Testing Refracted Gravity with the kinematics of stacked galaxy clusters

We test Refracted Gravity, a phenomenological modified gravity model that modifies the Poisson equation by introducing an effective gravitational permittivity $ε$ which depends on the baryonic matter density $ρ_{\rm bar}$ and mimics the. The new analysis still awaits peer review, but it already lays out the central claim clearly.

This matters because cosmology operates at the edge of what current instruments can measure, where systematic errors and model assumptions are never trivial. Small discrepancies between independent measurements have historically pointed toward missing physics rather than simple calibration errors, and the ongoing tension in the Hubble constant is a live example of how a persistent disagreement between methods can reshape the theoretical landscape. Each new dataset that approaches this territory with independent systematics adds real information to a problem that has resisted easy resolution for more than a decade. We test Refracted Gravity (RG), a phenomenological modified gravity model that modifies the Poisson equation by introducing an effective gravitational permittivity $ε(ρ_{\rm. We perform a kinematical analysis of member galaxies for a sample of 74 galaxy clusters, compiled from the Cluster Infall Regions in the Sloan Digital Sky Survey (CIRS).

We stack the clusters in three bins of increasing velocity dispersion to mitigate the effects of systematic errors induced by deviations from dynamical equilibrium and from the. By means of the \texttt{MG-MAMPOSSt} code, we reconstruct the gravitational potential in RG and infer the model parameters jointly with the gas and stellar mass-profile.

The estimates of the main parameter in Refracted Gravity, the vacuum permittivity $ε_0$, range from $0.04$ to $0. The constraints are mutually compatible within the quoted uncertainties and overlap with previous cluster-scale estimates.

The agreement becomes less sensitive to gas parametrisation when selecting the less-disturbed clusters for building the stacks. These results characterize the sensitivity of cluster-scale RG constraints to baryonic modelling and sample selection.

The relevance goes beyond one dataset because even small shifts in measured parameters can matter when the field is testing the limits of the standard cosmological model. The Lambda-CDM framework describes the observable universe with remarkable economy, but its success rests on two components, dark matter and dark energy, whose physical nature remains entirely unknown. Any credible measurement that tightens or loosens the constraints on those components moves the entire theoretical enterprise forward, regardless of whether the immediate result looks dramatic on its own terms.

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Because this is still a preprint, the result should be read with genuine interest and proportionate caution. Peer review is not a guarantee of correctness, but it is a process that forces authors to respond to technical criticism from specialists who have no stake in a particular outcome. Preprints that survive that process, often with substantive revisions, emerge with a stronger evidential base than the version that first appeared. Until that stage is complete, the responsible reading keeps uncertainty explicitly visible rather than treating the claims as established findings.

The next step is to see whether the effect survives when independent surveys, different calibration strategies and tighter control of systematic uncertainties enter the picture. Programmes such as Euclid, DESI and the Rubin Observatory will deliver datasets over the next several years that cover the same parameter space with largely independent methods. If the current signal persists through those tests, its theoretical implications will become impossible to set aside. Until peer review and independent follow-up address those open questions, skepticism is not a failure of appreciation for the work; it is part of how science decides what to keep.

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