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Hydrodynamic models of reheating

Juan Pablo Elía*, Lucas Cantarutti†, and Esteban Calzetta‡

Nahuel Mirón-Granese§

  • *Contact author: jp.elia@df.uba.ar
  • †Contact author: lcantarutti@df.uba.ar
  • ‡Contact author: calzetta@df.uba.ar
  • §Contact author: nahuelmg@df.uba.ar
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Phys. Rev. D 113, 023530 – Published 20 January, 2026

DOI: https://doi.org/10.1103/7zm8-b5v6

Abstract

We develop a causal hydrodynamic model that provides an effective macroscopic description of the field-theoretic dynamics during the early stages of reheating. The inflaton condensate is treated as a homogeneous background coupled to a relativistic fluid that represents its inhomogeneous fluctuations. Within the divergence-type theory framework derived from kinetic considerations, the model captures essential dissipative and nonequilibrium effects while remaining stable and causal. We find that the coupling between the oscillating condensate and the fluid induces a parametric resonance in the tensor sector, leading to the amplification of the viscous stress tensor and the generation of gravitational waves with a characteristic spectral peak. The predicted spectrum agrees with lattice simulations performed with CosmoLattice. This hydrodynamic approach offers an effective bridge between microscopic field dynamics and macroscopic cosmological observables.

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Phys. Rev. D 113, 023530– Published 20 January, 2026
  • Received 16 October 2025
  • Accepted 2 January 2026
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