This is very close to an idea known as "Black hole cosmology" -- basically the idea being that the visible universe is inside a black hole, leading to a sort of "nested multiverse".
A related theory, rather than being inside a black hole, is that the other side of a black hole is a "white hole". As matter collapses into a black hole, it is emitted from the white hole, creating another universe.
Here's an article from 2010 that expands on the idea, though this is definitely not the first time (or last time) it was discussed, it just happens to be an easily searchable article.
I'm sure it's not practical, but I always thought it would be interesting if instead of living "inside" a black hole, the visible universe was simply being consumed by a black hole so large it just encompassed everything outside the visible part. So no nesting, the universe just eventually gets consumed entirely by one black hole.
Unsure if the math actually checks out on this, but I was told that if you add up the observable mass/energy in our universe with the same average density we see now, you get a black hole with a Swarzchild radius around the size as the observable universe.
One could then quite reasonably argue that our universe is indeed inside or is itself a black hole.
In this "universe is inside a black hole" theory, is mass 1-to-1 with the "parent" universe? In this theory, are we inside a black hole containing billions of galaxies worth of matter?
An average black hole has 50 suns in it, 50 times the mass of our star, that would be a universe without much matter.
To preface, this is not my field (space stuff is just an interest of mine), and this specific theory is more on the fringes than most so I haven't spent a significant amount of time thinking about it.
>is mass 1-to-1 with the "parent" universe? In this theory, are we inside a black hole containing billions of galaxies worth of matter?
My understanding of the theory is that the proposed parent black hole which our universe is inside would necessarily have to contain all of the mass that we detect in our observable universe. So our universe would be 1-to-1 mass of the parent black hole. The parent universe would be larger (and may contain many black holes, each with a nested universe).
>An average black hole has 50 suns in it, 50 times the mass of our star, that would be a universe without much matter
Indeed! It's interesting to think that perhaps we are on one of the "lowest" layers of the nested multiverse, and perhaps there are only a few dozen (or whatever) layers below us until there is too little mass in that universe to create any more black holes. However, there could be an infinite amount of layers "above" us.
I am curious to where the 50 solar-mass figure comes from, though. Is this excluding super-massive and ultra-massive black holes (which are on the order of 10^6 to 10^11 solar masses)? My intuition says 50 solar-masses is orders of magnitude too low for the average mass of a black hole, but I've never really looked into it
https://en.wikipedia.org/wiki/Black-hole_cosmology
A related theory, rather than being inside a black hole, is that the other side of a black hole is a "white hole". As matter collapses into a black hole, it is emitted from the white hole, creating another universe.
Here's an article from 2010 that expands on the idea, though this is definitely not the first time (or last time) it was discussed, it just happens to be an easily searchable article.
https://www.space.com/8293-universe-born-black-hole-theory.h...