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LIGO detects most massive black hole merger to date

(www.caltech.edu)
360 points Eduard | 1 comments | 14 Jul 25 20:06 UTC | HN request time: 0s | source
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BurningFrog ◴[14 Jul 25 21:40 UTC] No.44565671[source]▶
I've always thought the event horizon for a black hole has to be spherical.

But my physics intuition tells me that as two of them merge, the resulting BH should have a "peanut" shape, at least initially.

And maybe it can keep having an irregular shape, depending on the mass distribution inside it?

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itishappy ◴[14 Jul 25 21:44 UTC] No.44565698[source]▶
It's only spherical in a Schwarzschild (non-rotating) black hole. A rotating black hole is called a Kerr black hole, and stuff gets weird, such as there being an oblate event horizon, a weird outer horizon called an ergosphere where spacetime gets dragged along such that it's impossible to stand still and you can accelerate objects using the black hole, a weirder inner horizon called the Cauchy horizon where time travel is possible, and a singularity in the shape of a ring. Your intuition is correct that during a merger it would be weirder still.

https://en.wikipedia.org/wiki/Kerr_metric

https://arxiv.org/pdf/0706.0622

https://en.wikipedia.org/wiki/Ergosphere

https://en.wikipedia.org/wiki/Cauchy_horizon

Edit: Updated the bit about about horizons as I research a bit more. It's complicated, and I'm still not positive I have it exactly right, but I think it's now as good as I can get it.

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TMEHpodcast ◴[14 Jul 25 23:27 UTC] No.44566444[source]▶
No matter how chaotic the merger looks, the event horizon must asymptotically become either spherical (Schwarzschild) or oblate (Kerr). The mass distribution inside doesn’t change this, general relativity doesn’t allow static “lumpy” horizons.

It’s wild how much happens in those milliseconds though. Numerical relativity papers like the one you shared from arxiv.org show the horizon “sloshing” before it stabilizes.

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pavel_lishin ◴[15 Jul 25 00:30 UTC] No.44566732[source]▶
Is it even sensible to talk about a "mass distribution" inside of an event horizon?
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kadoban ◴[15 Jul 25 03:24 UTC] No.44567586[source]▶
Sure, especially consider if singularities are not real. Then what's inside the event horizon is just some bunch of unknown material in some actual shape. Why wouldn't it be?

If singularities are real...same thing but more boring answer maybe? (the distribution just being: in the center).

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lisper ◴[15 Jul 25 06:13 UTC] No.44568326[source]▶
> Why wouldn't it be?

Because the whole concept of "shape" assumes properties of space that might not apply inside an event horizon?

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1. r0uv3n ◴[15 Jul 25 12:57 UTC] No.44570659[source]▶
Eh, space inside or outside the horizon is only different in so far as to whether it can reach our timelike infinity. Locally you cannot even tell where any horizon might be (just look at a small patch of a Penrose diagram near a horizon), they are very much something related to global properties of the spacetime. In particular it's not problematic to talk about some extended volume in spacetime occupied by mass, as long as the divergence of the stress energy tensor is 0.

The point where our notions of geometry would break down would be near the singularity, not near the horizon, and we don't even know if a volume enclosed by a horizon (i.e. anything you might call a black hole) necessarily has a singularity inside, it's just that our simple mathematical models all assume one.