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 $N^μ{}_{;μ}=nΓ$. Combining this open-system formulation with the unimodular diffusion term $Q$, we derive the exact identity $nT\dotσ=-3Hp_c-(ρ+p)Γ-\dot Q$, 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 $nT\dotσ=-\dot Q$, 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 $\dot Q\neq0$. The general identity, however, remains available as a starting point for relaxing this postulate in future work.