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If the universe is expanding, then at some time in the past, it must have started from a single point but why this point is not the center of the Universe. Just like the singularity of black holes is its center?

Qmechanic
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user17755
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    Everything came from that singularity. It doesn't correlate to a single point in the space. – Xiaolei Zhu Apr 17 '15 at 02:37
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    possible duplicate of Did the Big Bang happen at a point? – Javier Apr 17 '15 at 02:57
  • The Universe probably came into existence from a single point a.k.a singularity thereafter it is the space itself that expand with everything else moving away from everything. – user6760 Apr 17 '15 at 03:30
  • @user6760 What you just said is incorrect. – Jimmy360 Apr 17 '15 at 04:37
  • @user6760 If $x$ represents the current distance between two points that one could measure with a ruler, then in physics we say $a(t)x$ represents the distance between those same two points at time $t$. Today, $a(t)=1$. Due to expansion, $a(t)<1$ for all times before now. The big bang singularity is the time when $a(t)=0$. It is not a singularity as a point in space, but one formed because the distance between any points in space goes to zero – Jim Apr 17 '15 at 16:35

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I'll answer your question with an analogy. Imagine a really small balloon, so small that it occupies a point. Now, imagine that the balloon is expanding uniformly outward from that point. Note that that central point is not part of the balloon. It's the same idea as to what happened with the BB. In this analogy, the universe is the surface of the balloon. The central point is where the universe started, where the BB happened, but that point is actually not part of the universe (it's not on the surface of the balloon). Incidentally, if you look anywhere on the surface of the balloon (anywhere in the universe), you'll notice that the surrounding area is expanding around the point you're looking at so it looks as though any point is the centre of expansion. That is also true of the universe. If you focus your attention on any point in the universe, it looks as though everything is expanding outward from that point so that it looks as though that point is the centre of expansion.

wltrup
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    Are you saying that the big bang happened in a fourth spatial dimension? If not, I think the balloon analogy falls short. At one point, the universe occupied $1m^3$. Then it expanded to $10m^3$, while fully encapsulating the volume of the original $1m^3$. Why is it not accurate to find 'where' this original $1m^3$ was in relation to the current universe, and call that the center? – Ehryk Apr 17 '15 at 04:40
  • Or did the original $1m^3$ volume somehow vanish or disappear onto a different dimension? – Ehryk Apr 17 '15 at 04:41
  • @Ehryk Fair question. First, the balloon analogy is just that, an analogy. But here's some speculative thinking on my part. Imagine that the balloon is, again, a point in the 3+1 spacetime. Further, imagine that when the BB happens, it causes a rupture, a hole if you will, in that spacetime, at that point, much like a puncture in the balloon. The ballon/universe then expands, not into anything outside of it, but just expands, creating 3+1 spacetime as it does so. – wltrup Apr 17 '15 at 05:03
  • The puncture, although once a point in the then point-like universe-to-be, is no longer part of it. It's not in any containing space, it's just a point, so it occupies no volume. – wltrup Apr 17 '15 at 05:03
  • @Ehryk - It is possible that the universe is infinite in extent. In that case, there was never a time that the universe occupied 1 m$^3$. Even right after the big bang, the universe was infinite, it's just that everything was closer together. But, at the singularity, the universe took up zero volume. That instantaneous jump from zero volume to infinite is part of what makes it a singularity in the mathematical sense. And the thing is, there is no special point. An infinite space has no center. All of that being said, no one really knows how far past the cosmic horizon matter extends. – Mark Foskey Jun 15 '15 at 02:35
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it must have started from a single point

This is a common misconception popularized buy the media. Imagine this grid: enter image description here

Imagine each square getting larger. If you think about it, you will see that each point on the grid is expanding. The grid is the universe. Each point on the universe is its own "singularity".

Jimmy360
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The balloon analogy imagines the universe as a 2D surface expanding around a central point as it moves through a 3rd dimension of time. This may be the origin of confusion as in reality there is no 2D surface of expansion, like a wave front, but rather an expansion of 3D spacetime, wherein every point in space quite literally is its own central point from which the rest of the universe expanded. Hence the red shift of all distant galaxies, as would be the case regardless of your location. This is not as straightforward to illustrate, nor as easy to comprehend, as the balloon analogy, but we don't live on a balloon, we live within an expanding 3D space!

zak
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