In chapter 1/The Kerr spacetime-a brief introduction by Matt Visser of D. Wiltshire, M. Visser, S.M. Scott "The Kerr spacetime - Rotating black holes in general relativity" the author presents a schematic description of the relevant surfaces. In Boyer-Lindquist coordinates $(t, r, \theta, \phi)$:
Outer ergosurface $r_E^+ = m + \sqrt{m^2 - a^2 \cos^2 \theta}$
Outer event horizion $r_+ = m + \sqrt{m^2 - a^2}$
Inner event horizon $r_- = m - \sqrt{m^2 - a^2}$
Inner ergosurface $r_E^- = m - \sqrt{m^2 - a^2 \cos^2 \theta}$
$m$ black hole mass
$a = J/m$ angular momentum per unit mass
$J$ angular momentum
Each surface embedded in the previous one, that is:
$r_E^+ \ge r_+ \gt r_- \ge r_E^-$
The equal sign holds on the polar axis

Then the author states that the inner horizon and the inner ergosurface are not physical as there are reasons to suspect they are grossly unstable. Moreover causal pathologies (closed timelike curves), even if originating in the maximally extended $r \lt 0$ region, can reach out to the inner horizon.

My question is:
1. Which reasons argue that the inner horizon and inner ergosurface are unstable?
2. As for the maximally extended $r \lt 0$ region which is the consensus in the theoretical physicists community about its physical meaning?

  • $\begingroup$ This chapter is also available at arxiv.org/abs/0706.0622 $\endgroup$ – A.V.S. Mar 11 '18 at 12:22
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    $\begingroup$ You might want to have a look at Poplawski's "Radial motion into an Einstein-Rosen bridge", predating his cosmology (which has been controversial, probably because the extension of General Relativity which it uses--ECSK gravity--is not compatible with renormalization). It's coming into wider acceptance recently, as it provides for inflation without the hypothesized Higgs particle (lighter than the one found so far) which would provide for the field-based variety originated by Guth and others. I'm hoping you might then be able to "answer your own question", which strikes me as a good one. $\endgroup$ – Edouard May 20 '18 at 5:23

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