Three Super-Massive Black Holes Merging Together in Our Nearby Universe

One thing to keep in mind. What we see as the black hole is really the “event horizon”. To a local observer there’s nothing special at that location. You wouldn’t notice anything crossing it.

So really as the black holes merge, we see their event horizons merge. It’s not really valid to say one horizon falls into the other, as they are simply space time points like any other.

What happens to the singularities is probably a whole lot more complex and I don’t know if it’s well defined. In addition, there’s a lot more typically going on around most black holes since many have accretion disks of materials accelerated at relativistic speeds and are also rotating causing other gravitational complexities.

Where I think the article hits the nail on the head, is we need complex simulations and ultimately more data on observations to create more accurate models.

In the past/future you find all the stuff within the corresponding light cone. The stuff outside the cones can flip between being what seems (non-Relativistically) like it should be past and future, can be either depending on your motion relative to it, so an extra tense for spacelike separation is something I can imagine a sufficiently advanced civilisation using.

But then I started to think about it and it doesn’t seem exactly right either. Causality kind of implies interaction, and interaction implies mass (no?), so causality as such must necessarily be strictly slower than C. That is, the speed of the propagation of the wave function between causally connects to events is C but causality as such are the interesting things happening at those events.

If we have two alien neighbour planets aranged so, that we are in the middle between them.

Then we could see both aliens' election results at the same time, knowing that aliens still have to wait a bit before they know each other's results.

If you can imagine teleporting and still having a consistent timeline, then you can imagine going faster than light.

Is there any rational hope of picking up (e.g.) periodically varying red-shift of secondary radiation from the (presumably large) collection of black-hole pairs stuck in parsec-range orbits? Is there any process tending to align the rotational axes of accretion discs on bodies in this condition?

Somehow the word "universe" in the phrase "nearby universe" seems redundant (e.g. where ELSE would it be?)

Who knows how much “disconnected universe” there is. Unless that is somehow known, which seems difficult since it is by definition causally disconnected.

It's a question of definitions. He makes the argument that defining "universe" to mean "our visible universe", that is, a radius of around 140B ly, is the most useful; if you assume infinite space and infinite matter (which seem to follow from the theory of inflation, which has provided explanations and predictions elsewhere), there are likely infinitely many universes all around following this definition (for other visibility scopes).

It's a question of definitions. He makes the argument that defining "universe" to mean "our visible universe", that is, a radius of around 14B ly, is the most useful; if you assume infinite space and infinite matter (which seem to follow from the theory of inflation, which has provided explanations and predictions elsewhere), there are likely infinitely many universes all around following this definition (for other visibility scopes).

As a youtube-couch expert on astrophysics, I hear "universe" redefined as something like "a bubble of matter and spacetime that is 'quarantined' and can never communicate with the rest of the cosmos" whenever multiverse theory is discussed.