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Universal Bound on the Duration of a Kination Era

Cem Eröncel1,*, Yann Gouttenoire2,3,†, Ryosuke Sato4,‡, Géraldine Servant5,6,§, and Peera Simakachorn7,∥

  • 1İstinye University, Faculty of Engineering and Natural Sciences, 34396, İstanbul, Türkiye
  • 2School of Physics and Astronomy, Tel-Aviv University, Tel-Aviv 69978, Israel
  • 3PRISMA+ Cluster of Excellence and MITP, Johannes Gutenberg University, 55099 Mainz, Germany
  • 4Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan
  • 5Deutsches Elektronen-Synchrotron DESY, Notkestraße 85, 22607 Hamburg, Germany
  • 6II. Institute of Theoretical Physics, Universität Hamburg, 22761, Hamburg, Germany
  • 7IFIC, Universitat de València-CSIC, C/ Catedràtico José Beltràn 2, E-46980, Paterna, Spain

  • *Contact author: cem.eroncel@istinye.edu.tr
  • †Contact author: yann.gouttenoire@gmail.com
  • ‡Contact author: rsato@het.phys.sci.osaka-u.ac.jp
  • §Contact author: geraldine.servant@desy.de
  • ∥Contact author: peera.simakachorn@ific.uv.es
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Phys. Rev. Lett. 135, 101002 – Published 2 September, 2025

DOI: https://doi.org/10.1103/k7ty-gwjg

Abstract

We show that primordial adiabatic curvature fluctuations generate an instability of the scalar field sourcing a kination era. We demonstrate that the generated higher Fourier modes constitute a radiationlike component dominating over the kination background after about 11 e-folds of cosmic expansion. Current constraints on the extra number of neutrino flavors ΔNeff thus imply the observational bound of approximately 10 e-folds, representing the most stringent bound to date on the stiffness of the equation of state of the pre–Big Bang–nucleosynthesis universe.

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Phys. Rev. Lett. 135, 101002– Published 2 September, 2025
  • Received 12 February 2025
  • Accepted 1 August 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

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