A tale of two paradigms: the mutual incommensurability of ΛCDM and MOND1

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 re...