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Let's assume that at Bing Bang all the matter was in a very small area. Then it exploded, in other words all the pieces of matter started moving away from the s

Let's assume that at Bing Bang all the matter was in a very small area. Then it exploded, in other words all the pieces of matter started moving away from the starting area, presumably at an accelerating speed. That would simply mean that everything started moving away from everything else and that movement seems to be even accelerating. Fine. That is a good and easy to understand description. Why then do I have to add "BTW. space is expanding"? Things are moving away from each other I get it. Why do they need to add to that "Space itself is expanding"? I don't doubt there is a reason since best scientist are thinking about it. I just haven't seen a good explanation for it, yet.

drdeca 5y ago
AIUI, “Started moving away from the starting area” is wrong. The starting area is everywhere. It isn’t that there is some point where the center of the Big Bang was. The idea is that, stuff was basically evenly distributed everywhere , and very dense, and then later, without needing the amount of stuff to have decreased, stuff is nevertheless more spread out. Now, you might just explain this with like, “well, space is infinite in extent, so you can just spread it all out, without running out of room, by moving things out towards infinity” but you can describe the same sort of situation even if space is compact, turning back around in itself (as in the analogy of “imagine space is the surface of a balloon”)