2018/05/01 by Shahar Hod
Physics and Astronomy · #Astrophysical Phenomena and Observations #Black Holes and Theoretical Physics #Black hole (networking) #Classical mechanics #Cosmology and Gravitation Theories #Dimensionless quantity #Equation of state #Isotropy #Mathematical physics #Physics #Polytropic process #Quantum mechanics #SPHERES #Wormhole #astro-ph.HE #gr-qc #hep-th
paper · pdf · doi:10.1140/epjc/s10052-018-5905-y
published as The European Physical Journal C 78, 417 (2018) · 7 pages
openalex publication_date 2018/05/01 · arxiv created 2018/11/12 · arxiv updated 2018/11/21 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
Ultra-compact objects describe horizonless solutions of the Einstein field equations which, like black-hole spacetimes, possess null circular geodesics (closed light rings). We study analytically the physical properties of spherically symmetric ultra-compact isotropic fluid spheres with a polytropic equation of state. It is shown that these spatially regular horizonless spacetimes are generally characterized by two light rings \r^\text innerγ ,r^\text outerγ \ with the property C(r^\text innerγ )≤ C(r^\text outerγ ) , where C≡ m(r)/r is the dimensionless compactness parameter of the self-gravitating matter configurations. In particular, we prove that, while black-hole spacetimes are characterized by the lower bound C(r^\text innerγ )≥ 1/3 , horizonless ultra-compact objects may be characterized by the opposite dimensionless relation C(r^\text innerγ )≤ 1/4 . Our results provide a simple analytical explanation for the interesting numerical results that have recently presented by Novotný et al. (Phys Rev D 95:043009, 2017).