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Revisiting QCD-induced little inflation with chiral density wave state and its implications on pulsar timing array gravitational-wave signals

Tae Hyun Jung1,*, Seyong Kim2,†, Jong-Wan Lee1,‡, Chang Sub Shin3,1,4,§, and Hee Beom Yang5,∥

  • *Contact author: thjung0720@gmail.com
  • †Contact author: skim@sejong.ac.kr
  • ‡Contact author: j.w.lee@ibs.re.kr
  • §Contact author: csshin@cnu.ac.kr
  • ∥Contact author: qja1sk@gmail.com
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Phys. Rev. D 114, 023554 – Published 23 July, 2026

DOI: https://doi.org/10.1103/px47-qkxm

Abstract

We revisit QCD-induced little inflation in which the Universe starts with a large baryon chemical potential and undergoes a strong first-order QCD phase transition, generating an observable stochastic gravitational-wave background in the nano-Hz range relevant for pulsar timing array (PTA) observations. We point out that the conventional homogeneous transition from the quark-gluon plasma phase to the hadronic gas phase faces an unavoidable difficulty in achieving the required strength of supercooling for the observed baryon density. This motivates us to explore whether a qualitatively different phase structure at a large baryon chemical potential can alter the relation between the baryon density and the chemical potential, and thereby modify the supercooling history of the transition. Using the nucleon-meson model with isoscalar vector mesons, we determine the critical and spinodal structure of the chiral density wave (CDW) phase in the (μB,T) plane. We find that the CDW phase exhibits a nontrivial structure and can remain metastable down to a low baryon density in a certain region of the parameter space. Taking into account the subsequent liquid-gas transition and phase separation, however, the released latent heat is too small to realize a viable QCD-induced little inflation scenario and its associated PTA-scale gravitational-wave signal. Our analysis sharpens the conditions under which QCD phase transitions may act as cosmological sources of nano-Hz gravitational waves, while clarifying the possible cosmological relevance of inhomogeneous QCD phases.

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Phys. Rev. D 114, 023554– Published 23 July, 2026
  • Received 31 March 2026
  • Accepted 24 June 2026
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