It's worth knowing the context that we only have a few hundred black hole merges detected (which are the only practical detections methods of non-supermassive black hole available to us), so there just isn't enough data for any statistics yet. Scientists are saying here 'whoa, this is the biggest merger yet detected'.

[–] Anti_Iridium@lemmy.world 1 points 5 days ago* (last edited 5 days ago) (3 children)

I don't have any formal training in astrophysics, however I have watched all of PBS Spacetime. They (probably) formed by smaller black holes merging

However, these black holes’ masses aren’t the only mystery, as both were spinning between 80 and 90 percent of their top speed limit.

For a planet, angular momentum is distributed between the spin of the planet and its revolution in its orbit, and these are often exchanged by various mechanisms.

Furthermore from the Wikipedia article:

The conservation of angular momentum explains the angular acceleration of an ice skater as they bring their arms and legs close to the vertical axis of rotation. By bringing part of the mass of their body closer to the axis, they decrease their body's moment of inertia. Because angular momentum is the product of moment of inertia and angular velocity, if the angular momentum remains constant (is conserved), then the angular velocity (rotational speed) of the skater must increase.

The same phenomenon results in extremely fast spin of compact stars (like white dwarfs, neutron stars and black holes) when they are formed out of much larger and slower rotating stars.

[–] Evil_Shrubbery@lemmy.zip 1 points 5 days ago* (last edited 5 days ago) (2 children)

Yes, the size & the spin is explained by them merging before (an explanation so obvious that ofc it didn't take an eureka moment from the scientists), there is no other explanation really (except them theoretically being like from the ultra young space just after the big gang bang - but the location pretty much affirms this isn't the case even if those existed).

The spin isn't an additional mystery, it's the same one, just two different measurements the system is designed to estimate.

[–] Ragnor 1 points 5 days ago* (last edited 5 days ago) (1 children)

I don't think this is entirely correct. If the spin of the two black holes aren't in the same direction, I imagine that they will have less spin as a percentage of the maximum after the merger. It does seem odd to me - a layman with 30 years of interest in physics.

[–] Evil_Shrubbery@lemmy.zip 1 points 5 days ago* (last edited 5 days ago)

Yes, I agree - but in that case they would perhaps note a slow spin (still "the same explanation").

That's just semantics tho, we are saying the same thing.
A bit like saying 'we only have one method of travel from point A to B' and additionally nothing not to use that one method to travel in the opposite direction of B (bcs then it's no longer from A to B). That is absolutely correct & a relevant note to be precise (and absolutely more relevant than the title with the "shouldn't exist").

Also, I'm not sure, but it might be exceptionally rare for the two to orbit each other with opposite spins.

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