# Does the big bang model describe a first momment in time for the entire universe or just the observable universe

The big bang model describes the universe as contracting as we wind time backwards. Since the observable universe is of a finite size this ultimately sets the first moment of time at around 13.8 billion years ago.

My question is: Does this also include the non-observable universe? For instance if the non-observable universe is infinity large couldn't it contract indefinitely without a first moment of time?

• "For instance if the non-observable universe is infinity large couldn't it contract indefinitely without a first moment of time?" - why? May 29 '18 at 2:25

• @Alchimista Your first point seems corrrect, but you may want to ask it as a separate question on this site to let others provide an insight. You second point though is incorrect. Spacetime with energy-momentum are Fourier conjugates (two different sides of the same coin) meaning they cannot exist without each other. If spacetime was there before the Big Bang, then energy also was there. And if so, the universe would simply start expanding earlier. So no, energy and time had to come to existence together at $t=0$. Time starts at the Big Bang, so there is no such time as "before" the Big Bang. Jan 22 '19 at 9:31