Primordial gravitational waves from excited states

TL;DR

This paper demonstrates that excited scalar states during inflation can significantly enhance tensor modes, producing distinctive oscillations in the gravitational wave background, with potential observational signatures surpassing standard predictions.

Contribution

It introduces a new mechanism where excited states from sharp inflationary features amplify gravitational waves and provides a universal formula for tensor spectra in multifield inflation.

Findings

Enhanced tensor modes from excited states during inflation.

Characteristic oscillations in the stochastic gravitational wave background.

Potential detectability of inflationary gravitational waves exceeding standard sources.

Abstract

We show that a scalar excited state with large occupation numbers during inflation leads to an enhancement of tensor modes and a characteristic pattern of order-one oscillations in the associated stochastic gravitational wave background (SGWB) sourced during inflation. An effective excited state, i.e. a departure from the Bunch-Davies vacuum, can emerge dynamically as the result of a transient non-adiabatic evolution, e.g. a sharp feature along the inflationary history. We provide an explicit example in a multifield context where the sharp feature triggering the excited state is identified with a strong turn in the inflationary trajectory. En passant, we derive a universal expression for the tensor power spectrum sourced at second order by an arbitrary number of scalar degrees of freedom during inflation, crucially taking into account the nontrivial structure of the Hilbert space in multifield setups. The SGWB sourced during inflation can overcome the standard scalar-induced SGWB sourced at horizon re-entry of the fluctuations after inflation, while being less constrained by perturbativity and backreaction bounds. In addition, one may entertain the possibility of detecting both since they peak at different frequencies exhibiting oscillations with distinct periods.