Will a person be held accountable for the doubts that cross his mind? And is it possible that previous civilizations had attained cosmic sciences mentioned in the Qur’an?

A person is not held accountable for the mere occurrence of thoughts in their heart, unless those thoughts lead to something legally prohibited [in Islam]. As for reaching scientific facts about previous civilizations based on studies grounded in sound principles, there is nothing wrong with it. This is in contrast to speculation and conjecture, which are false. The miraculous nature of the Qur'an lies in its reporting of unseen scientific facts that unfold exactly as it reported, because it is a revelation from Allah, the All-Knowing, the All-Aware, whereas humankind is weak and ignorant.