Comment by pdonis

Comment by pdonis 10 months ago

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> 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 10 months 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 10 months 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 10 months 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 10 months 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 10 months 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 10 months 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 10 months 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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    • ordu 10 months ago

      > you should just ignore it.

      I believe we shouldn't ignore it. I know about physics from pop-science mostly, so I have limited choices, either "space stretching", or (if I just ignore pop-science) "I have no clue what is happening", or I should stop doing all I'm doing now and dig into physics textbooks, to get real understanding. The last option is not really tempting, I have better ways to spend my free time, the second option doesn't seem constructive at all, so the only viable option is to not ignore vague pop-science description.

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      • pdonis 10 months ago

        > I believe we shouldn't ignore it.

        To be more precisely, you should ignore it if you want to actually understand the science. Pop science presentations will not help you understand the science. That's not what they're for. Being as charitable as possible (i.e., ignoring the obvious money-making and eyeball-capturing motives), pop science is for getting people interested in a science topic--so that at least some of them will be motivated to learn more about it, from sources like textbooks or peer-reviewed papers or class lecture notes and other teaching materials (many universities now have those available online for free) which can help you actually understand the science.

        > the only viable option is to not ignore vague pop-science description.

        As long as you are ok with not understanding the actual science. Nature doesn't care how much time and effort it takes to actually understand something in science. So it is no argument at all to say that you have better ways to spend your time, if you actually want to understand the science. The time required to do that is not dictated by your convenience.

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    • jefb 10 months ago

      My point was to illustrate that our physics models don't agree on the nature of this expansion (Hubble tension) so using it to dismiss the fact that the observable universe is dense enough to form an event horizon seems like a stretch.

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      • pdonis 10 months ago

        > My point was to illustrate that our physics models don't agree on the nature of this expansion (Hubble tension)

        The Hubble tension is not an uncertainty about the "nature" of the expansion. No matter how that tension gets resolved, our underlying mathematical model of "the expanding universe" will not change. All that will change is that the value we use for one particular parameter in that model will be more accurately known.

        > using it to dismiss the fact that the observable universe is dense enough to form an event horizon

        I have not dismissed that fact at all. I have simply pointed out that, as a matter of physics, that fact does not mean our universe actually has an event horizon. "Dense enough to form a event horizon" is just a mathematical calculation. Whether that calculation actually means something, physically, does not just depend on the value it gives you. It also depends on the underlying spacetime model, and our underlying spacetime model for the universe as a whole (which, as noted above, is not in dispute at all, Hubble tension or no) is not the one in which the mathematical calculation of "dense enough to form an event horizon" has any physical meaning. (In more technical language, that calculation only has physical meaning in the Kerr-Newman family of spacetimes, but the FRW spacetime used to model our universe as a whole is not in that family.)

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  • Jerrrrrrry 10 months ago

    magnitude difference between dark energy and the schwarzschild radius

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