Why can't the big bang the first cause?

An other explanation could be that there is really an infinite past. I read that in the after the universe origined the metric expansion of space was exponential. And an exponential function has an asymptote at the x-axis. So there could be an infinite line of cause.

However, inventing a "god" or "designer" who lived "outside of causality" and created the universe is very ridiculous.