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Peer-reviewedCC BY 4.0Journal of High Energy Physics

Cosmology, decoherence and the second law

  • Sebastián Céspedes1,2
  • Senarath de Alwis3
  • Fernando Quevedo4,5
5 affiliations
  1. 1Imperial College London
  2. 2Universidad de Chile
  3. 3University of Colorado
  4. 4New York University Abu Dhabi
  5. 5DAMTP University of Cambridge
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Abstract

A bstract : We consider quantum decoherence and entropy increase in early universe cosmology. We first study decoherence in a discrete bipartite quantum system for which a single qubit gets entangled with an environment and the entropy increase is correlated with the decay of the off-diagonal terms of the reduced density matrix. We compare this system with continuous systems relevant for cosmology for which there is a natural external intervention, corresponding to the time-dependent separation between the sub- and super-horizon infla-tionary fluctuations. We find, in this case, that the off-diagonal terms of the density matrix, in a field basis, do not decay as sometimes assumed in cosmological set-ups. Nevertheless, following a recent treatment in terms of open Effective Field Theories (EFTs), we compute the entanglement entropy for a Gaussian state and show that it actually increases monotonically ( Ṡ>0 ) during the accelerated phases ( ä>0 with a ( t ) the scale factor). We generalise this result to include non-Gaussian states and briefly discuss the relevance of computing the von Neumann entropy as compared to the thermodynamic entropy.

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Cite

BibTeXRIS
BibTeX
@article{cespedes2026cosmology,
  title = {{Cosmology, decoherence and the second law}},
  author = {Sebastián Céspedes and Senarath de Alwis and Fernando Quevedo},
  journal = {Journal of High Energy Physics},
  year = {2026},
  month = oct,
  doi = {10.1007/jhep10(2026)176},
  url = {https://link.springer.com/article/10.1007/JHEP10(2026)176},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - Cosmology, decoherence and the second law
AU  - Sebastián Céspedes
AU  - Senarath de Alwis
AU  - Fernando Quevedo
PY  - 2026
DA  - 2026/10/15
JO  - Journal of High Energy Physics
DO  - 10.1007/jhep10(2026)176
UR  - https://link.springer.com/article/10.1007/JHEP10(2026)176
AB  - A bstract : We consider quantum decoherence and entropy increase in early universe cosmology. We first study decoherence in a discrete bipartite quantum system for which a single qubit gets entangled with an environment and the entropy increase is correlated with the decay of the off-diagonal terms of the reduced density matrix. We compare this system with continuous systems relevant for cosmology for which there is a natural external intervention, corresponding to the time-dependent separation between the sub- and super-horizon infla-tionary fluctuations. We find, in this case, that the off-diagonal terms of the density matrix, in a field basis, do not decay as sometimes assumed in cosmological set-ups. Nevertheless, following a recent treatment in terms of open Effective Field Theories (EFTs), we compute the entanglement entropy for a Gaussian state and show that it actually increases monotonically ( Ṡ>0 ) during the accelerated phases ( ä>0 with a ( t ) the scale factor). We generalise this result to include non-Gaussian states and briefly discuss the relevance of computing the von Neumann entropy as compared to the thermodynamic entropy.
SN  - 1029-8479
ER  -