The road to MOND--a novel perspective

TL;DR

This paper presents a new perspective on MOND, framing it as a specific case within a broader class of scale-invariant modified gravity theories that explain galactic mass discrepancies.

Contribution

It introduces a novel formulation of modified dynamics theories emphasizing scale invariance, positioning MOND as a particular instance supported by recent data.

Findings

MOND is a special case of scale-invariant dynamics.

Galactic discrepancies can be explained by replacing G with a scale-invariant constant.

Data supports the acceleration-based boundary between standard and modified dynamics.

Abstract

Accepting that galactic mass discrepancies are due to modified dynamics, I show why it is specifically the MOND paradigm that is pointed to cogently. MOND is thus discussed here as a special case of a larger class of modified dynamics theories whereby galactic systems with large mass discrepancies are described by scale-invariant dynamics. This is a novel presentation that uses more recent, after-the-fact insights and data (largely predicted beforehand by MOND). Starting from a purist set of tenets, I follow the path that leads specifically to the MOND basic tenets. The main signposts are: (i) Space-time scale invariance underlies the dynamics of systems with large mass discrepancies. (ii) In these dynamics, $G$ must be replaced by a single "scale-invariant" gravitational constant, Q0 (in MOND, Q0=A0=Ga0, where a0 is MOND's acceleration constant). (iii) Universality of free fall points to the constant q0=Q0/G as the boundary between the G-controlled, standard dynamics, and the Q0-controlled, scale-invariant dynamics (in MOND, q0=a0). (iv) Data clinches the case for q0 being an acceleration (MOND).