Density fluctuations in Brans-Dicke inflation

TL;DR

This paper calculates the density perturbation spectrum in Brans-Dicke inflation, showing that adiabatic modes dominate post-inflation and are similar to Einstein gravity, but isocurvature modes are needed for accuracy.

Contribution

It provides the first detailed calculation of density fluctuations in Brans-Dicke inflation, highlighting the role of isocurvature modes for precise spectrum predictions.

Findings

Adiabatic mode amplitude similar to Einstein gravity.

Isocurvature modes are negligible after inflation.

Including isocurvature modes yields accurate spectra.

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.