# Is space itself expanding or is it just momentum from the Big Bang carrying things apart? [duplicate]

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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• Possible duplicate of Did the Big Bang happen at a point? – pela May 4 at 23:52
• "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. – safesphere May 5 at 4:27
• Here is essentially the same question answered: physics.stackexchange.com/questions/423536/… – safesphere May 5 at 4:55
• @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. – safesphere May 5 at 5:00

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.