Given there have recently been several questions put on hold as "off-topic" when they're clearly about cosmology/astrophysics (which is defined as on topic in the Help Centre), I thought it might be worth resurrecting this meta issue. Have we recently become too narrow in our focus, or are there other factors involved?
I've included three examples at the end, and proposed some actions...
1. The meta issue
The Help Centre says that
Astronomy Stack Exchange is a Question and Answer site about astronomy and astronomy related topics; the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties
The purpose of this site is to provide expert level answers to questions on:
- Setting up, using and maintaining your astronomy related equipment
- Astronomical observations, for all celestial objects across the entire spectral range.
- Astrophysics and Cosmology [my emphasis]
The inclusion of astrophysics and cosmology is deliberate, but from the start it has raised questions of application.
Back in private beta stage, in September 2013, manishearth explained that we don't need to draw a line between Astrophysics and Physics: "It's perfectly OK for sites to have an overlap."
In response to the same OP, SE's Director of Community Development, Robert Cartaino posted a link to the instructive 2012 blog post — Respect the community – your own, and others’ — which is well worth a read. In the blog, Community Manager Josh Heyer suggests:
Be a bit jealous of your site – don’t blithely turn askers away simply because their question could be asked somewhere else. Don’t hit them over the head with your scope, help them tailor their question to fit into it – and if that means your site’s scope overlaps a bit with another site’s, so be it. ... Close questions with an eye toward improvement and re-opening, not driving users away.
Also in Sept. 2013, SE Community Manager Jon Ericson unequivocally responded to the question "Do cosmological questions belong here or not?":
Yes. If astronomers and astrophysics study such questions (and they do), ask your cosmology questions here.
In another contemporary post - Let's ask more meaty questions! - Jon Ericson urged users:
Please ask deeper and more-specific questions. And when you see commonly-asked questions, either write strong answers that improve on easily discovered sources or vote to close (as "too broad", most likely).
3. Not clear enough?
In October 2013, seeking further clarity, called2voyage posted the question How much is astrophysics, cosmology, and theoretical astrophysics on topic?:
"There have been some concerns raised about cosmology and theoretical astrophysics, as well as astrophysics in general.
The concern is that this community does not support astrophysics questions, especially cosmology and theoretical astrophysics.
If you think this community can support astrophysics questions, make your case for it in an answer here."
The single answer listed 9 examples of good cosmology/astrophysics questions on our site, plus 8 users with known expertise in these fields, as evidence that Astronomy SE can indeed support such questions.
A more recent (2016) question asked Are Questions related to Cosmology and ancient history of astronomy allowed in Astronomy SE?. Regarding the "cosmology" half of the question, there was one short answer by called2voyage that seems to me to be definitive:
Yes, scientific cosmology is perfectly acceptable here.
4. Current examples - action needed!
The following three closures have occurred in just the last two weeks...
Assessment: This is without doubt a cosmology question. Rory voted to close this question as off-topic "because it really belongs on Physics (although it is probably a dupe of a few there)", but that's not a reason to close. I voted to close too, but I'm still learning the intricacies of my newly-acquired privilege and in hindsight probably made the wrong decision. It's a poor question, as it shows no research effort (for instance this topic been covered ad nauseam on Physics SE), but that's a reason to downvote rather than close.
Status: Currently closed. Zero votes; two answers, both with positive votes, one answer accepted.
Solution: (1) Reopen as a canonical question on the topic, so that future duplicates can be directed to this post. A third answer (perhaps as a community wiki?) listing some of the best Physics SE answers on this topic would add further value. Otherwise, (2) Reopen and vote to delete: if it's not good enough as a canonical post, it's probably not good enough to retain at all, and it doesn't meet SE's criteria for the system to delete it automatically so voting would be required.
Assessment: This is a theoretical astrophysics question about the thermodynamics of the Recombination Epoch. It was put on hold by a moderator for being "off-topic", which would appear to be incorrect. Its core request "How can I derive this equation?" is probably better suited to Physics SE, but that's no reason to close. Zephyr has provided a link and brief response in a comment.
Status: Currently on hold. Votes: +1. No answers.
Solution: I recommend reopening, as it doesn't meet the criteria for closure. However, I don't think this question adds a lot to our site, and Zephyr's comment may well be the best answer the OP is going to get, in which case 2 downvotes (to bring it to -1 score) would be sufficient for this question to be automatically deleted in about 20 days as a dead question [I think death from natural causes is probably nicer than deletion by voting].
Assessment: This post asks for help understanding how an expression relating to CMB and redshift was derived. This is theoretical astrophysics, which is on topic. It was put on hold by a moderator as off-topic for being about Earth Science, which makes no sense; I can only guess that perhaps the moderator confused Compton scattering in the early universe with Rayleigh scattering in the Earth's atmosphere. I've voted to reopen.
Status: Currently on hold. Votes: +5. Answers: zero. However, the same question has been cross-posted at Physics SE, where the OP has provided an answer to their own question.
Solution: The closure was for an invalid reason, so the question should be reopened. Given it's been cross-posted to a site more appropriate to the content, the best solution would be for the OP to delete their Astronomy post (and FWIW, become only the 16th site user to earn a Disciplined badge).