So a week or so ago, I made the tag; made a tag-wiki for it too. Recently, a certain someone (not relevant who) made it a synonym of . For the record, it is currently the only synonym of . I think this was not valid. I'd like to state my disagreement with this action and request that it be undone.

Cosmology is a broad field that applies to a very large number of topics. It is true that would be a subset of , but it is not a synonym. only applies to topics that include a scale factor, are about the GR model of the universe we live in, and make some reference to the slightly more advance concepts like Friedmann equations, Hubble radius, co-moving quantities, or the like. Whereas cosmology can be applied to a much broader range, such as AdS/CFT, Schwarzschild space, asymptotically de Sitter solutions, and many others.

You could have a question that asks about the problem with using de Sitter space as a model for the universe we live in. That would be tagged as , but it should not be . Similarily, a question asking about cosmological redshift is a cosmology question and it is probably within an FRW-universe, but the question shouldn't be tagged as any more than a question about radioisotope dating should be tagged with Additionally, a question that asks about finding the Ricci tensor for a higher-dimensional FRW metric could be tagged under (albeit, it would be more appropriate as , but those would more likely be synonyms) and , but I wouldn't necessarily put it under .

Furthermore, if this is a simple case that 99% of the time when is used, will also be used, then I'd like to point out the existence of , , , or possibly the worst for that particular crime: . These are all allowed and often used without being recast as synonyms.

Based on all this, I think the synonymizing should be reversed.



  1. if the Phys.SE community thinks a tag would be useful, and

  2. if somebody would like to be custodian over the tag, then it seems like a good idea to undo the synonym.

In an ideal world you are right: Let's tag all the questions as precise as possible. The problem is that we are only allowed to use 5 tags. And many users would not know the tag, and not use it. The fact is that currently almost none of the FRW-related questions are tagged as such. Thus it would require a custodian to

  1. initially find all the FRW-related questions and tag them appropriately, and

  2. in the future to tag new FRW-related questions consistently.

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    $\begingroup$ I've been slowly working at retagging old questions to frw-universe. And I try to tag new posts that deserve it, but in general most askers that don't know what it is don't ask questions that should be tagged by it $\endgroup$ – Jim Apr 23 '14 at 21:35
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    $\begingroup$ What tag would people think to use when they are asking a question about the FRW metric? I think it would make sense to have that tag, or make that a synonym of frw-universe. (Personally I would have called it flrw-metric but I'm not in that field) $\endgroup$ – David Z Apr 23 '14 at 22:03
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    $\begingroup$ @DavidZ yeah, I was torn between metric and universe. I ended up choosing universe because I figured a question about the metric is also about the FRW-universe solution. But a question about the FRW-universe need not specifically reference the metric. That said, I'm not married to the choice $\endgroup$ – Jim Apr 23 '14 at 22:09

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