Do The Many Problems of Contemporary Cosmology Have a Single Cause? A Research Program

Abstract

A number of key predictions of the standard cosmological model are in tension with observations, including the apparent need for dark matter and dark energy, the Hubble tension, the S₈ tension, and the existence of seemingly “impossible” early galaxies. Together, these issues imply that only about 5% of the Universe’s matter–energy content is understood, yet they are typically examined in isolation. In this work, we explore a single unifying hypothesis: these diverse discrepancies may arise from a systematic misestimation of the speed of light on cosmological scales. We propose that the effective cosmological light speed is reduced by a factor of approximately 2.4 relative to its local vacuum value. This framework introduces no new forms of matter, fields, or laws; instead, it is grounded in general relativity through an extension of the Shapiro effect. From this assumption, we derive a set of quantitative predictions and compare them with observations across roughly a dozen major cosmological problems. The agreement obtained is consistently favorable, and the coherence of the approach strengthens its plausibility. While it may be premature to dismiss the standard model outright, our results suggest that its interpretation par-ticularly its assumption that the speed of light is universally equal to its local value requires reconsideration. We advocate pursuing this hypothesis in parallel with the ΛCDM model, as it offers a simple, unified alternative to the numerous ad hoc components currently invoked.