Exact relativistic treatment of stationary counter-rotating dust disks III. Physical Properties

TL;DR

This paper analyzes explicit solutions for stationary counter-rotating dust disks in Einstein's equations, exploring their physical properties, limits, and relation to known solutions like Kerr, using analytical and numerical methods.

Contribution

It provides a detailed physical analysis of a class of solutions with constant angular velocity and density, including their limits and relativistic features, expanding understanding of such disk models.

Findings

Solutions include ergospheres in relativistic regimes

Ultrarelativistic limit yields diverging central density with finite mass

Exterior can resemble the extreme Kerr solution

Abstract

This is the third in a series of papers on the construction of explicit solutions to the stationary axisymmetric Einstein equations which can be interpreted as counter-rotating disks of dust. We discuss the physical properties of a class of solutions to the Einstein equations for disks with constant angular velocity and constant relative density which was constructed in the first part. The metric for these spacetimes is given in terms of theta functions on a Riemann surface of genus 2. It is parameterized by two physical parameters, the central redshift and the relative density of the two counter-rotating streams in the disk. We discuss the dependence of the metric on these parameters using a combination of analytical and numerical methods. Interesting limiting cases are the Maclaurin disk in the Newtonian limit, the static limit which gives a solution of the Morgan and Morgan class and the limit of a disk without counter-rotation. We study the mass and the angular momentum of the spacetime. At the disk we discuss the energy-momentum tensor, i.e. the angular velocities of the dust streams and the energy density of the disk. The solutions have ergospheres in strongly relativistic situations. The ultrarelativistic limit of the solution in which the central redshift diverges is discussed in detail: In the case of two counter-rotating dust components in the disk, the solutions describe a disk with diverging central density but finite mass. In the case of a disk made up of one component, the exterior of the disks can be interpreted as the extreme Kerr solution.