Galaxies as simple dynamical systems: observational data disfavor dark matter and stochastic star formation

According to modern theory, galactic evolution is driven by the dynamics of dark matter and stochastic star formation, but galaxies are observed to be simple systems. The existence of dark matter particles is a key hypothesis in present-day cosmology and galactic dynamics. Given the large body of high-quality work within the standard model of cosmology (SMoC), the validity of this hypothesis is challenged significantly by two independent arguments: (i) The dual dwarf galaxy theorem must be true in any realistic cosmological model. But the data now available appear to falsify this postulate when the dark-matter-based model is compared with the observational data. A consistency test for this conclusion comes from the significantly anisotropic distributions of satellite galaxies (baryonic mass <10⁸M⊙) that orbit in the same direction around their hosting galaxies in disk-like structures, which cannot be derived from dark matter models. (ii) The action of dynamical friction due to expansive and massive dark matter halos must be evident in the galaxy population. The evidence for dynamical friction is poor or even absent. Indendently of this, the long history of failures of the SMoC have reduced the likelihood that it describes the observed Universe to less than 10^-4%. The implication for fundamental physics is that exotic dark matter particles do not exist and that consequently effective gravitational physics on the scales of galaxies and beyond ought to be non-Newtonian and (or) non-Einsteinian. An analysis of the kinematic data in galaxies shows them to be described elegantly in the weak-gravitational regime by scale-invariant dynamics, as discovered by Milgrom. The full classical regime of gravitation is effectively described by Milgromian dynamics. This leads to a natural emergence of the simple laws that galaxies are indeed observed to obey. Such success has not been forthcoming in dark-matter-based models. Observations of stellar populations in galaxies suggest that the galaxy-wide initial mass function varies with star formation rate and that stochastic descriptions of star formation are inconsistent with the data. This requires a reinterpretation of the stellar mass assembly in galaxies and thus of the accretion rates onto galaxies. A consequence of this understanding of galactic astrophysics is that most dwarf satellite galaxies are formed as tidal dwarf galaxies in galaxy-galaxy encounters, that they follow the mass-metallicity relation, that galactic mergers are rare, that galaxies immersed in external potentials are physically larger than isolated galaxies, and that star-forming galaxies form a main sequence. Eight predictions are offered that will allow the concepts raised here to be tested. A very conservative, cold- and warm-dark-matter-free cosmological model may be emerging from these considerations. (author)