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

Among the predictions of the standard model of cosmology, known as ΛCDM, a number contradict observations. These are referred to as problems (those of dark matter, dark energy, etc.) or tensions (the Hubble tension, the S8 tension, etc.). We only know in total about 5% of the matter and energy in the universe. These problems are currently examined separately from one another, each on its own. In this paper, we examine a discordant hypothesis: all these problems are symptoms of a single problem, that of a miscalculation of the speed of light on a cosmological scale. We propose that it be lowered by a factor of about 2.4 compared to its value in the local vacuum (solar system). Our model requires no new matter nor new laws; based on general relativity (Schwarzschild metric, extended Shapiro effect), it allows predictions to be made that can be compared with observations. For the ten or so problems discussed, the comparison is favorable, including from a quantitative point of view. Its validity is reinforced by the simplicity and uniqueness of the perspective taken on different physical situations. Should we advocate a paradigm shift and abandon the standard model (which appears to have been refuted)? This model continues to explain a great many facts. At this stage, we argue for further exploration of our hypothesis and detailed examination of the various difficult cases, in coexistence with the standard model, whose interpretation needs to be renewed.