for the work is the continuing debate over whether dark energy is truly constant. Several recent analyses of large-scale structure and supernova data have reported hints that the properties of dark energy may evolve, although the statistical significance and interpretation remain under discussion. A varying cosmological term could mimic some of the observational signatures associated with dynamical dark energy, while a changing (G) could alter the inferred expansion history without behaving like a conventional dark-energy fluid. This distinction matters because cosmological parameters are not measured in isolation. Inferences about the amount and nature of dark energy depend on assumptions about gravity, the standard ruler, the calibration of supernovae, and the conservation of matter and energy. By varying (G) and (\Lambda) together, the study explores a broader theoretical landscape in which apparent tensions may arise from interactions between gravitational physics and cosmic acceleration.
The authors also test the late-time behavior of the model by examining the present-day rate of change of the gravitational constant, expressed as (\dot G/G), and by applying information criteria to compare the model with (\Lambda)CDM. The quantity (\dot G/G) is especially important because local experiments place stringent limits on any present variation in (G), using lunar laser ranging, planetary dynamics, pulsar timing, and other precision measurements. A cosmological model can fit large-scale observations yet remain physically problematic if it predicts a modern rate of change that violates these local bounds. Information criteria provide a complementary perspective: they penalize models for adding parameters, asking whether improved agreement with observations is substantial enough to justify the extra complexity. The study therefore treats a successful cosmology not simply as one capable of matching data, but as one that remains compatible with local gravitational tests and earns its additional freedom statistically.
The broader significance of the research lies in its attempt to connect two major questions in contemporary cosmology: why the universe is accelerating and whether the laws governing gravity are immutable. The analysis does not overturn general relativity or establish that (G) and (\Lambda) definitely vary. Instead, it supplies observational constraints on a specific phenomenological framework and evaluates how its predicted expansion history compares with the established cosmological model. As DESI continues mapping the universe and future supernova, galaxy, and gravitational-wave surveys expand the available data, models with evolving constants will face increasingly precise tests. If their parameters converge toward zero variation, the result will reinforce (\Lambda)CDM and place tighter limits on alternatives. If consistent evidence for evolution emerges across independent datasets, cosmology could be forced to reconsider whether the constants written into its foundational equations are truly constant—or only appear so across the limited cosmic history we have measured.
Subject of Research: Varying gravitational constant and cosmological term in observational cosmology
Article Title: Observational constraints on varying-((G,\Lambda)) cosmology with DESI DR2
Article References: Mandal, S., Singh, A. & Chaubey, R. Astrophysics and Space Science 371, 44 (2026). https://doi.org/10.1007/s10509-026-04576-2
Image Credits: AI Generated
DOI: 10.1007/s10509-026-04576-2
Keywords: Varying dark energy, gravitational constant, cosmological constant, DESI DR2, Cosmic Chronometers, Pantheon+SH0ES, cosmology, cosmic acceleration
