The same black hole can collide with its kin multiple times, lopsided merger suggests

Jul 10, 2020
If there are regions of space "packed with black holes", then how can any type of black hole collision be considered unusual?

I also don't understand why anyone would just assume a black hole couldn't merge more than once. It seems it would happen as often as it happens. It doesn't seem any different than any other two objects colliding.
The big news is the big black hole merger involving 70 - 150 solar mass black holes.

[The research paper, "GW190521: A Binary Black Hole Coalescence with a Total Mass of 150 Solar Masses," was published in Physical Review Letters on September 2, 2020: DOI: 10.1103/PhysRevLett.125.101102
The research paper, "Properties and Astrophysical Implications of the 150 Solar Mass Binary Black Hole Merger GW190521," was published in Astrophysical Journal Letters on September 2, 2020: DOI: 10.3847/2041-8213/aba493
The research paper, "GW190814: Gravitational Waves from the Coalescence of a 23 Solar Mass Black Hole with a 2.6 Solar Mass Compact Object," was published in Astrophysical Journal Letters on June 23, 2020.
The research paper, "GW190412: Observation of a Binary-Black-Hole Coalescence with Asymmetric Masses," has been accepted for publication in Physical Review D, and was published on Arxiv on April 17, 2020:]

That we can find black holes in these ranges means there is no principle problem of growing super massive black holes as regards size, even if we still don’t know how they grow so large so quickly.

But there are more clues to hierarchical growth conditions in this accompanying commentary article in Physical Review Letters.

"“If the object was able to merge again (in this case, to produce GW190412), it would mean the kick that it received was not enough to escape the stellar cluster in which it formed. If GW190412 indeed is a product of hierarchical merging, the team calculated that it would have occurred in an environment with an escape velocity higher than 150 kilometers per second. For perspective, the escape velocity of most globular clusters is about 50 kilometers per second.

This means that whatever environment GW190412 arose from had an immense gravitational pull, and the team believes that such an environment could have been either the disk of gas around a supermassive black hole, or a “nuclear cluster” — an incredibly dense region of the universe, packed with tens of millions of stars.”"

The merger radiated away 8 solar masses, which is 2-4 times more than typical earlier mergers, so it is no wonder that they saw twice as far. Even if it was a low frequency chirp with 10-20 Hz main frequency, it seems the upgraded Advanced LIGO could observe that low.

The event had an expected rate of ~ 0.1 Gpc^-3 yr^-1, so the 100 Gpc^3 volume would see these about every month – we will soon get good statistics on what seems to be happening in the center of galaxies.
If there are regions of space "packed with black holes", then how can any type of black hole collision be considered unusual?

I also don't understand why anyone would just assume a black hole couldn't merge more than once. It seems it would happen as often as it happens. It doesn't seem any different than any other two objects colliding.
Well, you would have to read the paper (and its references) for the entire story.

But besides that we don't know how larger mass black holes got started in the early universe, there has been "mass gaps" that troubled scientists. From star models it wasn't clear if collapsing stars between 2 and 5 solar masses would become neutron stars or black holes. Another problem from such models come in the mass range between 50ish (says some researchers) to 100ish solar masses where x rays released during normal star end life collapse becomes so energetic that they generate electron-positron pairs and had a runaway disintegration instead of collapse. And finally there was no black holes observed between a few solar masses and 10,000ish solar masses (the intermediate range).

From observations it seems all these mass gaps have been closed, though how we don't know yet. The 2-5 solar mass gap has seen 1-2 merger results that looks like black holes, i.e. no light (but we have to wait a few years in case dust obscures any neutron star). The pair production instability mass gap has now a star inside, likely an earlier merger and the collapse models may survive. And we have a 100+ solar mass merger result, only 1-3 orders of magnitude left on the growth mass gap.

The problem with hierarchical merger theory is many I think (is it feasible? does it explain what we see?), but my previous comment touch on what I recently learned may have been the largest problem. If a merger happens the resulting black holes gets a kick and tend to disappear from dense clusters. The resolution for late universe black hole growth at intermediate (and super massive) masses may be growth within an active galaxy nucleus. If that is correct, a few years of gravity wave research should be able to elaborate on and test that.

How to explain early hierarchical growth seems more iffy.
Sep 6, 2020
Is there a model in which two black holes temporarily overlap event horizons but never actually merge? I was picturing what might happen if two black holes are both approaching a given point in space very fast and from opposite directions. It would be wrong to call what results a "collision," since all the actual matter is at the center of the spherical event horizon, and in my scenario, those cores wouldn't hit. I'm imagining the two black holes only grazing past each other, overlapping event horizons temporarily, deflecting each other's trajectories, but still separating. Is that situation even consistent with GR? If so, just how much of an overlap is possible? Could it happen (say, with supermassive black holes approaching each other at speeds close to c) that the center of one of the black holes is temporarily within the event horizon of the other, but then gets out?

I think I know why a more ordinary object could not pass through the event horizon and emerge: the black hole warps space so much that the only "straight" path for the object is a spiral toward the black hole's center. But when it's two black holes warping the same volume of space, might not the second black hole have an "unwarping" effect? The whole point of this speculation is that it might give us a theoretical handle on extracting information from the interior of the event horizon - i.e., whether there is a "firewall" there or just nothing special. Also: what if some ordinary object C were orbiting just outside the event horizon of black hole A, which grazes the event horizon of black hole B. Could C temporarily cross B's event horizon but still emerge?