Conformal transformations in classical gravitational theories and in cosmology

TL;DR

Conformal transformations provide a unifying mathematical framework across gravity theories by mapping non-Einstein frames to Einstein gravity with additional scalar sectors. The Einstein frame is advocated as the physical frame in many classical theories due to positive energy and stability, but this choice is not universal, especially when matter is present. In cosmology, frame dependence affects inflationary dynamics and perturbations, complicating comparisons with observations unless predictions are anchored in the physical frame. The review also discusses nonminimal couplings, higher-dimensional reductions, and the implications for experiments, emphasizing the need for frame-consistent interpretations in both classical and quantum regimes.

Abstract

In recent years, the use of conformal transformation techniques has become widespread in the literature on gravitational theories alternative to general relativity, on cosmology, and on nonminimally coupled scalar fields. Typically, the transformation to the Einstein frame is generated by a fundamental scalar field already present in the theory. In this context, the problem of which conformal frame is the physical one has to be dealt with and, in the general case, it has been clarified only recently; the formulation of a theory in the ``new'' conformal frame leads to departures from canonical Einstein gravity. In this article, we review the literature on conformal transformations in classical gravitational theories and in cosmology, seen both as purely mathematical tools and as maps with physically relevant aspects. It appears particularly urgent to refer the analysis of experimental tests of Brans-Dicke and scalar-tensor theories of gravity, as well as the predictions of cosmological inflationary scenarios, to the physical conformal frame, in order to have a meaningful comparison with the observations.