I chose the proton mass because it dominates the ordinary matter component of the universe. Chemistry and the absorption lines or astrophysical maser emissions

I chose the proton mass because it dominates the ordinary matter component of the universe. Chemistry and the absorption lines or astrophysical maser emissions of molecular clouds (of water, ammonia, and so on) works in all directions and at all scales, or we would very excitedly notice differences in the equivalents of the Lyman-alpha forest and wonder what was happening to protons in different parts of the universe. As far as we can tell, protons interact gravitationally, electromagnetically, and via weak interactions in identical fashion everywhere and everywhen. We could certainly talk about electrons instead of protons. Focusing on baryons and discussing baryonic matter, rather than things like leptons, is more cultural than anything. For example: https://astronomy.swin.edu.au/cosmos/B/Baryonic+Matter https://astronomy.swin.edu.au/cosmos/B/Baryonic+Matter (Electrons and neutrons are practically always dancing with protons; for the most part it's only protons that you find going solo in large numbers in ionized clouds. Likewise, where do you find most of the universe's quarks and gluons? In protons.) Of course one could also say that as far as we can tell from observation, the Standard Model works well everywhere in the observed universe. There may be some small correcting terms yet to be added to make it work better or in regimes or regions not yet observed, but "small" is important there. (Personally, when I hear "SM" I hear relativistic QFT on curved spacetime, and start thinking of the behaviour of dozens of fields, and in that context electrons are already messy (cf. dressed electrons), and protons are almost incomprehensible: https://profmattstrassler.com/articles-and-posts/largehadroncolliderfaq/whats-a-proton-anyway/ https://profmattstrassler.com/articles-and-posts/largehadron... just scratches the surface!)