Then, as we shall see in more detail in Section 4.3, another coincidence, which is central to this whole review, is the appearance of a characteristic scale – dubbed – in the behavior of the dark matter sector, a scale with units of acceleration. This acceleration scale appears in various seemingly unrelated galactic scaling relations, mostly unpredicted by the CDM model (see Section 4.3). The value of this scale is , which yields in natural units8, (or, more precisely, ). It is perhaps even more meaningful [51, 298, 304] to note that, in these same units:9. Whether these numerical coincidences are physically relevant or just true (insignificant) coincidences remains an open question, closely related to the nature of the dark sector, which we are going to elaborate on in Sections 5 – 10. But, at this stage, it is in any case striking that the dark matter and dark energy sectors do have such a common scale. This coincidence of scales, together with the coincidence of energy densities at redshift zero, might perhaps be a strong indication that one should cease to consider dark energy as an additional component physically independent from the dark matter sector , and/or cease to consider that GR correctly describes gravity on the largest scales and in extremely weak gravitational fields, in order to perhaps address the two above coincidence problems at the same time.
Finally, let us note that the existence of the -scale is actually not the only dark-matter–related coincidence, as there is also, in principle, absolutely no reason why the mechanism leading to the baryon asymmetry (between baryonic matter and antimatter) would simultaneously leave both the baryon and dark matter densities with a similar order of magnitude (). If the effects we attribute to dark matter are actually also due to a breakdown of GR on cosmological scales, then such a coincidence might perhaps appear more natural as the baryons would then be the actual source of the effect attributed to the dark matter sector.
Living Rev. Relativity 15, (2012), 10
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