There are a couple of different things that are happening here, and I'm not sure that I will quite disentangle them while I'm writing this answer, but here goes.

First, there's the question of topicality. I've been on record nearly since I joined with the opinion that interesting questions deserve more leeway than boring questions, and that it's a privilege of the human members of our community to decide which types of questions are "interesting." I thought this particular question was super-interesting, so I answered it --- which means that for this discussion I'm participating with my user hat on, and my diamond-moderator hat hanging in the hallway.

Second, the goal of the bounty is slightly different from the original question. The bounty text is

Looking for an answer drawing from credible and/or official sources.

This problem has presumably been conclusively settled in the literature. Answers here should provide a self-contained derivation and an explanation of the physics, but also suitable references to confirm their material in the literature, particularly given the disagreements between and about the different existing answers.

We have a protocol for asking questions where the primary goal is a literature reference: the resource-recommendations tag. Such questions are made community-wiki, and there are standards for answers which are enforced collectively by the community and the moderators. A pure resource-recommendations version of this question would absolutely be on-topic, and the mostly-complete homework-like solutions (again: I'm currently guilty here) would be off-topic answers to such a question. However, the question wasn't edited into this form before the bounty was added.

Third is the issue raised in the moderator flag response. In general, a question with a high score and no close votes (especially absent the HNQ effect, which didn't happen to this question) is a question that the community has decided to like. However, the bounty was added early in the question's history, and it's impossible to cast a close vote on a bountied question. It's possible that one or more users saw the question after it was bountied, tried and failed to cast a close vote, and gave up on recording their opinion rather than cast a custom flag or questioning the on-topic-ness in a comment there. Some better guidance in the user interface could have helped here; I'll draft (or find and upvote) a proposal on the mother Meta.

Again, I am bowing out of expressing an opinion about whether the question should or shouldn't remain open, since I wrote an answer which superficially breaks the guidelines. But wow, I had a ton of fun constructing that answer; I learned a lot writing it, and now I'm in a position to appreciate what I'll find in the literature when I make time to answer the resource-recommendation part of the question. I'm glad the question was posed, and I'm glad for the tension that was pointed out between the early answers --- whether it's eventually closed or not.

There are a couple of different things that are happening here, and I'm not sure that I will quite disentangle them while I'm writing this answer, but here goes.

First, there's the question of topicality. I've been on record nearly since I joined with the opinion that interesting questions deserve more leeway than boring questions, and that it's a privilege of the human members of our community to decide which types of questions are "interesting." I thought this particular question was super-interesting, so I answered it --- which means that for this discussion I'm participating with my user hat on, and my diamond-moderator hat hanging in the hallway.

Second, the goal of the bounty is slightly different from the original question. The bounty text is

Looking for an answer drawing from credible and/or official sources.

This problem has presumably been conclusively settled in the literature. Answers here should provide a self-contained derivation and an explanation of the physics, but also suitable references to confirm their material in the literature, particularly given the disagreements between and about the different existing answers.

We have a protocol for asking questions where the primary goal is a literature reference: the resource-recommendations tag. Such questions are made community-wiki, and there are standards for answers which are enforced collectively by the community and the moderators. A pure resource-recommendations version of this question would absolutely be on-topic, and the mostly-complete homework-like solutions (again: I'm currently guilty here) would be off-topic answers to such a question. However, the question wasn't edited into this form before the bounty was added.

Third is the issue raised in the moderator flag response. In general, a question with a high score and no close votes (especially absent the HNQ effect, which didn't happen to this question) is a question that the community has decided to like. However, the bounty was added early in the question's history, and it's impossible to cast a close vote on a bountied question. It's possible that one or more users saw the question after it was bountied, tried and failed to cast a close vote, and gave up on recording their opinion rather than cast a custom flag or questioning the on-topic-ness in a comment there. Some better guidance in the user interface could have helped here.

Again, I am bowing out of expressing an opinion about whether the question should or shouldn't remain open, since I wrote an answer which superficially breaks the guidelines. But wow, I had a ton of fun constructing that answer; I learned a lot writing it, and now I'm in a position to appreciate what I'll find in the literature when I make time to answer the resource-recommendation part of the question. I'm glad the question was posed, and I'm glad for the tension that was pointed out between the early answers --- whether it's eventually closed or not.