Is the use of Christoffel connection in gravity theories conceptually correct?

TL;DR

This paper questions the conceptual correctness of using Christoffel connections in gravity theories, especially in regions dominated by massless particles like the early universe, highlighting potential inconsistencies.

Contribution

It demonstrates that the identification of Christoffel connections with particle orbits is inconsistent in the absence of massive particles, challenging their universal applicability in gravity theories.

Findings

Christoffel connection does not necessarily represent particle orbits in massless-only regions.

In the absence of massive particles, the connection cannot be uniquely defined.

Implications for the use of Christoffel connection in early universe and high-energy astrophysical regions.

Abstract

Christoffel connection did not enter gravity as an axiom of minimal length for the free fall of particles (where anyway length action is not defined for massless particles), nor out of economy, but from the weak equivalence principle (gravitational force is equivalent to acceleration according to Einstein) together with the identification of the local inertial frame with the local Lorentz one. This identification implies that the orbits of all particles are given by the geodesics of the Christoffel connection. Here, we show that in the presence of only massless particles (absence of massive particles) the above identification is inconsistent and does not lead to any connection. The proof is based on the existence of projectively equivalent connections and the absence of proper time for null particles. If a connection derived by some kinematical principles for the particles is to be applied in the world, it is better these principles to be valid in all relevant spacetime rather than different principles to give different connections in different spacetime regions. Therefore, our result stated above may imply a conceptual insufficiency of the use of the Christoffel connection in the early universe where only massless particles are expected to be present (whenever at least some notions, like orbits, are meaningful), and thus of the total use of this connection. If in the early universe the notion of a massive particle, which appears latter in time, cannot be used, in an analogous way in a causally disconnected high-energy region (maybe deep interior of astrophysical objects or black holes) the same conclusions could be extracted if only massless particles are present.