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Please help me reconcile what I see as contradicting theories:

  1. The universe began with the Big Bang and expanded from an infinitesimally small point.
  2. The universe is infinite.

How can something infinitely big "expand" from an infinitely small point? There has to be some sort of a 'blast wave', right?

Also, if we think we know the total amount of matter the Big Bang must have produced - how can a limited amount of matter fill an infinitely big universe?

P.S. One idea I have is that the universe may be infinite, but the Big Bang didn't create it - it created matter within the infinitely big universe. This would explain the expansion we're observing now: the matter will continue being distributed across the infinitely large universe. So I guess my question is more of a "why this assumption is incorrect"... :)

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    $\begingroup$ Where have you searched for an answer ? $\endgroup$
    – user34599
    Commented Aug 5, 2020 at 16:53
  • $\begingroup$ Google, naturally ;) No article I could find puts the two together: either they talk about Big Bang originating from a small point, or that the universe is infinite and why. Hence my question: how something originating from a small point become infinitely large? $\endgroup$
    – dniq
    Commented Aug 5, 2020 at 17:39
  • $\begingroup$ Related question on our sister site: physics.stackexchange.com/q/136860/123208 $\endgroup$
    – PM 2Ring
    Commented Aug 6, 2020 at 10:20
  • $\begingroup$ TLDR; Space-time doesn't give 2 s**ts about the speed of causality. $\endgroup$
    – Alastor
    Commented Mar 31 at 15:47

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The universe started with "big bang" is a slogan for "There was a time when the universe was hot and dense, and this time may be considered to be the start of the universe, and the start of time."

The universe has (as far as we know) always been infinitely large. That's not certain, but we have no evidence contradicting it. The universe was never an infinitesimally small point. It was always infinite at every time t>0.

We know how much matter the big bang has produced in the observable universe, and we can model this. (We don't really know why it produced this much matter, but we have enough parameters in our models to get a universe like ours).

We also think that the universe is kind of the same everywhere. Again, there is no proof of this, but nothing contradicts it, and so there would be an infinite amount of matter in the universe.

There was no "blast wave". Because the whole universe was hot and dense at the same time. The whole infinite universe, filled with an infinite amount of matter, which then expanded. (and yes, an infinite thing can expand, it just means that things within it get further apart.)

As for t=0 or for t<0 We have no idea. I don't know if those are even things that we can talk about. It seems that we can talk about the state of the universe at time t=1 second. But we can't talk about the state of the universe at t=0 seconds, at least, not with the mathematical and physical knowledge that we currently have. Time t=0 is a singularity.

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  • $\begingroup$ From exploratorium.edu/origins/cern/ideas/bang.html: "The universe began, scientists believe, with every speck of its energy jammed into a very tiny point. This extremely dense point exploded with unimaginable force, creating matter and propelling it outward to make the billions of galaxies of our vast universe. Astrophysicists dubbed this titanic explosion the Big Bang." $\endgroup$
    – dniq
    Commented Aug 5, 2020 at 17:30
  • $\begingroup$ Just to make it clear: I'm not an astronomer nor an astro-physicist. My interest in this is purely as a layman who gets confused by contradicting statements from different sources. $\endgroup$
    – dniq
    Commented Aug 5, 2020 at 17:32
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    $\begingroup$ @dniq: We all must live with analogies. These may be more or less easy to grasp. But really, so many sites discuss this. One misconcption in your question, though, is that the universe exploded into something. That is not the case, its expansion creates time and space, there is no inside and outside that our 3 dimensional surface dweller brains that developed in the steppe landscape demand to see. $\endgroup$
    – user34599
    Commented Aug 5, 2020 at 17:38
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    $\begingroup$ I think that exploratorium is a "lie to children". Lies to children are at the heart of all good teaching, but I'm not sure about this one. It repeats many of the misconceptions that are repeatedly aired here: that the big bang was an explosion in space. $\endgroup$
    – James K
    Commented Aug 5, 2020 at 17:41
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    $\begingroup$ To clear up one detail, we know how much matter is in our observable universe, but that amount of matter can't produce models which match what we see today. There seems to be missing mass, known as dark matter, needed to create a universe with galaxies we can see today. Dark matter and dark energy appear to be far more abundant than the "normal" matter and energy we can measure. Cosmology is a fascinating field! $\endgroup$
    – AstroShannon
    Commented Aug 5, 2020 at 18:51

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