“Champagne” is the legal name of the stuff they make in a certain area of France. In California they can make it from the very same grapes (and they do) but they must call it “sparkling wine”. There’s plenty of that on the shelves, buy American!
IIRC, the closest CA vintners can come to calling their bubbly “Champagne” is to call it a “Champagne method” sparkling wine. I’m not into sparkling wines, but I understand that CA varieties are as good or better than anything the French produce.