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Consider then the following reasoning:

Schwarzschild Metric describes the spacetime of a black hole. A gravitational collapse is a mechanism to produce Schwarzschild Black Holes. Conversely, a Schwarzschild metric describes exterior spacetime prior the collapse as well.

Now, I read [1] that this reasoning cannot be applied to Kerr black holes. I mean, Kerr metric does not describe the spacetime of a rotating body, just a rotating black hole. But a star is never a spherical body, and the collapse is never perfectly spherically symmetrical. Therefore the resultant black hole has some rotation, and therefore the spacetime is a Kerr one.

Why cannot we say that the exterior spacetime of a rotating body (not supposed to be a black hole) is described by Kerr solution?

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$[1]$ RAINE.D, THOMAS.E; Black Holes. Imperial College Press. pg 132. 2015.

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First of all, the Schwarzschild metric is the most general spherically symmetric vacuum solution to the Einstein field equations. A Schwarzschild black hole is a black hole that, having neither electric charge nor angular momentum, is described by the Schwarzschild metric.

The Kerr metric has a few "problems" and cannot be used to describe realistic stars apart from asymptotically far away. That is because realistic stars have:

  • a) an interior. There are no acceptable interior solutions to the Kerr metric as they cannot satisfy required boundary conditions.
  • b) close to their surface, their mass distribution and hence the surrounding spacetime have multiple moments in the multipole series expansions, which in principle can be independent. In the Kerr solution, the multipole terms are actually closely related to each other, so in order to match the star's $n^{\text{th}}$ pole to that of the Kerr solution, it would require an unphysical gravitational collapse evolution which selectively radiates only certain poles.

A black hole has neither an interior nor it is undergoing gravitational collapse. So the Kerr metric may well describe the spacetime outside a rotating one. Hence the name Kerr black hole.

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  • $\begingroup$ Can you tell me some reference for the interior non-matching property you mentioned? It's quite amazing since for matching interior of Schwarzschild we just use RW metric. $\endgroup$
    – aitfel
    Oct 4, 2020 at 5:59
  • $\begingroup$ @aitfel see references at the end of page 2 here $\endgroup$
    – SuperCiocia
    Oct 4, 2020 at 6:02

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