Graviton Production from Inflaton Condensate: Boltzmann vs Bogoliubov

TL;DR

This paper compares Boltzmann and Bogoliubov methods for graviton production during reheating, revealing that Bogoliubov captures non-perturbative effects and non-adiabatic transitions absent in Boltzmann, especially for steeper potentials.

Contribution

It systematically analyzes graviton production using both frameworks, clarifying their regimes of validity and highlighting the importance of non-adiabatic effects in steep potentials.

Findings

For quadratic potentials, both methods agree on short-wavelength spectra.

Steeper potentials show significant non-adiabatic contributions captured only by Bogoliubov.

Analytic approximations elucidate the spectrum's physical origin and scaling behavior.

Abstract

We study graviton production from an oscillating inflaton condensate during reheating by systematically comparing Boltzmann and Bogoliubov descriptions for inflaton potentials of the form $V(\phi)\propto\phi^n$ around the minimum. The Bogoliubov framework provides a unified description of graviton production, capturing both perturbative and non-perturbative effects across short and long wavelengths, whereas the Boltzmann approach is restricted to perturbative production at short wavelengths. For the quadratic case ($n=2$), we find that the two approaches yield identical graviton spectra at short wavelengths, indicating that the Boltzmann treatments fully captures perturbative gravitational production in this regime. For steeper potentials ($n>2$), however, we identify a sizable contribution arising from the non-adiabatic transition between inflation and reheating. This component is naturally incorporated in the Bogoliubov formalism but absent in the Boltzmann description, and we show that it is important over a broad range of momenta. We derive analytic approximations within both frameworks that clarify the physical origin and scaling behavior of the spectrum. Our results delineate the regime of validity of Boltzmann approaches and show that, for steeper inflaton potentials, graviton production is governed by non-adiabatic transition dynamics for which the Bogoliubov formalism provides the most appropriate description.