Questions about Schwarzschild Cosmology and White Hole Big Bangs

[Tanya Li]

TL;DR

Asking if the Big Bang could be a white hole expanding out of a black hole from a parent universe.

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 see in our night sky right now are creating new universes on the other side?

Thanks so much for your help!

 

[Dale]

Nikodem Popławski’s model (where our universe expands like a white hole inside a parent black hole)

I know that Popławski works a lot with Einstein-Cartan gravity, but I don’t know about a white hole solution in Einstein-Cartan gravity. It would surely be different from the standard GR version.

 

[PeroK]

TL;DR: Asking if the Big Bang could be a white hole expanding out of a black hole from a parent universe.

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 see in our night sky right now are creating new universes on the other side?

Thanks so much for your help!

I'm not an expert, but that sounds more like science fiction to me. A better understanding of what happens inside the event horizon of a black hole depends on a quantum theory of gravity. So, no one knows.

Also, a singularity is not a physical thing. It is literally where a mathematical model breaks down. Saying our universe started with a singularity is not physically meaningful. Instead, the Big Bang model only runs back to $t \approx 10^{-10} s$ (*), where the classical model breaks down and we need a theory of quantum gravity.

(*) Or, some small number.

 

[Marcus Jensen]

This is one of the more elegant "singularity-avoidance" proposals out there, though it's worth being upfront that it's a minority/speculative model, not mainstream cosmology.


1. The collapse-to-bounce mechanism

Poplawski's model doesn't use standard General Relativity; it uses Einstein-Cartan gravity, an extension of GR that includes a property called torsion, which standard GR sets to zero by assumption.

• In ordinary GR, spacetime is only curved. In Einstein-Cartan theory, spacetime can also be twisted; that's torsion.
• Torsion effectively acts like a repulsive force that only becomes significant at extremely high densities (fermion density, specifically, since torsion couples to the spin of matter, not just its mass/energy).
• As matter collapses toward a black hole singularity, this torsion-generated repulsion grows and eventually overwhelms gravitational attraction before the density becomes truly infinite.
• Instead of crushing to a singularity, the collapsing matter "bounces"; it reaches a maximum density, then rebounds and expands.
• From the outside, this looks like an ordinary black hole (event horizon and all). From the inside, the bounce looks like a Big Bang: the expanding matter becomes a new, causally disconnected region of spacetime, a baby universe on the "other side" of the horizon, which behaves like a white hole.

This is elegant because it uses the same mechanism (torsion) to explain why both black hole singularities and the Big Bang singularity might not actually be true, physical infinities.

2. Does every black hole spawn a universe?

If the model is correct, yes, in principle every black hole would be the "seed" of a new expanding universe on its interior side. A few important caveats, though:

• These would be causally disconnected from us. Nothing crosses back out through the horizon, so there's no way to observe, contact, or receive information from a universe formed this way. We can't test it directly from our side.
• This means we can't currently confirm or rule it out through the "output"; we'd need indirect evidence instead.
• Poplawski has proposed some (contested) indirect signatures, e.g., this framework predicts specific things about the large-scale structure or rotation of our own universe, if we originated inside a black hole in some larger parent universe. Some of that connects to ideas about why the universe has a preferred handedness or specific inflation-like behavior without needing a separate inflaton field.
• It's worth noting this is not the consensus view. Most cosmologists still work with standard GR + inflationary Big Bang models, and Einstein-Cartan torsion-based bounces remain a minority research program, though a mathematically serious one.

 

[PeroK]

Two obvious questions are:

1) Our universe contains billions of galaxies. That must have been some monster black hole in the parent universe. Is a star of that physical size possible? The universes in the black holes within our universe would only have the total matter of a large star. These wouldn't be universes like ours. Just a baby universe with the total mass of a single large star.

2) Our universe is so large that it appears to be infinite. What causes the flatness and the appearance of infinite extent if we are the remnants of a collapsed star in a parent universe? This ups the ante for the laws of physics in the parent universe.

 

[Marcus Jensen]

Two obvious questions are:

1) Our universe contains billions of galaxies. That must have been some monster black hole in the parent universe. Is a star of that physical size possible? The universes in the black holes within our universe would only have the total matter of a large star. These wouldn't be universes like ours. Just a baby universe with the total mass of a single large star.

2) Our universe is so large that it appears to be infinite. What causes the flatness and the appearance of infinite extent if we are the remnants of a collapsed star in a parent universe? This ups the ante for the laws of physics in the parent universe.

These two objections hit right at the heart of why many cosmologists are initially skeptical of "baby universe" models like Popławski’s. However, within the specific framework of Einstein-Cartan gravity + Cosmic Inflation, there are theoretical mechanisms that address both points:

1. "A star the size of our entire universe?" (The Mass Problem)

You are completely right that a 30-solar-mass star does not contain enough matter to build a universe with billions of galaxies. However, in this model, the parent star only provides the initial trigger, not the final mass:

• Mass/Energy isn't strictly conserved in expanding space: In General Relativity and Einstein-Cartan gravity, global energy conservation doesn't work like classical mechanics. Matter/energy density changes as space expands or contracts.
• The "Zero-Energy Universe": Standard inflationary models show that the positive energy of matter and radiation is precisely balanced by the negative energy of the gravitational field. The net energy of a universe can effectively be zero.
• Torsion-driven expansion: Once Popławski's torsion mechanism causes the bounce, the extremely dense, hot matter rebounds and enters an explosive expansion phase (similar to cosmic inflation). The immense vacuum energy and particle creation during this rapid expansion build the matter content of the new universe "from the inside." The parent star is just the spark that ignites the engine.

2. "Why is our universe flat and seemingly infinite?"

This is the classic flatness problem. If we originated inside a collapsed star/black hole, shouldn't space be strongly curved and finite?

• Inflation flattens space: Immediately following the torsion bounce, the new region undergoes exponential expansion, stretching space by a factor of 10^26 or more in a fraction of a second.
• The Balloon Analogy: If you stand on a small round balloon, you easily notice its curve. If that balloon is blown up to the size of the Sun, the local patch you stand on looks completely flat and practically infinite. The overall curvature may still exist on a hyper-large scale, but our entire observable universe is such a tiny fraction of the total geometry that it measures as flat.
• No new laws required: This doesn't require modifying the fundamental laws of physics in the parent universe. It only requires the same torsion and quantum-scale interactions that created our own bounce to apply there as well.

I hope this was helpful!