Whether a question exists elsewhere -- another Stack Exchange site, Quora, an academic site, etc -- should not affect whether the question is welcome on our site. We should judge each question on its own merits. So closing as off-topic just because it also exists on Physics isn't the right approach, except maybe in the degenerate case of an exact duplicate (somebody asked the same exact question on both sites with no tailoring).
Worldbuilding has a broad scope, and so there are a lot of SE sites that overlap ours. That's fine. It happens in other clusters of sites too; some questions could easily be asked on either DBA or SO, questions about SF movies could be asked on either Science Fiction & Fantasy or Movies & TV, there's a lot of apparent overlap between Ask Ubuntu and Unix & Linux, and then there are all the bitcoin sites. Here on Worldbuilding we sometimes overlap with Physics, Chemistry, Biology, Astronomy, the religion sites, Writing, RPG, and probably others.
The best answers here support their claims with sources or other supporting evidence. Does it matter if those sources are on Physics.SE as opposed to in a relevant journal, blog, Wikipedia page, or academic course? The quality of some sources is higher than others (peer review increases credibility), but the validity of using them on this site is IMO the same. If an answer here links to relevant answers on Physics, then readers can click through to see those posts in their original context (and can vote on them).
Another factor to consider is that while some subject matter can belong on either site, the audiences are different. An ideal answer synthesizes (sourced) information from elsewhere and presents it in a form that fits the current audience. While Worldbuilding probably has advanced physicists among its users, it also has a lot of users who aren't -- presumably way more than Physics.SE does. An answer on Physics.SE might therefore be too technical or assume too much background knowledge to work here, but could be "translated". On Mi Yodeya (the site about Judaism) I see this sort of thing all the time, where somebody cites or quotes a complicated explanation in a legal source (probably written in Hebrew or Aramaic) and then breaks it down and explains it for English-speaking readers who are not fluent in all the legal nuances.
While I don't want to see excessive quoting or gratuitous links (to anywhere), we're part of a larger network and I'm happy to see cooperation and collaboration across sites when it makes sense.