APOD: Ninety Gravitational Wave and... (2021 Dec 07)

[VictorBorun]

How could it? The entire mass of a black hole is in a dimensionless point (or maybe a subatomic-sized volume) at the center. There is nothing going on between there and the event horizon. Two black holes should be able to pass each other with their event horizons overlapping and continue on unchanged. The event horizon isn't part of the structure of a black hole.

But suppose the donor BH is yet not all in the center of its event horzon; suppose there are some ring singularity or more complex cluster of singular points, filaments and branes, and perhaps some matter is still swarming around that central region deep under the event horizon

Online discussions definitively say the regardless of relative velocity between two black holes, if their even horizons overlap at all, their fate is sealed in a merger.
Black holes: Is merger inevitable when horizons touch?
If two black hole event horizons overlap (touch) can they ever separate again?
Maybe a way to think about this is once a bridge is formed between two separate event horizons, there is only one, contiguous event horizon, and since the relative BH velocity cannot reach the speed of light, neither BH can escape the common event horizon, however small the bridge is. I don't know, but superficially, breaking a closed-surface event horizon seems contrary to BH fundamentals.

Thanks, but I am still not satisfied.
Some commentors are vulgar and confuse a small mass object crossing a BH's event horizon with a more complex story of moving, growing (and receding?) event horizons.
Some commentors picture a BH as a simplistic non-rotating 1 point singularity with a spherical event horizon. But more realistic BHs should be quite complex under their event horizons; even a ring singularity is too perfect to be true.
Some commentors refer censorship conjectures that won't let us see a speckle of dust ever again once it goes under some BH's event horizon.
Now what of the binary BH pair before the merge, when the two horizons are reaching toward each other? Are not those horizons receding at other regions?

[johnnydeep]

While reading today's APOD explanation, I encountered the sentence,

"Humanity has only had the technology to hear these unusual chirps for
the past seven years, but since then we have heard about 90 -- during
the first three observing runs."
...
Ninety waves in seven years is not all that big a number (given the scale
of the universe and the number of black holes to be found in it), but if the
runs are only six months out of each year, the number is much more
impressive.
...

Well, these are black hole collisions that are being detected, which I presume aren't all that frequent, and they last for a very small percentage of the life time of the black holes themselves, which gives us much less of a window in which to detect any particular one. Hmm, I wonder what the estimate is for the average number of black hole collisions per galaxy. For comparison, I believe that supernova happen about once every 100 years per large galaxy like the Milky Way.

[neufer]

Well, these are black hole collisions that are being detected, which I presume aren't all that frequent, and they last for a very small percentage of the life time of the black holes themselves, which gives us much less of a window in which to detect any particular one. Hmm, I wonder what the estimate is for the average number of black hole collisions per galaxy. For comparison, I believe that supernova happen about once every 100 years per large galaxy like the Milky Way.

The Laniakea Supercluster is the galaxy supercluster that is home to the Milky Way and approximately 100,000 other nearby galaxies. It has a major axis of 159 Mpc.

We currently observe about 40 BH collisions a year within 2,000 Mpc which corresponds to a volume ~16,000 larger than the Laniakea Supercluster.

Therefore a detectable BH collision occurs about once every 40 million years per galaxy [i.e., (16,000 x 100,000)/40.]

[A detectable BH collision occurs within our own Laniakea Supercluster about once every 400 years.]

[johnnydeep]

Well, these are black hole collisions that are being detected, which I presume aren't all that frequent, and they last for a very small percentage of the life time of the black holes themselves, which gives us much less of a window in which to detect any particular one. Hmm, I wonder what the estimate is for the average number of black hole collisions per galaxy. For comparison, I believe that supernova happen about once every 100 years per large galaxy like the Milky Way.

The Laniakea Supercluster is the galaxy supercluster that is home to the Milky Way and approximately 100,000 other nearby galaxies. It has a major axis of 159 Mpc.

We currently observe about 40 BH collisions a year within 2,000 Mpc which corresponds to a volume ~16,000 larger than the Laniakea Supercluster.

Therefore a detectable BH collision occurs about once every 40 million years per galaxy [i.e., (16,000 x 100,000)/40.]

[A detectable BH collision occurs within our own Laniakea Supercluster about once every 400 years.]

Thanks. I agree that your math checks out, but somehow you always manage to state things in such a deceptively straightforward looking manner that nevertheless ends up taking me far too long to convince myself is correct. I really did think I was better at math and science than that. Time has clearly taken its toll on my mental flexibility (not to mention my physical prowess). <sigh>

But about that "40 BH collisions a year within 2,000 Mpc" figure. Is that supposed to be within a sphere of radius 2000 Mpc centered on the Earth, or only within our perhaps much more restricted observing cones? And is the distance limit due to the sensitivity of the detectors?