4.2 Unobserved predictions

Apart from the above puzzling coincidences, the concordance ΛCDM model also has a few more concrete empirical challenges to address, in the sense of having made a few predictions in contradiction with observations (with the caveat in mind that the model itself is not always that predictive on small scales). These include the following non-exhaustive list:
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Figure 2: The fraction of the expected baryons that are detected as a function of potential-well depth (bottom axis) and mass (top). Measurements are referenced to the radius R500, where the enclosed density is 500 times the cosmic mean [284Jump To The Next Citation Point]. The detected baryon fraction fd = Mb ∕ (0.17M500 ), where Mb is the detected baryonic mass, 0.17 is the universal baryon fraction [229Jump To The Next Citation Point], and M500 is the dynamical mass (baryonic + dark mass) enclosed by R500. Each point is a bin representing many objects. Gray triangles represent galaxy clusters, which come close to containing the cosmic fraction. The detected baryon fraction declines systematically for smaller systems. Dark-blue circles represent star-dominated spiral galaxies. Light-blue circles represent gas-dominated disk galaxies. Orange squares represent Local Group dwarf satellites for which the baryon content can be less than 1% of the cosmic value. Where these missing baryons reside is one of the challenges currently faced by ΛCDM.

However, let us note that, while challenges 1 to 3 are not real smoking guns yet for the ΛCDM model, challenges 4 to 10 are concerned with processes happening on kpc scales, for which it is fair to consider that the model is not very predictive because the baryon physics should play a more important role, and this is hard to take into account rigorously. However, it is not sufficient to qualitatively invoke handwavy baryon physics to avoid confronting predictions of ΛCDM with observations. It is also mandatory to show that the feedback from the baryons, which is needed to solve the observational problems, is what would quantitatively happen in a physical galaxy. This, presently, is not yet the case for the aforementioned challenges. However, these challenges are “model-dependent problems”, in the sense of being failed predictions of a given model, but would not have appeared a priori surprising without the standard concordance model at hand. This means that subtly changing some parameters of the model (like, e.g., swapping CDM for WDM, making DM more self-interacting, etc.) might help solving at least a few of them. But what is even more challenging is a set of observations that appear surprising independently of any specific dark matter model, as they involve a fine-tuned relation between the distribution of visible and dark matter. These are what we call hereafter “unpredicted observations”.

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