I've noticed that some studies are named with a 論 suffix in Japanese, and effectively in Korean as well. They are otherwise simply suffixed with 学 in Chinese and Vietnamese. Compare these examples:
Japanese 宇宙論 vs Chinese 宇宙学 (cosmology)
Japanese 音韻論 vs Chinese 音位学 (phonology)
There are also words like 集合論 (set theory) that are used in both Japanese and Chinese, but the English translation is more likely to contain "theory".
So I wonder what makes some studies 論 or 学. Do 論-studies involve much more theories or suggestions, and those theories or suggestions are, well, prone to being debatable? For instance, phonologists have had different opinions on the number of phonemes of Japanese, which makes their study (phonology) the 論 type? And cosmologists only speculate about the Big Bang theory, and no one has ever witnessed the creation of the universe since we didn't exist then, which makes cosmology the 論 type? I also wonder why studies like archeology (考古学) or paleontology (古生物学) are the 学 type, since they surely involve a lot of theories. We have yet to be able to travel back in time to confirm what dinosaurs really looked like, haven't we?