Black holes are usually created when massive stars use up all their fuel and collapse due to gravitational collapse. All stars rotate. However, since angular momentum must be conserved even when they become black holes, if they become very small in size as black holes, their rotation speed will become very fast. I think this rotation speed will be very fast, close to the speed of light. At this time, the black hole will have anisotropy where the polar and equatorial parts are not the same due to the rotation phenomenon. Since the black hole rotates, additional mass is created in the equatorial part due to the rotation speed, so can't the event horizon of the black hole become elliptical?
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$\begingroup$ Related question that might help: physics.stackexchange.com/q/640971 $\endgroup$– Franklin LuisCommented Aug 27 at 8:57
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4 Answers
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Most Black Holes in our ways are rotating, because of the reasoning you provided. You can solve Einsteins Equation for such a stationary case. The Solution for this is the Kerr metric.
It has 2 Event Horizon. Both are oblate spheroids, being more compressed on the polar regions. You can look them up on Wikipedia under "Kerr metric".
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$\begingroup$ The stationary solution is Schwarzchild solution. The rotating one is Kerr. $\endgroup$ Commented Aug 27 at 13:42
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$\begingroup$ The Kerr metric does not change with time. There is no mathematical dependence on a time coordinate. It is fair to call it stationary. $\endgroup$– DarenWCommented Aug 27 at 19:15
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$\begingroup$ Yes, DarenW you are right. The Schwarzschild Solution is called static. I do not know why the Kerr metric is called stationary, but this is how I learned it. $\endgroup$– KenCommented Aug 28 at 14:48
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Black holes are not necessarily spherical in shape. The shape of a black hole can vary depending on the specific conditions and characteristics of the black hole. When a black hole rotates, it becomes oval-shaped. Imagine two black holes colliding. Then you can easily tell whether they are spherical or not. In particular, supermassive black holes, such as the one at the center of our galaxy, can be very irregular in shape.
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7$\begingroup$ what is the source for the claim about SMBHs being irregular? that sounds wrong. $\endgroup$ Commented Aug 27 at 15:15
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Black holes are four-dimensional objects that can be spherical or nearly spherical in shape. The shape of a black hole depends on whether it's rotating:
Non-rotating black holes: Have a precisely spherical event horizon.
Rotating black holes: Have an oblate event horizon, which means they're slightly flattened along their rotational axis. This is similar to how planets in our solar system are wider at their equators than at their poles due to rotation.
However, supermassive black holes are very irregular in shape (Atleast to our knowledge, till date).
And yes although you didn't ask for this, in higher dimensions, mathematicians have found that an infinite number of black hole configurations are possible. If we were to discover black holes with non-spherical shapes, it could indicate that our universe has more than three dimensions of space.
Now, that's interesting! (I hope you like this fact too!)
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$\begingroup$ Why do you think so, please? $\endgroup$ Commented Aug 27 at 10:00
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2$\begingroup$ You correctly stated that black holes are four-dimensional, but it means that for the exterior space-time, they are cylinders with spheroid base, not that they are spherical (they are in 3D). Second, probing or not higher dimensions is a matter of compact dimensions, for which the exterior aspect of a large black hole is not impacted at all. It would require large additional dimensions to have i.e. toroidal black holes. $\endgroup$ Commented Aug 27 at 10:12
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4$\begingroup$ Again, what is the source of the claim that SMBHs are irregular? That sounds wrong. $\endgroup$ Commented Aug 27 at 16:20
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Black holes are spherical but their circular disc is not spherical. Most celestial bodies form spherical structures due to surface tension and equal gravity on each axis and angle. Their disk revolves around the black hole and creates a spiral oval around it.
