Could a black hole interior contain an entire universe?

[krishnamurthy]

TL;DR

A thought experiment: Could our universe exist inside a black hole in a parent universe, with black holes in each universe containing further universes? Has anything similar been explored in physics?

I would like to share a thought experiment that occurred to me while thinking about black holes and the geometry of spacetime. I am not a physicist, and I am not proposing a formal theory. I am curious whether this possibility has been explored in theoretical physics.

The thought experiment

What if our entire universe exists inside a black hole in a larger parent universe?

In turn, black holes within our universe could contain other universes, whose black holes could contain further universes, potentially forming an infinite hierarchy.

The intuition that led me to this idea concerns the geometry inside a black hole.

We usually describe a black hole from the perspective of an external observer, using properties such as its mass and event-horizon radius. But inside the event horizon, spacetime geometry can be very different from our everyday intuition. Could the interior spatial geometry and volume be vastly different from what we might expect based on the exterior dimensions?

I wonder whether the interior of a black hole could contain a cosmological region with its own spatial extent and evolution, potentially much larger or more complex than its exterior appearance suggests.

I understand that this does not establish that another universe exists inside a black hole. The idea is purely speculative, and I may be overlooking fundamental constraints.

What I would like to understand

1. Are there solutions of general relativity in which a black-hole interior can develop into a separate expanding cosmological region?
2. How independent can the interior spatial geometry and volume be from the properties measured outside the event horizon?
3. Has anyone investigated the possibility of recursively nested universes of this kind?
4. What are the main mathematical or physical objections to such a scenario?

I would be grateful for references to relevant research or explanations of why the idea might or might not be physically possible.

I am sharing this simply as a thought experiment and an opportunity to learn.

 

[PeroK]

This was discussed recently:

T

High School Thread 'Questions about Schwarzschild Cosmology and White Hole Big Bangs'

Sep 3, 2026

Hi everyone,
I'm a student who is really interested in cosmology. I was thinking about how both the Big Bang and black holes start with a singularity, and I ended up finding out about Schwarzschild Cosmology and Nikodem Popławski’s model (where our universe expands like a white hole inside a parent black hole).
Since I'm just a beginner, I wanted to ask the experts here a couple of questions to help me understand:

1. How does a collapsing black hole turn into an expanding white hole? What is the physics behind that "bounce"?
2. If this theory is right, does it mean the black holes we...

• Tanya Li
• Replies: 7
• Forum: Astronomy, Astrophysics, Cosmology

 

[PeterDonis]

Are there solutions of general relativity in which a black-hole interior can develop into a separate expanding cosmological region?

No.

How independent can the interior spatial geometry and volume be from the properties measured outside the event horizon?

Not at all. A black hole spacetime has only three parameters, mass, charge, and angular momentum, and those are the same inside and outside the horizon.

Has anyone investigated the possibility of recursively nested universes of this kind?

There have been speculations about black holes spawning "baby universes" (Hawking wrote a popular book with a similar title), but they are only speculations and are outside the framework of standard GR (they basically involve some sort of speculative hypothesis about quantum gravity).

 

[krishnamurthy]

Thank you both for the references and explanations. I'll look into the existing models, particularly Schwarzschild cosmology and Popławski's work.

My intention wasn't to claim that a black-hole interior necessarily develops into a new universe, but to understand whether the geometry permits such a possibility and whether it could be extended recursively.

I'll read more before drawing any conclusions.

 

[pervect]

Not in general relativity, but in Einstein Cartan theory there are proposals that are not exactly the same as what you propose, but similar enough to be noteworthy.

A somewhat gee-whiz popularization can be found at https://www.aip.org/inside-science/every-black-hole-contains-a-new-universe.

A serious non-popularized treatment would probably require a graduate level thread, unfortunately.l

Einstein Cartan theory is very similar to General Relativity in the realm we can test experimentally, but it makes different predictions for black hole collapse. To oversimplify, Einstein-Cartan theory allows black holes to "bounce" and re-expand after they collapse, but they do not and cannot re-expand in the same universe they collapsed in. One nice feature of Einstein-Cartan theory is it doesn't have the same problem with singularities that GR does. The main difference between the theories is that in Einstein-Cartan theory, the property of matter called spin creates a property of space-time called torsion, which can act a bit like repulsive gravity, avoiding the singularity and allowing black holes to "bounce" rather than collapse to a singularity. General relativity assumes that there is no torsion. There's no experimental eveidence that the torsion predicted by Einstein-Cartan theory exists, because normally its effects are too small to measure.