Density fluctuations in Brans-Dicke inflation

TL;DR

This work analyzes density perturbations in Brans-Dicke inflation with two dynamical scalars, the inflaton and the Brans-Dicke dilaton. By performing a conformal transformation to the Einstein frame, the authors show that perturbations decompose into adiabatic and isocurvature modes, with the adiabatic mode dominating after inflation and the isocurvature mode providing only small corrections to the spectrum. The result is that the amplitude of the adiabatic perturbations is not significantly different from the Einstein-gravity case, but a correct treatment requires including the isocurvature component. Applying the framework to chaotic and new inflation, the required end-of-inflation parameter values to match COBE data are essentially the same as in GR for large ω, though the analysis corrects prior simplifications that neglected isocurvature contributions.

Abstract

Spectrum of density perturbations in the Universe generated from quantum-gravitational fluctuations in slow-roll-over inflationary scenarios with the Brans-Dicke gravity is calculated. It is shown that after inflation the isocurvature mode of perturbations may be neglected as compared to the adiabatic mode, and that an amplitude of the latter mode is not significantly different from that in the Einstein gravity. However, the account of the isocurvature mode is necessary to obtain the quantitatively correct spectrum of adiabatic perturbations.