[whiteboardr]

While interesting and putting perspectives into place I’m having a really hard time following how there can be a black hole merger area (let alone the black hole era) having the model of redshift engrained in my brain.

Serious question: wouldn’t all mass that had time to gravitate towards and convene into galaxies, stars and black holes been drifting too far apart by then to be even remotely close for their gravities overcome these - then - unimaginable distances?

[LorenPechtel]

Any group of galaxies that is gravitationally bound will end up in effect putting the black holes into orbit about each other. The orbits will be across incredible distances and move very slowly, but it will happen. Orbital energy over time gets converted to gravitational waves and thus they spiral inwards. (All orbits actually spiral inwards, it's just the effect is so tiny that in all but extreme cases you won't notice it within the current age of the universe.)

Galaxies which are not gravitationally bound won't encounter each other, so it doesn't end up merging the whole universe into a single black hole.