A tale of two paradigms: the mutual incommensurability of ΛCDM and MOND

Abstract

The concordance model of cosmology, ΛCDM, provides a satisfactory description of the evolution of the universe and the growth of large-scale structure. Despite considerable effort, this model does not at present provide a satisfactory description of small-scale structure and the dynamics of bound objects like individual galaxies. In contrast, MOND provides a unique and predictively successful description of galaxy dynamics, but is mute on the subject of cosmology. Here I briefly review these contradictory world views, emphasizing the wealth of distinct interlocking lines of evidence that went into the development of ΛCDM while highlighting the practical impossibility that it can provide a satisfactory explanation of the observed MOND phenomenology in galaxy dynamics. I also briefly review the baryon budget in groups and clusters of galaxies where neither paradigm provides an entirely satisfactory description of the data. Relatively little effort has been devoted to the formation of structure in MOND; I review some of what has been done. While it is impossible to predict the power spectrum of the microwave background temperature fluctuations in the absence of a complete relativistic theory, the amplitude ratio of the first to second peaks was correctly predicted a priori. However, the simple model that makes this predictions does not explain the observed amplitude of the third and subsequent peaks (3e, 4e, …). MOND anticipates that structure forms more quickly than in ΛCDM. This motivated the prediction that reionization would happen earlier in MOND than originally expected in ΛCDM, as subsequently observed. This also provides a natural explanation for massive, early clusters of galaxies and large, empty voids. However, it is far from obvious that the mass spectrum of galaxy clusters or the power spectrum of galaxies can be explained in MOND, two things that ΛCDM does well. Critical outstanding issues are the development of an acceptable relativistic parent theory for MOND, and the reality of the non-baryonic dark matter of ΛCDM. Do suitable dark matter particles exist, or are they a modern æther?