Comment by jefb
Comment by 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 ;)
Comment by 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 ;)
> 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.
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
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.
> 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.
> 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.
> 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.
> 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.)
magnitude difference between dark energy and the schwarzschild radius
> 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.