We haven't. Cosmology (and even QM and relativity) has been attacked many times on these threads, and on creationist websites, etc. According to most of the evolution antagonists, though, cosmology, geological gradualism and evolutionary biology are all part of the same theory called "evolution", though. Get used to it if you continue to read these threads - when a young-earth creationist refers to "evolution", they really mean any finding in any branch of science with implications reaching further back in history than the 6000 years literal Biblical history allows for.
Which is everything.
Guess I'll start worrying when I'm required to teach "Creation physics" or "Biblical physics" alongside PHYS 101.