Periapsis shifts in dark matter distribution with a dense core

Abstract

We consider the periapsis shifts in dark matter distribution with a dense core. We model the dark matter distribution as an isotropic gas sphere, the Emden polytropic sphere of index 5 in general relativity. This model has a parameter range where all the energy conditions are satisfied in the entire region. Within the parameter range, the asymptotic analysis for stellar motion allows us to identify two competing effects on the bounded motion: the general-relativistic effect and a local-density effect of matter. Furthermore, using nearly circular bound orbits, we demonstrate that retrograde periapsis shifts occur near the center, where the local-density effect dominates over the general-relativistic effect, whereas prograde periapsis shifts occur in the far region, where the general-relativistic effect dominates over the local-density effect. This result means that a natural explanation for the retrograde periapsis shifts is not the existence of exotic objects (e.g., naked singularities or wormholes) but the local distribution of physically reasonable matter on the stellar orbit. Furthermore, it also implies that the periapsis shift plays a crucial role in distinguishing black hole alternatives, such as dark matter cores, from a pure black hole.