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So, we observe that galaxies are moving apart (on average) and the further away from us they are the faster they are moving away from us. If space was expanding that would explain this.

However, I imagine that if a primordial explosion happened then at some point close to where the explosion happened things would not move much, and they would be moving faster the further away you were from "ground zero".

But if we were not near ground zero there should be some anisotropy in the speeds we observe, is that correct? There should be a "ground zero" in the "explosion model", right??

However, regardless of the "ground zero" issue, do we (or could we) distinguish between a uniform "explosion" model and the idea that space is expanding?

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    $\begingroup$ Possible duplicate of Did the Big Bang happen at a point? $\endgroup$
    – pela
    May 4, 2019 at 23:52
  • $\begingroup$ "There should be a 'ground zero' in the "explosion model", right??" - Correct, but it does not mean that this point can be easily identified. "But if we were not near ground zero there should be some anisotropy in the speeds we observe, is that correct?" - No, this is incorrect. The most popular "explosion model" is by Milne, whose student was Walker (the W in FLRW): en.wikipedia.org/wiki/Milne_model - This Wiki article does not do justice to his fascinating model, in which the universe expands inside the Big Bang singularity. There is no speed anizotropy in this model. $\endgroup$
    – safesphere
    May 5, 2019 at 4:27
  • $\begingroup$ Here is essentially the same question answered: physics.stackexchange.com/questions/423536/… $\endgroup$
    – safesphere
    May 5, 2019 at 4:55
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    $\begingroup$ @pela The question you've linked is NOT a duplicate. There the discussion is about the unproven theoretical speculations of the FLRW model. In contrast, here the OP is asking about a direct experimental evidence, of which none exists, to distinguish between the space expansion and galaxies just flying apart on inertia. $\endgroup$
    – safesphere
    May 5, 2019 at 5:00

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It is a misconception that the big bang happened at a point. It did not. There is no ground zero point (in space) where you could point and say, this is where the explosion happened. Space is expanding everywhere at the same rate, and everything is getting farther away from everything.

Now this is not completely true, because galaxies are getting usually farther away from each other, because in the intergalactic space, expansion is dominant.

But inside galaxies, gravity dominates, so space does not expand inside galaxies.

So on the bigger scale, between the intergalactic voids of space, where expansion dominates space itself is expanding, and this expansion seems to be accelerating. Now an explosion as you say could not do this, because to have an ever accelerating expansion you need more and more energy (whatever is causing expansion, like dark energy or negative density). An explosion cannot do that.

So the big bang did not happen at a point in space, but it happened everywhere at the same time.

There is no center of the universe or as you say a ground zero point.

The universe is not like a ball expanding, there is nothing outside the universe that it is expanding into.

Please see here:

Did the Big Bang happen at a point?

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  • $\begingroup$ Got it - that the expansion seems to be accelerating seems very key to me $\endgroup$
    – Paul Young
    May 4, 2019 at 23:32
  • $\begingroup$ Statements in your answer are not based on a direct experimental evidence, but on conclusions of one particular cosmological model developed first by Alexander Friedman of Russia and known as FLRW or, with additional details, as Lambda-CDM. There is no experimental evidence that space expands or that "the Big Bang did not happen at a point" or of the existence of "dark energy". All these are popular, but unproven theoretical speculations based on a very problematic Friedman model. The evidence is that space is flat, for which this model gives non-physical results at time zero (infinite mass). $\endgroup$
    – safesphere
    May 5, 2019 at 4:19
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    $\begingroup$ @safesphere There is a huge body of evidence showing that space is expanding and all existing evidence points to that. Exactly how fast it is happening, and whether or not the rate of expansion will accelerate (or decelerate) is not something we know. $\endgroup$
    – forest
    May 5, 2019 at 8:16
  • $\begingroup$ @forest Welcome to physics Mr. Security :) Unfortunately your comment is incorrect on both points. It looks like you are confusing the expansion of the universe, for which there indeed is a plenty of evidence, with the expansion of space, for which no direct evidence currently exists. These two concepts are related, but different, because the universe can conceptually expand without the expansion of space, e.g. search for the Milne model. Such evidence is possible, we just don't have it yet. if you believe we do, then perhaps you could clarify what it is. $\endgroup$
    – safesphere
    May 5, 2019 at 15:18
  • $\begingroup$ @forest You second point is also incorrect. The space expansion is a conclusion of the currrent model of cosmology first developed by Friedman. This model clearly states exactly how the universe expands, with what acceleration or deceleration (except for a brief initial inflation). So, if you believe the official cosmological model known as Lambda-CDM (or FLRW), then we indeed know exactly how space expands, although only theoretically, without a direct experimental evidence. We also have experimental data measuring the acceleration of the expansion of the universe, but not necessarily space. $\endgroup$
    – safesphere
    May 5, 2019 at 15:30

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