Dynamical axisymmetric compact objects in General Relativity

Abstract

The search for exact solutions describing asymptotically FLRW compact objects in General Relativity remains a challenging problem. Progress has largely been limited to the spherically symmetric case, with notable exceptions such as the Kerr--de Sitter and Thakurta solutions. In this work, we present two new results that advance the description of axisymmetric compact objects embedded in a cosmological background. First, we introduce a new solution-generating technique that allows for the construction of nonstationary, axisymmetric solutions of the self-interacting Einstein-scalar system. Using this method, we obtain the first exact solution that can describe a dynamical axisymmetric compact object in a FLRW cosmology. We then outline how a detailed analysis of its properties, particularly dynamical trapping (or anti-trapping) horizons, can be carried out. For this purpose, we employ the mean curvature vector (MCV), which provides a natural extension of the Kodama vector beyond spherical symmetry. The norm of the MCV defines a foliation-independent, though embedding-dependent, quantity that can be used to identify trapped, anti-trapped, and untrapped regions, and to characterise the causal structure of the geometry without relying on specific symmetry assumptions. The embedding dependence must be treated carefully, as it determines the extent to which the analysis can be performed analytically while minimising the use of numerical methods. Overall, the solution-generating approach and the associated analysis tools offer a framework to further investigate dynamical axisymmetric compact objects, including black holes in cosmological settings and scenarios involving dynamical scalar accretion.