Abstract
We investigate the properties of a special class of singular solutions for a self-gravitating perfect fluid in general relativity: the singular isothermal sphere. For arbitrary constant equation-of-state parameter w=p/ρw=p/ρ<math><mrow><mi>w</mi> <mo>=</mo> <mi>p</mi> <mo>/</mo> <mi>ρ</mi></mrow> </math> , there exist static, spherically-symmetric solutions with density profile ∝1/r2∝1/r2<math><mrow><mo>∝</mo> <mn>1</mn> <mo>/</mo> <msup><mi>r</mi> <mn>2</mn></msup> </mrow> </math> , with the constant of proportionality fixed to be a special function of w. Like black holes, singular isothermal spheres possess a fixed mass-to-radius ratio independent of size, but no horizon cloaking the curvature singularity at r=0r=0<math><mrow><mi>r</mi> <mo>=</mo> <mn>0</mn></mrow> </math> . For w=1w=1<math><mrow><mi>w</mi> <mo>=</mo> <mn>1</mn></mrow> </math> , these solutions can be constructed from a homogeneous dilaton background, where the metric spontaneously breaks spatial homogeneity. We study the perturbative structure of these solutions, finding the radial modes and tidal Love numbers, and also find interesting properties in the geodesic structure of this geometry. Finally, connections are discussed between these geometries and dark matter profiles, the double copy, and holographic entropy, as well as how the swampland distance conjecture can obscure the naked singularity.