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  • Open Access

Graviton stimulated emission in squeezed vacuum states

Atsuhisa Ota1,* and Yuhang Zhu2,†

  • 1Department of Physics and Chongqing Key Laboratory for Strongly Coupled Physics, Chongqing University, Chongqing 401331, People’s Republic of China
  • 2Cosmology, Gravity and Astroparticle Physics Group, Center for Theoretical Physics of the Universe, Institute for Basic Science, Daejeon 34126, Korea

  • *Contact author: aota@cqu.edu.cn
  • †Contact author: yhzhu@ibs.re.kr
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Phys. Rev. D 112, 103513 – Published 7 November, 2025

DOI: https://doi.org/10.1103/3mzq-lt9d

Abstract

We study the dynamics of gravitons in a squeezed vacuum state under a thermal radiation background. Unlike traditional treatments that rely on the Boltzmann equation, we employ the Heisenberg equation and average it over general quantum states. In contrast to the usual Boltzmann-based descriptions, our approach captures the subtleties arising from quantum coherence in different number eigenstates, which is essential for soft graviton modes in the squeezed vacuum state. Our new method successfully reproduces the previous one-loop results within the in-in formalism when the expansion parameter is small and deviates significantly as the parameter increases, indicating that our results extend beyond the one-loop in-in formalism. We examine the implications of graviton emission effects stimulated by quantum coherence in both flat and expanding backgrounds. In the flat background, it is found that backreaction of radiation on the spacetime dynamics is crucial for significant stimulated emission. In the expanding background, to avoid the subtleties associated with superhorizon modes, we investigate the effect of emission within the horizon immediately after reheating and find a significant effect. We also examined the IR graviton evolution from a symmetry perspective and propose a regularization prescription to eliminate the secular growth problem.

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Outline

Information

Phys. Rev. D 112, 103513– Published 7 November, 2025
  • Received 3 July 2025
  • Accepted 30 September 2025
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