The transition to phenomenological behaviour of static solutions of the Einstein-Dirac system for an increasing number of fermions

Journal article, 2025

Static spherically symmetric solutions to the Einstein-Dirac system were constructed numerically for the first time in 1999 by Finster et al (1999 Phys. Rev. D 59 104020) in the case of two fermions. In 2020 this result was generalized by Leith et al (2020 Phys. Rev. D 101 106012) to a system consisting of an even number κ of fermions. They constructed solutions for 2 ⩽ κ ⩽ 90 . The purpose of the present investigation is to compare the properties of static solutions of the Einstein-Dirac system with static solutions of the Einstein-Vlasov system as the number of fermions increases, that is, for 2 ⩽ κ ⩽ 180 . Since the Einstein-Vlasov system is a fully classical physical model, whereas the Einstein-Dirac system is semiclassical and thus has a quantum signature, this framework provides an excellent opportunity to study the transition from quantum to classical behaviour. It turns out that even for a comparatively small number of particles, the features of the solutions are remarkably similar. For both systems, we find highly relativistic solutions having a multi-peak structure with strikingly similar characteristics. We also investigate the maximum compactness ratio sup 2 m / r of the solutions. The solutions of both systems share the fundamental properties regarding the maximum compactness ratio and obey the inequality derived in Andréasson (2008 J. Differ. Equ. 245 2243-66). Furthermore, we investigate the sign of the pressure components of solutions of the Einstein-Dirac system. For small values of κ, there are regions where the radial pressure is negative. These regions disappear as κ increases. This supports the interpretation we make as a transition from quantum to classical behaviour as the number of fermions increases.