# 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

## 1 Answer

The Big Bang model describes early moments of time for the entire universe. The official Lambda-CDM model allows for two possibilities. (1) If the universe is closed (global space has a positive curvature), then the universe is expanding from smaller to larger. (2) If the universe is open (global space is flat or negatively curved), then the universe is infinite and has always been infinite. In this case, the universe was initially infinitely large, with the infinite mass, and infinite density everywhere.

Infinities in physics are a problem, as infinite solutions are generally considered non-physical. A well known example is how a concern with infinite solutions in classical physics has lead to the development of quantum mechanics that avoided such infinities. Accordingly, the hope is that quantum gravity will bring more light on the first moments of the Big Bang and what the universe looked like back then.

• So basically in flat space there were points - at overwhelming distances beyond the observable U and even beyond the nearby, ie inflation accounted - which never were in contact to each other? Or in other words is not true that space came into existence but was there, shrinked but infinite? Jan 22 '19 at 8:55
• @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
• Yes let us start from when energy was there of course. Else nothing would have happened. I am afraid are these kind of things too complicated. I can just shift the Q to did this space and energy appears everywhere well beyond horizons. Then someone could say it is very huge we'll never access that and so on. In fact I do have problem with rolling back, rather than the far far away parts of U. I am already happy you got the first point. Unfortunately I am not use to chat else we could write a Q. When I've tried I got to know what more or less knew already. My doubt was immediately back.... Jan 22 '19 at 10:55
• I think that obviously I must roll back including inflation so that the scale goes faster down overcoming the simple sum of tiny specks of distance. With time I will try a question indeed. Jan 23 '19 at 8:32
• @Alchimista Yes, it's your question. Wasn't easy to phrase :) Hopefully I'd doesn't get downvoted, but you never know on this site. Jan 23 '19 at 8:44