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> As the size of the black hole goes up, its overall density must decrease. The center of a black hole is infinitely dense. That's why it even exists. The ev

> As the size of the black hole goes up, its overall density must decrease. The center of a black hole is infinitely dense. That's why it even exists. The event horizon is not the black hole. > and the fixed density of our observed universe Our universe is expanding. It's density is not fixed. You really want to be thinking about this in terms of entropy and not matter.

DrammBA 1y ago
> The center of a black hole is infinitely dense. That's why it even exists. The event horizon is not the black hole. Arguing semantics is rather boring when it's obvious you understood the point he was trying to make. > Our universe is expanding. It's density is not fixed. None of that precludes uniform density at large scales.
- daedrdev 1y ago
  I think a point they are trying to make is that the border of a black hole is only to us outside observers, if you yourself fell into one you wouldn't notice anything specific when you crossed the boundary. The popular example of hawking radiation references a border and pairs of particles, however its actually only to help people understand the idea of what is going on
  - viraptor 1y ago
    > if you yourself fell into one you wouldn't notice anything specific when you crossed the boundary. Wouldn't you notice that fairly suddenly everything's getting brighter because all the light/radiation is sucked back in?
    - PantaloonFlames 1y ago
      I learned recently on [a video](https://youtu.be/a4vHwY0wMjs?t=246 https://youtu.be/a4vHwY0wMjs?t=246) that for very large black holes , we suspect there is no difference. For smaller BH, the gravity gradient is higher and it would be detectable.
- lutusp 1y ago
  >> Our universe is expanding. It's density is not fixed. > None of that precludes uniform density at large scales. According to observation, the universe is expanding. An argument that it's really static at a large scale would require contradicting observational evidence, but none exists. A theory that requires abandoning observational evidence bears a special burden, which this theory lacks.
  - Jensson 1y ago
    Black holes are capable of expanding, they do it by eating material from outside.
    - lutusp 1y ago
      The universe's expansion, and a black hole's increase in mass over time, are unrelated phenomena. We could have one without the other. In fact, because of Hawking radiation, in the far future we might see a larger universe accompanied by smaller black holes.
kannanvijayan 1y ago
Yeah I was referencing the event horizon as the most meaningful measure of size. And whether the density is fixed over time or not doesn't affect the question. Let's take the universe at its current average mass/energy density - whatever the "true" measure of that is. To the best of our understanding, at large scales the density is uniform. So if we consider a suitably large spherical volume of space within our (presumably infinite) universe.. that volume will have an average mass/energy content greater than the threshold amount for a black hole of that apparent volume (again, using the external event horizon frame). So that suggested to me that either we live in a finite universe, or we must be on the inside of an event horizon. It seems like an unavoidable conclusion.
postalrat 1y ago
It's a mathematical model, not reality. I don't believe scientists believe an actual infinitely dense object exists at the center of black holes.
lutusp 1y ago
>> As the size of the black hole goes up, its overall density must decrease. > The center of a black hole is infinitely dense. That's why it even exists. The event horizon is not the black hole. >> and the fixed density of our observed universe > Our universe is expanding. It's density is not fixed. These are both correct and germane points. So why was this post downvoted? Physics isn't a popularity contest, it relies on evidence.

DrammBA 1y ago

> The center of a black hole is infinitely dense. That's why it even exists. The event horizon is not the black hole. Arguing semantics is rather boring when it's obvious you understood the point he was trying to make. > Our universe is expanding. It's density is not fixed. None of that precludes uniform density at large scales.

daedrdev 1y ago
I think a point they are trying to make is that the border of a black hole is only to us outside observers, if you yourself fell into one you wouldn't notice anything specific when you crossed the boundary. The popular example of hawking radiation references a border and pairs of particles, however its actually only to help people understand the idea of what is going on
- viraptor 1y ago
  > if you yourself fell into one you wouldn't notice anything specific when you crossed the boundary. Wouldn't you notice that fairly suddenly everything's getting brighter because all the light/radiation is sucked back in?
  - PantaloonFlames 1y ago
    I learned recently on [a video](https://youtu.be/a4vHwY0wMjs?t=246 https://youtu.be/a4vHwY0wMjs?t=246) that for very large black holes , we suspect there is no difference. For smaller BH, the gravity gradient is higher and it would be detectable.
lutusp 1y ago
>> Our universe is expanding. It's density is not fixed. > None of that precludes uniform density at large scales. According to observation, the universe is expanding. An argument that it's really static at a large scale would require contradicting observational evidence, but none exists. A theory that requires abandoning observational evidence bears a special burden, which this theory lacks.
- Jensson 1y ago
  Black holes are capable of expanding, they do it by eating material from outside.
  - lutusp 1y ago
    The universe's expansion, and a black hole's increase in mass over time, are unrelated phenomena. We could have one without the other. In fact, because of Hawking radiation, in the far future we might see a larger universe accompanied by smaller black holes.

kannanvijayan 1y ago

Yeah I was referencing the event horizon as the most meaningful measure of size. And whether the density is fixed over time or not doesn't affect the question. Let's take the universe at its current average mass/energy density - whatever the "true" measure of that is. To the best of our understanding, at large scales the density is uniform. So if we consider a suitably large spherical volume of space within our (presumably infinite) universe.. that volume will have an average mass/energy content greater than the threshold amount for a black hole of that apparent volume (again, using the external event horizon frame). So that suggested to me that either we live in a finite universe, or we must be on the inside of an event horizon. It seems like an unavoidable conclusion.

postalrat 1y ago

It's a mathematical model, not reality. I don't believe scientists believe an actual infinitely dense object exists at the center of black holes.

lutusp 1y ago

>> As the size of the black hole goes up, its overall density must decrease. > The center of a black hole is infinitely dense. That's why it even exists. The event horizon is not the black hole. >> and the fixed density of our observed universe > Our universe is expanding. It's density is not fixed. These are both correct and germane points. So why was this post downvoted? Physics isn't a popularity contest, it relies on evidence.