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My most recent physics rabbit hole was the black hole hole. They are fascinating. My favorite is the idea of primordial black holes which formed in the instant

by api 2y ago

My most recent physics rabbit hole was the black hole hole. They are fascinating. My favorite is the idea of primordial black holes which formed in the instants after the Big Bang. Many models and theories predict them and they could be an excellent dark matter candidate. The universe could be full of black holes in the asteroid mass range the size of hydrogen atoms. There is also a hypothesis that the predicted (by many solar system simulations and models) planet nine far beyond Neptune could be a captured primordial black hole in the 1-5 Earth mass range and about the size of a golf ball to a tennis ball. I really really hope that exists because if it did it would be within probe range. Going and checking out a black hole could allow us to solve physics and develop a complete tested unified theory. Then there’s spooky shit like: https://en.m.wikipedia.org/wiki/Black_hole_electron https://en.m.wikipedia.org/wiki/Black_hole_electron

renox 2y ago
> The universe could be full of black holes in the asteroid mass range the size of hydrogen atoms. ? Not if there's Hawking radiations.
- api 2y ago
  If the Hawking temperature is below the CMB no net evaporation happens. This means there is a mass cutoff and it’s below asteroid mass. Any smaller PBHs would have evaporated by now assuming we are right about Hawking radiation. The math says it should exist but we have AFAIK not proven it. The big black holes will last insanely long amounts of time.
  - ithkuil 2y ago
    I'm confused. Hawking temperature is inversely proportional to the mass. I assume most black holes except the very small ones would thus have a hawking temperature lower than the CMB. Does that mean that effectively no black holes will ever evaporate not even a tiny bit well until the future time when the CMB will be so red shifted that black holes will start to have net radiation?
    - LegionMammal978 2y ago
      Wikipedia suggests that this is the case [0]: "However, since the universe contains the cosmic microwave background radiation, in order for the black hole to dissipate, the black hole must have a temperature greater than that of the present-day blackbody radiation of the universe of 2.7 K. A study suggests that M must be less than 0.8% of the mass of the Earth – approximately the mass of the Moon." I'm not sure where the discrepancy between the mass of the Moon vs. an asteroid comes from, though. [0] https://en.wikipedia.org/wiki/Hawking_radiation#Black_hole_evaporation https://en.wikipedia.org/wiki/Hawking_radiation#Black_hole_e...
    - mr_toad 2y ago
      The CMB temperature is declining as the universe expands. When it was first created it was the temperature of incandescent plasma, and it shone like the Sun.
ck2 2y ago
Black holes can't be dark matter. Dark matter is a WIMP The universe was built on a scaffold of dark matter like an old spiderweb that slowly collects dust.
- api 2y ago
  That's another leading hypothesis. Some classes of WIMPs have been ruled out but the whole idea hasn't been. BTW primordial black holes could still exist even if they aren't a significant dark matter component.
pavel_lishin 2y ago
> Then there’s spooky shit like: > https://en.m.wikipedia.org/wiki/Black_hole_electron https://en.m.wikipedia.org/wiki/Black_hole_electron From the article: > However, Carter's calculations also show that a would-be black hole with these parameters would be "super-extremal". Thus, unlike a true black hole, this object would display a naked singularity, meaning a singularity in spacetime not hidden behind an event horizon. It would also give rise to closed timelike curves. I wonder if there's a fun sci-fi story in the discovery that all electrons are in fact naked singularities.
- ithkuil 2y ago
  In quantum field theory electrons are excitations in the electron field. If they also were tiny black holes, what would it mean to be an excitation in the electron field which when the wave function collapses it would behave like a black hole. Does it mean that it's not a black hole anymore when the wave function spreads out?
jefb 2y ago
Let's not forget that the radius of the observable universe is ~= the Schwarzschild radius i.e. we're all almost certainly inside a black hole ;) https://en.wikipedia.org/wiki/Black_hole_cosmology https://en.wikipedia.org/wiki/Black_hole_cosmology
- gerad 2y ago
  I guess that'd explain the big bang nicely. It does make it the "outer universe" disappointingly unobservable though.
- pdonis 2y ago
  > we're all almost certainly inside a black hole No, we're not. The universe is rapidly expanding. Equating the Schwarzschild radius for a given blob of matter with the event horizon of a black hole requires that the matter be static or collapsing. The "black hole cosmology" models referred to in the Wikipedia article are misnamed. It is theoretically possible that our observable universe is a patch of a Schwarzschild spacetime, which is what the models referred to are asserting, but if it is, then, since the universe is expanding, it would be a patch of the white hole portion of the spacetime, not the black hole portion. And the "horizon" would be a white hole horizon, i.e., one from which the universe's expansion would eventually cause us to pass out of. However, such a model is extremely unlikely because it has no way of explaining where the white hole horizon came from. A black hole horizon can come into being from gravitational collapse, but a white hole horizon would have to have been "built in" to the overall universe from the very beginning. Nobody has any reason to think that is actually possible, even if we have a theoretical mathematical model that includes it.
  - api 2y ago
    What if we're expanding because we are in a black hole that is being fed by a collapsing star or other object in a many orders of magnitude larger scale universe? Of course these kinds of things are probably 100% untestable.
    - pdonis 2y ago
treis 2y ago
Why wouldn't the primordial black holes condense into bigger ones like the matter that makes up planets and stars did?
- kadoban 2y ago
  Some of them would, some wouldn't, depends on where they were and how they were moving. This is a bit like asking why Jupiter isn't part of the Sun but other stuff is.
- itishappy 2y ago
  Some would, but the vast majority of matter is not found in planets and stars. Most stuff just floats around on it's own out there.
skykooler 2y ago
If we did have a tennis-ball-sized black hole out beyond Neptune, it would be far beyond our capabilities to locate and track - we can barely track debris that small in low Earth orbit, and black holes aren't even courteous enough to provide a radar return. We would not be able to send probes near it any time soon.

by api 2y ago

renox 2y ago

> The universe could be full of black holes in the asteroid mass range the size of hydrogen atoms. ? Not if there's Hawking radiations.

api 2y ago
If the Hawking temperature is below the CMB no net evaporation happens. This means there is a mass cutoff and it’s below asteroid mass. Any smaller PBHs would have evaporated by now assuming we are right about Hawking radiation. The math says it should exist but we have AFAIK not proven it. The big black holes will last insanely long amounts of time.
- ithkuil 2y ago
  I'm confused. Hawking temperature is inversely proportional to the mass. I assume most black holes except the very small ones would thus have a hawking temperature lower than the CMB. Does that mean that effectively no black holes will ever evaporate not even a tiny bit well until the future time when the CMB will be so red shifted that black holes will start to have net radiation?
  - LegionMammal978 2y ago
    Wikipedia suggests that this is the case [0]: "However, since the universe contains the cosmic microwave background radiation, in order for the black hole to dissipate, the black hole must have a temperature greater than that of the present-day blackbody radiation of the universe of 2.7 K. A study suggests that M must be less than 0.8% of the mass of the Earth – approximately the mass of the Moon." I'm not sure where the discrepancy between the mass of the Moon vs. an asteroid comes from, though. [0] https://en.wikipedia.org/wiki/Hawking_radiation#Black_hole_evaporation https://en.wikipedia.org/wiki/Hawking_radiation#Black_hole_e...
  - mr_toad 2y ago
    The CMB temperature is declining as the universe expands. When it was first created it was the temperature of incandescent plasma, and it shone like the Sun.

ck2 2y ago

Black holes can't be dark matter. Dark matter is a WIMP The universe was built on a scaffold of dark matter like an old spiderweb that slowly collects dust.

api 2y ago
That's another leading hypothesis. Some classes of WIMPs have been ruled out but the whole idea hasn't been. BTW primordial black holes could still exist even if they aren't a significant dark matter component.

pavel_lishin 2y ago

> Then there’s spooky shit like: > https://en.m.wikipedia.org/wiki/Black_hole_electron https://en.m.wikipedia.org/wiki/Black_hole_electron From the article: > However, Carter's calculations also show that a would-be black hole with these parameters would be "super-extremal". Thus, unlike a true black hole, this object would display a naked singularity, meaning a singularity in spacetime not hidden behind an event horizon. It would also give rise to closed timelike curves. I wonder if there's a fun sci-fi story in the discovery that all electrons are in fact naked singularities.

ithkuil 2y ago
In quantum field theory electrons are excitations in the electron field. If they also were tiny black holes, what would it mean to be an excitation in the electron field which when the wave function collapses it would behave like a black hole. Does it mean that it's not a black hole anymore when the wave function spreads out?

jefb 2y ago

Let's not forget that the radius of the observable universe is ~= the Schwarzschild radius i.e. we're all almost certainly inside a black hole ;) https://en.wikipedia.org/wiki/Black_hole_cosmology https://en.wikipedia.org/wiki/Black_hole_cosmology

gerad 2y ago
I guess that'd explain the big bang nicely. It does make it the "outer universe" disappointingly unobservable though.
pdonis 2y ago
> we're all almost certainly inside a black hole No, we're not. The universe is rapidly expanding. Equating the Schwarzschild radius for a given blob of matter with the event horizon of a black hole requires that the matter be static or collapsing. The "black hole cosmology" models referred to in the Wikipedia article are misnamed. It is theoretically possible that our observable universe is a patch of a Schwarzschild spacetime, which is what the models referred to are asserting, but if it is, then, since the universe is expanding, it would be a patch of the white hole portion of the spacetime, not the black hole portion. And the "horizon" would be a white hole horizon, i.e., one from which the universe's expansion would eventually cause us to pass out of. However, such a model is extremely unlikely because it has no way of explaining where the white hole horizon came from. A black hole horizon can come into being from gravitational collapse, but a white hole horizon would have to have been "built in" to the overall universe from the very beginning. Nobody has any reason to think that is actually possible, even if we have a theoretical mathematical model that includes it.
- api 2y ago
  What if we're expanding because we are in a black hole that is being fed by a collapsing star or other object in a many orders of magnitude larger scale universe? Of course these kinds of things are probably 100% untestable.
  - pdonis 2y ago

treis 2y ago

Why wouldn't the primordial black holes condense into bigger ones like the matter that makes up planets and stars did?

kadoban 2y ago
Some of them would, some wouldn't, depends on where they were and how they were moving. This is a bit like asking why Jupiter isn't part of the Sun but other stuff is.
itishappy 2y ago
Some would, but the vast majority of matter is not found in planets and stars. Most stuff just floats around on it's own out there.

skykooler 2y ago

If we did have a tennis-ball-sized black hole out beyond Neptune, it would be far beyond our capabilities to locate and track - we can barely track debris that small in low Earth orbit, and black holes aren't even courteous enough to provide a radar return. We would not be able to send probes near it any time soon.