Some advice on the topic given by the Duke physics graduate admissions FAQ:
Duke University wrote:4.7. Does it matter if I list my specializations in the application?
Maybe. At the beginning, we will determined the rank without your specialization. Then we will enter in consideration the specializations, and it could be filled spaces in your specialization(s). From year to year this availability of specializations can differ, and often there are no filled spaces in specializations at all.
4.8. Should I gamble on my specializations?
No, that is not recommended. We look at your personal statement and your recommendation letters, to check your “specializations”. If these are different, we would look for an explanation.
4.9. Should I say “all” specializations?
No, unless you really want to work on any specializations. Usually you would reduce your chances of being selected because you would be too generic.
I agree with the notion that whatever you declare, you should be able to back up that decision in your personal statement. Another thing to consider is that if you can't get your specialization of choice at a school, do you really want to go there?