Conformal frames in cosmology

TL;DR

This paper reviews conformal frames in cosmology, emphasizing their classical equivalence and exploring how different frame choices can impact cosmological models, including potential imprints on the CMB.

Contribution

It provides a comprehensive review of conformal frames in cosmology and introduces a model where frame choice affects observable cosmological signatures.

Findings

Conformal frames are classically equivalent in cosmology.

Frame choice can influence the coupling of fields like the curvaton.

Potential observable imprints on the CMB from frame-dependent couplings.

Abstract

From higher dimensional theories, e.g. string theory, one expects the presence of non-minimally coupled scalar fields. We review the notion of conformal frames in cosmology and emphasize their physical equivalence, which holds at least at a classical level. Furthermore, if there is a field, or fields, which dominates the universe, as it is often the case in cosmology, we can use such notion of frames to treat our system, matter and gravity, as two different sectors. On one hand, the gravity sector which describes the dynamics of the geometry and on the other hand the matter sector which has such geometry as a playground. We use this interpretation to build a model where the fact that a curvaton couples to a particular frame metric could leave an imprint in the CMB.