On the viability of the Palatini form of 1/R gravity

TL;DR

This paper defends the viability of the Palatini 1/R gravity model by clarifying misconceptions about frame equivalence and experimental constraints, arguing that previous claims ruling it out are based on a misinterpretation.

Contribution

It clarifies the physical interpretation of conformal frames in Palatini 1/R gravity, countering claims that it is experimentally ruled out.

Findings

Conformal transformations are mathematically but not physically equivalent.

The physical frame must be chosen in the vacuum theory before adding matter.

Palatini 1/R gravity remains consistent with experimental tests when properly interpreted.

Abstract

Recently Flanagan [astro-ph/0308111] has argued that the Palatini form of 1/R gravity is ruled out by experiments such as electron-electron scattering. His argument involves adding minimally coupled fermions in the Jordan frame and transforming to the Einstein frame. This produces additional terms that are ruled out experimentally. Here I argue that this conclusion is false. It is well known that conformally related theories are mathematically equivalent but not physically equivalent. As discussed by Magnano and Sokolowski [2] one must decide, in the vacuum theory, which frame is the physical frame and add the minimally coupled Lagrangian in this frame. If this procedure is followed the resulting theory is not ruled out experimentally. The discussions in this paper also show that the equivalence between the generalized gravitational theories and scalar tensor theories discussed by Flanagan [gr-qc/0309015] is only mathematical, not physical.