Conformal transformations in classical gravitational theories and in cosmology

TL;DR

This paper reviews the use of conformal transformations in alternative gravitational theories and cosmology, emphasizing their mathematical and physical significance, especially in relation to experimental tests and cosmological models.

Contribution

It provides a comprehensive review of conformal transformations in gravitational theories and cosmology, highlighting their physical implications and importance for experimental and observational comparisons.

Findings

Conformal transformations are crucial in analyzing alternative gravity theories.

The physical conformal frame must be identified for meaningful experimental comparisons.

Conformal techniques are essential in cosmological inflation and scalar-tensor theories.

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.