...In this framework, inflation is driven by a de Sitter space time, where tensor metric fluctuations (i.e., gravitational waves) naturally arise from quantum vacuum oscillations, and scalar fluctuations are generated via second-order tensor effects. We compute the power spectrum of such scalar fluctuations and show it to be consistent with near scale invariance...
Delta_V in space
Inflation without an inflaton
https://journals.aps.org/prresearch/abstract/10.1103/vfny-pgc2