we learn that the entire observable universe – the area that sits within the “Hubble radius” is also on that line. In other words, if a black hole was as large as the universe we can see, it would have the same density as the universe.

Not saying any of this makes sense, but you seem to be working with different assumptions.

Another narrative could be: As mass gets added to the black hole, it (the universe) grows.

[–] MuThyme@lemmy.world 5 points 1 year ago (1 children)

That also isn't how it works, I'm afraid.

The black hole gets less dense because the so called "event horizon" becomes larger, the space inside doesn't stretch like we see in the universe.

It's a little removed from my area, but as I understand it there are similarities in the maths. The observable universe has a horizon in some sense, but it's not a point if no return like a black hole, so much as a point of "the universe hasn't existed long enough for anything to be able to travel that far yet"

This has a similar effect as a black hole, but it's really just a concept we see in perfectly flat empty space, light taking time to move.

[–] Leate_Wonceslace@lemmy.dbzer0.com 5 points 1 year ago (1 children)

There's also an event horizon where space is expanding away from us rapidly enough that we can never get that far no matter how long we travel.

[–] MuThyme@lemmy.world 4 points 1 year ago

Oh yeah, that's also a great point, the Hubble horizon!

It turns out there are a few different horizons in cosmology like that, they really do show up everywhere.