By the same standard you use for an eternal or uncaused universe.
which is basically established by saying "Because I said so."
No, not because I say so but because it logically and ontologically necessary.
Yet you demand answers for science beyond that.
False dichotomy. God and science are not antithetical. God and materialist philosophy are opposed to one another but not God and science. If science can shed some light on the subject on whether or not the universe had a beginning, then more power to it. But thus far you have demonstrated that you are not interested in discussing whether or science can illuminate the issue of whether or not the universe had a beginning, and if so whether or not the beginning was caused.
Cordially,
Science is always a work in progress and doesn't claim to have the ultimate answers. You do claim to have the ultimate answer to these scientific questions, based on a book written by people who thought the Earth is flat.