A passage from a paper:

"If one imagines running the clock backward in time, any given region of the universe shrinks and all galaxies in it get closer and closer until they smash together in a cosmic traffic jam—the big bang"

I can not imagine how this can happen in a flat space, specially when author says in the continue:

"The totality of space could be infinite. Shrink an infinite space by an arbitrary amount, and it is still infinite."

How can a flat space be shrunk at a specific moment to be called Big Bang?

There are many reasons in cosmology that space is flat. When I think of a flat space shrinking, I can not imagine a definitive moment of time when all things smash together. A flat space can never be jammed. With shrinking (thinking backward in time), things certainly smash but that never ends in a certain moment of time. That continues forever! Since the space is flat (it's more like a flat paper sheet instead of a ball), things always can be found in infinitely far distances, and when thinking backward in time, things constantly smash together, but, this will never ends in a specific moment! Unlike the case for a shrinking ball, where all things smash together at a specific moment.

What's the catch?

(Confusion remover: The Big Bang happened everywhere and I do not have a problem with that. My problem is with the moment of Big Bang in a flat space. When you shrink a sphere, everything at the surface comes close to each other until at a specific moment, everything smash together, Big Bang happens everywhere. This is hard to imagine in a flat space.)

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    $\begingroup$ The big bang doesn't require that all things were in one singular point at some time, it only requires that in the beginning everything was much smaller and that the density and temperature were much higher. How small and how dense and hot that depends on the model for the big bang and we have a number of different ones, we just can't tell them apart with the data that we have. So what's the catch? Generally very poor visualization of the big bang model on tv and in other media which gave you the wrong idea about the assumptions of modern cosmology. $\endgroup$
    – CuriousOne
    Commented Jun 10, 2015 at 8:15
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    $\begingroup$ This might be useful background reading, though it isn't a duplicate. $\endgroup$ Commented Jun 10, 2015 at 8:20
  • $\begingroup$ Thank you CuriousOne. My problem is that I can not imagine the moment of Big bang in a flat space. In a closed universe, it is obvious. There is a certain moment of time. But in an open universe, I can not imagine how I get to a specific moment of time where all thing smash together because a flat space essentially never shrinks even if we imagine that the proper distance shrinks with time. $\endgroup$
    – user65852
    Commented Jun 10, 2015 at 8:23
  • $\begingroup$ This one might help. physics.stackexchange.com/q/4921 I think, and I say I think cause I'm not an expert, but I think, an infinite universe means an infinite big bang - just, obviously, much much more dense in the beginning. Infinity can stretch or contract and remain infinite the whole time. $\endgroup$
    – userLTK
    Commented Jun 10, 2015 at 8:37
  • $\begingroup$ I think you are too focused on this popular idea that the universe had to be some tiny spot at some time in the past. That's not necessarily the case. We know that the visible universe was a lot smaller, but that doesn't say anything about the size and shape of the entire universe. The entire universe may not even be homogeneous, let alone flat, we just don't know about it because we can't interact with it. What we do know is that the part we can interact with is, within the precision of our best measurements, consistent with the hypothesis that the visible universe is flat. $\endgroup$
    – CuriousOne
    Commented Jun 10, 2015 at 8:44