6 ms·

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.wi

by 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
    > 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.
  - Jerrrrrrry 2y 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
- jefb 2y 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?
  - pdonis 2y 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.
    - ordu 2y 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.
      - pdonis 2y 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.
- ndsipa_pomu 2y 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.
  - pdonis 2y 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.
    - ndsipa_pomu 2y ago
      Thanks - that makes it a bit more precise.

by jefb 2y ago

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
  > 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.
- Jerrrrrrry 2y 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
jefb 2y 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?
- pdonis 2y 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.
  - ordu 2y 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.
    - pdonis 2y 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.
ndsipa_pomu 2y 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.
- pdonis 2y 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.
  - ndsipa_pomu 2y ago
    Thanks - that makes it a bit more precise.