Conformal Frame Dependence of Inflation

TL;DR

This paper investigates how different conformal frames in scalar-tensor gravity affect matter observations during inflation, revealing frame-dependent features in the curvature perturbation spectrum.

Contribution

It demonstrates the frame dependence of matter-related observables in inflation, analyzing specific models with a spectator curvaton in different Jordan frames.

Findings

Blue tilt at short scales in super-inflationary Jordan frame

Blue tilt at large scales in bouncing Jordan frame

Frame choice influences curvature perturbation spectrum

Abstract

Physical equivalence between different conformal frames in scalar-tensor theory of gravity is a known fact. However, assuming that matter minimally couples to the metric of a particular frame, which we call the matter Jordan frame, the matter point of view of the universe may vary from frame to frame. Thus, there is a clear distinction between gravitational sector (curvature and scalar field) and matter sector. In this paper, focusing on a simple power-law inflation model in the Einstein frame, two examples are considered; a super-inflationary and a bouncing universe Jordan frames. Then we consider a spectator curvaton minimally coupled to a Jordan frame, and compute its contribution to the curvature perturbation power spectrum. In these specific examples, we find a blue tilt at short scales for the super-inflationary case, and a blue tilt at large scales for the bouncing case.