Non-Gaussianity from Excited Initial Inflationary States

TL;DR

The paper evaluates whether excited initial inflationary states, modeled as Bogoliubov transforms of the Bunch-Davies vacuum, can generate observable squeezed-limit non-Gaussianity $f_{NL}$. By enforcing subhorizon onset, negligible backreaction, negligible superhorizon evolution, and at least three decades of near-scale invariance, the authors show that most such states yield unobservable $f_{NL}$; a narrow mixed scenario with $0.01<|\beta_k|\leq0.1$ can produce order-unity signals, while concrete slow-roll examples with large $|\beta_k|$ remain unobservable due to small $\epsilon$. The results constrain how pre-inflationary physics and excited initial conditions could imprint measurable non-Gaussianity, informing future data analyses and model-building. Overall, the work narrows the viable parameter space for excited-initial-state inflation to a small region and emphasizes the role of $\epsilon$ and backreaction in determining observables.

Abstract

We study squeezed limit $f_{NL}$ generation by excited initial inflationary states in a model independent way. We restrict "excited" to mean a Bogoliubov transformation of the Bunch Davies state. We simultaneously impose the constraints that the observable power spectrum is nearly scale invariant over at least three decades and that the observable modes today be subhorizon at the beginning of inflation while not causing significant backreaction. We show that most excited initial inflationary states for single field inflationary models with negligible superhorizon evolution do not produce an observable squeezed limit $f_{NL}$. The case in which one mode is in the Bunch Davies state while the other two modes are in an excited state with $0.01<|β_k|\leq0.1$ may generate a squeezed limit $f_{NL}$ which is detectable with future experiments.