Bernard GUYResearcher Retired from Mines Saint-Etienne, Institut Mines Télécom, 158 Cours Fauriel, 42023 Saint-Etienne Cedex 2, Francebernard.guy15@wanadoo.frInternational Journal of Fundamental Physical Sciences (IJFPS)https://www.fundamentaljournals.org/index.php/ijfps14224-402231-818610.14331/ijfps.2024.330166https://doi.org/10.14331/ijfps.2024.330166https://creativecommons.org/licenses/by-nc/4.0/Cosmology is currently facing some major challenges. In addition to dark matter and dark energy, the issue of ‘impossible’ galaxies has been brought to the fore by the James Webb Telescope. This paper proposes that on the cosmological scale, the speed of light should be considered lower than the standard vacuum value, introducing a refractive index. The author extends the Shapiro effect using the Schwarzschild metric and derives a cosmological refractive index (nc). The study suggests that this approach could explain various cosmic phenomena without the need to invoke dark matter or dark energy and proposes a revised estimate of the universe's age—up to 33 billion years. The proposed theory unifies velocity scaling, gravitational lensing, and cosmic microwave background interpretations.Cosmologydark matterdark energyShapiro effectSchwarzschild metric