Universal Modified Newtonian Dynamics Relation between the Baryonic and "Dynamical" Central Surface Densities of Disc Galaxies

Cosmological realism

I discuss the relevance of the current predicament in cosmology to the debate over scientific realism. I argue that the existence of two, empirically successful but ontologically inconsistent cosmological theories presents difficulties for the realist position.

Testing galaxy formation and dark matter with low surface brightness galaxies

Galaxies are the basic structural element of the universe; galaxy formation theory seeks to explain how these structures came to be. I trace some of the foundational ideas in galaxy formation, with emphasis on the need for non-baryonic cold dark matter. Many elements of early theory did not survive contact with observations of low surface brightness galaxies, leading to the need for auxiliary hypotheses like feedback. The failure points often trace to the surprising predictive successes of an alternative to dark matter, the Modified Newtonian Dynamics (MOND). While dark matter models are flexible in accommodating observations, they do not provide the predictive capacity of MOND. If the universe is made of cold dark matter, why does MOND get any predictions right?

Interposing a Varying Gravitational Constant Between Modified Newtonian Dynamics and Weak Weyl Gravity

The Newtonian gravitational constant G obeys the dimensional relation [G][M][a] = [v]⁴, where M, a, and v denote mass, acceleration, and speed, respectively. Since the baryonic Tully-Fisher (BTF) and Faber-Jackson (BFJ) relations are observed facts, this relation implies that G a = constant. This result cannot be obtained in Newtonian dynamics which cannot explain the origin of the BTF and BFJ relations. An alternative, modified Newtonian dynamics (MOND) assumes that G = G ₀ is constant in space and derives naturally a characteristic constant acceleration a = a ₀, as well as the BTF and BFJ relations. This is overkill and it comes with a penalty: MOND cannot explain the origin of a ₀. A solid physical resolution of this issue is that G ∝ a ^-1, which implies that in lower-acceleration environments the gravitational force is boosted relative to its Newtonian value because G increases. This eliminates all problems related to MOND's empirical cutoff a ₀ and yields a quantitative method for mapping the detailed variations of G(a) across each individual galaxy as well as on larger and smaller scales. On the opposite end, the large accelerations produced by G(a) appear to be linked to the weak-field limit of the fourth-order theory of conformal Weyl gravity.