Comment by api

Comment by api 3 days ago

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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 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

jefb 3 days 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

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  • pdonis 3 days 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.

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    • api 3 days 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.

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      • pdonis 3 days 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?

        Expanding and collapsing are two different things. So I don't see how your suggestion here makes any sense.

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      • Jerrrrrrry 3 days ago

        complexity of life's scale somehow trillions of magnitudes "smaller" than a similarly constructed universe is not only completely irreconcilably untestable (outside of one thought one) but also reminiscent of m-theory (11 dimensions) and the plot of men in black

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    • ndsipa_pomu 3 days ago

      I recall seeing something (likely a youtube video on cosmology) that suggested that the Big Bang would be the white hole horizon (i.e. a singularity in out past) and that does make some kind of sense as it'd be impossible to go inside the Big Bang. I recall there being some good reasons as to why that's not believed to be the case though and also why the visible universe doesn't have an event horizon.

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      • pdonis 3 days ago

        > the Big Bang would be the white hole horizon (i.e. a singularity in out past)

        The white hole horizon is not the same thing as the white hole singularity. The "Big Bang" as an initial singularity in our universe (which is not actually the correct usage of the term "Big Bang", but that's a whole other discussion) would be the white hole singularity, not the horizon.

        Note also that in a white hole model of our universe, we would be inside the white hole horizon, not outside it.

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    • jefb 3 days ago

      > 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.

      If the space containing the matter is stretching does that still count as expansion?

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      • pdonis 3 days ago

        > If the space containing the matter is stretching does that still count as expansion?

        "Space stretching" is a vague pop science description that doesn't really correspond to anything in the actual physics model. So it doesn't count as anything; you should just ignore it.

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      • Jerrrrrrry 3 days ago

        magnitude difference between dark energy and the schwarzschild radius

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  • gerad 3 days ago

    I guess that'd explain the big bang nicely. It does make it the "outer universe" disappointingly unobservable though.

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pavel_lishin 3 days ago

> Then there’s spooky shit like: > 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.

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  • ithkuil 3 days 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?

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skykooler 3 days 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.

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treis 3 days ago

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

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  • itishappy 3 days 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.

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  • kadoban 3 days 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.

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renox 3 days 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.

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  • api 3 days 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.

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    • ithkuil 3 days 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?

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      • LegionMammal978 3 days 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_e...

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      • mr_toad 2 days 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.

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ck2 3 days 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.

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  • api 3 days 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.

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