It's said that only a few -ku forms perfom as a noun; 遠く, 近く, 早く, 遅く, 古く or 多く. Some other -ku forms, e.g., 高く, 深く also can be used in some cases, e.g. with adverbs or nouns; 風船は、かなり高くまで上がって割れた, 胸の奥深くが痛んだ. As you can see, they are locative or temporal adjectives except 多く. Plus, interestingly, the couterparts of the pairs 新しく, 低く, 浅く or 少なく can't have this usage.