The Geometry Of Modified Newtonian Dynamics

TL;DR

This paper explores the geometric structures of modified gravity theories like AQUAL, TeVeS, and GEA, revealing unique spacetime features and their effects on particle trajectories, with implications for astrophysical phenomena.

Contribution

It provides a comprehensive geometric analysis of three modified gravity theories, uncovering novel features and generic behaviors in their spacetime structures.

Findings

AQUAL and TeVeS exhibit excess or deficit solid angles similar to monopole spacetimes.

In GEA, a disformal symmetry appears in the weak field limit.

Massive particles cannot reach infinity; photons can only do so after infinite redshift.

Abstract

Modified Newtonian Dynamics is an empirical modification to Poisson's equation which has had success in accounting for the `gravitational field' $\Phi$ in a variety of astrophysical systems. The field $\Phi$ may be interpreted in terms of the weak field limit of a variety of spacetime geometries. Here we consider three of these geometries in a more comprehensive manner and look at the effect on timelike and null geodesics. In particular we consider the Aquadratic Lagrangian (AQUAL) theory, Tensor-Vector-Scalar (TeVeS) theory and Generalized Einstein-{\AE}ther (GEA) theory. We uncover a number of novel features, some of which are specific to the theory considered while others are generic. In the case of AQUAL and TeVeS theories, the spacetime exhibits an excess (AQUAL) or deficit (TeVeS) solid angle akin to the case of a Barriola-Vilenkin global monopole. In the case of GEA, a disformal symmetry of the action emerges in the limit of $\grad\Phi\rightarrow 0$. Finally, in all theories studied, massive particles can never reach spatial infinity while photons can do so only after experiencing infinite redshift.