[jefb]

> 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]

> 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]

> 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]

> 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.

[ordu]

It seems to me as too black-and-white view: either you understand the science, or you don't understand it, with no ground in between.

I want to understand nature, but I have limited amount of time to spend on this goal. So what? Wouldn't be my chosen strategy appropriate? Yeah, I know, my understanding will be limited and sometimes wrong, but it is understanding, isn't it? Isn't it better than total ignorance?

It works not only with nature, there are legal laws for example. Knowledge of laws have a much bigger potential to have an impact on my life, than a nuanced understanding nature. Still I'm not trying to become a lawyer using the same excuse: I have not enough time for that. Instead I maintain some vague understanding of laws and rely on it.

It works for health related issues. I can treat some minor illnesses on my own, because I have some understanding how my body works. I benefit from my limited knowledge of medicine and if my knowledge was better, I would benefit more, but still I have a limited time to study biology and medicine, so while I'm always ready to absorb some more facts, I'm not ready to get a formal education in medicine. Moreover I'm not sure it is possible, to know all the medicine, because qualified doctors are specializing, and I have no chance to be on par with all these specialists.

To my mind it is ok, but with one condition: if you know the limits of your understanding. You need to know when the time has come to seek help of a qualified specialist.

[pdonis]

> either you understand the science, or you don't understand it, with no ground in between.

"Understand" in the sense of being able to make accurate predictions about events that have not yet been observed, or more generally in the sense of having a generative model that can give accurate explanations of things you haven't encountered before, even if they are things that have been observed (by others), is black and white: either you can do it or you can't.

If you have limited time to spend on understanding in the above sense, then your ability to do the things described above will be limited. And note that that is not just true of science; it's true of the other areas you mention (law and health) as well. If your knowledge of the law is limited, your ability to predict the legal risks involved with a planned action, or the likely outcome of a legal dispute, will also be limited. Similarly, if your knowledge of medicine is limited, your ability to judge what doctors and other medical professionals tell you--whether it has an actual firm basis or is just them guessing (and the latter is far more prevalent than many people like to think)--will be limited.

> if you know the limits of your understanding. You need to know when the time has come to seek help of a qualified specialist.

You're assuming that there is a qualified specialist in the area in question. And you're also assuming that you can trust the qualified specialist, or at least that you can spot when the qualified specialist, because of some other agenda involved besides helping you, is giving you information that you shoudn't trust.

None of those assumptions are likely to be valid in cases where it matters. First, if "qualified specialist" means someone who does understand the domain in question in the sense I described above--they can make accurate predictions and they have generative models that give them accurate explanations--then there are no qualified specialists in most domains of interest. Certainly that is the case for the law (lawyers would say they are "qualified specialists", but that doesn't mean they can actually do the things I described above--when they predict an outcome, what they're actually doing is telling you they believe they can manipulate the outcome that way, and that depends on how much money you have to spend and how good the opposing lawyers are). It is also the case for many areas of medicine. (Some areas of medicine, such as particular surgical procedures or particular well-understood diseases, do have qualified specialists who can do those things. But that is a small subset of all of medicine.)

Second, even if we take a domain like physics, where in many areas there are qualified specialists, that doesn't mean that you can read pop science books by those qualified specialists and get an understanding of the physics from them, even if you accept that any such understanding will be limited. Many of the things even Nobel Prize winning physicists say in pop science books and articles and videos are not well established physics, they are just that particular physicist's opinons. And if you yourself aren't a qualified specialist, you have no way of knowing when the physicist is telling you well established physics and when they are just giving their opinions. So even in this hardest of hard sciences, "seek help of a qualified specialist" doesn't actually work well as a strategy.

[ordu]

> You're assuming that there is a qualified specialist in the area in question.

This is a necessary assumption, because you cannot be a specialist in the most fields. You can probably be a specialist in one narrow field, if you spend your life to become one.

> So even in this hardest of hard sciences, "seek help of a qualified specialist" doesn't actually work well as a strategy.

In my experience it works. The trick is to talk with the specialist, to lay out your understanding of the problem to them, to get their critique, fix your understanding and then do several iterations of this. If you really need to be sure that your understanding is adequate for the task ahead of you, you could try to talk with several specialists.

And in overall I have the same feeling of black-and-white worldview on your part. Trying to guess what is different between you and me I come to this:

Truth is not Real, it is Ideal. You cannot reach it. Any understanding is Real, so it is not ideal, it is not perfect. Any prediction is probabilistic. There is Reality itself and there is my limited understanding of it, and there is a vast ocean of information on how others understand Reality. This ocean of information is not the Ideal understanding either. So the crucial skill is to learn how to drink from the ocean a couple of gulps that will be enough for my current task. And it is not just my preference how to deal with the ocean, it is the only viable way to deal with it, because I cannot drink all the ocean. I can't even drink it faster than it gets new information, so even if I had infinite time to drink it, I would be able to bottom it up.

[jefb]

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.

[pdonis]

> 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.)

[Jerrrrrrry]

magnitude difference between dark energy and the schwarzschild radius