[beltsazar]

Yes, cosmological principle is probably the most fundamental assumption in astronomy.

Most people don't realize that science—and even everything in life—has to start from some axioms/assumptions, just like math. I first realized this fact when I was reading the Relativity book written by Einstein himself, who challenges the assumptions in classical physics.

As time goes, some of the assumptions could be proved to be unnecessary or even wrong. There must be still some assumptions left, though—because without them, we can't talk about science, or anything, really.