Requires answers backed up by equations, empirical evidence, scientific papers, other citations, etc. Contrast with: science-based and reality-check tags. This tag should never be the only tag on a question because this tag frames how a question should be answered, not the topic. Review the tag info before using this tag, and flag your question for moderator attention once posted to have the hard-science notice added.
Question using this tag should ensure they contain the information necessary to solve the question. If the question does not possess the required information, closure is not necessary but comments should be included that request the required information.
All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Speculative or unreferenced answers, as well as those not supported by strong scientific theory, are not welcome. Long, comprehensive answers are desirable, but length and quality aren't always correlated.
The answers should be based on current, undisputed science. This means no subjective sciences, though fields like sociology are mostly considered acceptable (See this meta post). Ideally, answers should be backed up by equations, relevant theories, and citations where possible - arXiv can be quite good for citations, though Wikipedia is usually OK too. (See this meta post.)
Answers that do not meet the requirements of this tag but still answered the question should not be deleted. A good course of action is to leave a comment and downvote if necessary.
Avoid using this tag as the only tag on a question. Instead, use it in combination with subject-specific tags.
After posting a question with this tag, you should "flag" your question for moderator attention and point out that it is tagged hard-science. A moderator will then add the hard-science notice to your question, to draw answerers' attention to the fact that you are looking for hard science.
Suggested default comment:
This answer does not satisfy the requirements of the hard-science tag.
This answer does not satisfy the requirements of the [tag:hard-science] tag.
If you merely want plausibility, go for reality-check instead.
If you want scientifically correct answers but don't need scientific citations, consider using science-based instead. Do not use science-based on a question that has hard-science; it is redundant. However, do not remove hard-science from a question that has the tag and science-based. Instead remove science-based, because hard-science holds answers to an even higher standard.