There is a distinction between primary and secondary causes, with the first cause being the most primary one. The universe cannot be its own cause, and the Big Bang is not the same thing as creation - as noted by Fr. Lemaître himself( i.e. the man who developed the Big Bang theory in the first place). Perhaps it should also be noted that Aquinas operated under the presumption of an eternal universe.
So there's some real fundamental misunderstandings here.