Matter Creation in Unimodular Gravity Cosmology
Abstract
We investigate matter creation in the cosmological framework of unimodular gravity. We keep the energy density ρ, pressure p, particle number density n, and particle number N=nV as the physical thermodynamic variables of the matter sector, and introduce particle creation directly through the balance equation Nμ;μ=nΓ. Combining this open-system formulation with the unimodular diffusion term Q, we derive the exact identity nTσ̇=−3Hpc−(ρ+p)Γ−Q̇, which shows that the creation pressure pc, the creation rate Γ, and the diffusion term Q are, in general, three independent sources of entropy production per particle. Adopting the standard postulate that particle creation is itself adiabatic, the first two terms cancel identically and the relation reduces to the particular case nTσ̇=−Q̇, so that the entropy per particle is constant only when Q is constant. Thus, even when the creation process associated with Γ is adiabatic in the usual Prigogine sense, the full matter evolution in unimodular gravity is non-adiabatic whenever Q̇≠0. The general identity, however, remains available as a starting point for relaxing this postulate in future work.
Versions
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BibTeX
@misc{cardenas2026matter,
title = {{Matter Creation in Unimodular Gravity Cosmology}},
author = {Víctor H. Cárdenas and Miguel Cruz and Samuel Lepe},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.29569},
eprint = {2609.29569},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.29569},
}
RIS
TY - UNPB
TI - Matter Creation in Unimodular Gravity Cosmology
AU - Víctor H. Cárdenas
AU - Miguel Cruz
AU - Samuel Lepe
PY - 2026
DA - 2026/09/16
PB - arXiv
DO - 10.48550/arxiv.2609.29569
UR - https://arxiv.org/abs/2609.29569
AB - We investigate matter creation in the cosmological framework of unimodular gravity. We keep the energy density ρ, pressure p, particle number density n, and particle number N=nV as the physical thermodynamic variables of the matter sector, and introduce particle creation directly through the balance equation N^μ_;μ=nΓ. Combining this open-system formulation with the unimodular diffusion term Q, we derive the exact identity nTσ̇=−3Hp_c−(ρ+p)Γ−Q̇, which shows that the creation pressure p_c, the creation rate Γ, and the diffusion term Q are, in general, three independent sources of entropy production per particle. Adopting the standard postulate that particle creation is itself adiabatic, the first two terms cancel identically and the relation reduces to the particular case nTσ̇=−Q̇, so that the entropy per particle is constant only when Q is constant. Thus, even when the creation process associated with Γ is adiabatic in the usual Prigogine sense, the full matter evolution in unimodular gravity is non-adiabatic whenever Q̇≠0. The general identity, however, remains available as a starting point for relaxing this postulate in future work.
ER -