Hello everyone: I'm a doctoral student in particle physics, confused about something pretty fundamental and need your help. From what I know, the only evidence we have from the Big Bang is the Cosmic Microwave Background radiation (CMB), which showed up around 300k years after the hypthesised Big Bang happened. According to this, I see people claiming that the universe started from the size of a "proton" and expanded to that level that produced the CMB. Where did that come from? is that just simply an extrapolation? and what does make that extrapolation over 300k years valid? Now my question is: is there evidence for what's claimed to be true about the universe before the universe was 300k years old? Why can't we say that the universe would expand and crunch continuously without stop? Of course, I know that the universe is flat, but this is still not over, and the universe's expansion could be accelecrating, and then could decelerate again and go to the big crunch again, and there we'll have a new big bang when all matter crunches. So you see, I have some unclear issues about this topic. Could someone please clarify the story on the basis of what is proven by evidence and what is just hypothesised? Thanks for any efforts.