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Large black holes are the most spherical things in the relative sense, not in an absolute sense; although more so than pico-black holes, which are not very spherical.

Light cones/the 4d-radii of causality are* perfectly spherical.

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How big a factor is the rotation of the black hole? Maybe look for a large black hole not rotating too much if you want to be sure. :-)

Aren't they warped by the curvature of space/time that they are spreading over?

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