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Could it be that the Universe is truly infinite, and the Big Bang was merely a singular, specific event that resulted in the birth of a 'bubble,' i.e. what we perceive as 'the observable universe'? Alternatively, is there a widely accepted astronomical fact that heavily contradicts this hypothesis and disproves it?

Throughout the history of science, we've discovered that humans and our world/planet are not the focal point of everything (e.g. realizing that we are not the center of our solar system). Could it also be plausible that our Universe is just a tiny fraction of the boundless reality, where 'Big Bangs' occur frequently, continually generating new segments of existence? An infinite wellspring of energy and matter.

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    $\begingroup$ If there were a "widely accepted astronomical fact that heavily contradicts the big bang theory", then it would not be the widely accepted theory for the description of the universe. Of course it's clear we haven't understood everything, and a big overhaul like Einstein's general relativity 100 years ago may occur - maybe needs to occur - to explain the parts we don't understand... e.g.: what's dark matter? what's dark energy? $\endgroup$
    – planetmaker
    Commented Jan 4 at 11:23
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    $\begingroup$ Regarding your opening paragraph: It is not only possible, but probable. Empty infinite space must have always existed. Occam's razor aggressively supports this hypothesis. $\endgroup$
    – White Prime
    Commented Jan 4 at 13:20
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    $\begingroup$ @WhitePrime The stretching of space itself is core to our understanding of cosmology. If what we understand as space is expanding, then the big bang cannot have been matter exploding into a preexisting space -- at least not space as we know it. There can't have been infinite empty space before space came into being and expanded out from a point. Occam's razor only applies if there are multiple possibilities that all explain the evidence equally well; and in this case "matter came from one point into an infinite empty universe" doesn't explain the evidence. $\endgroup$
    – Darth Pseudonym
    Commented Jan 4 at 14:48
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    $\begingroup$ Note that it is possible that the Big Bang occurred within a larger (perhaps infinite) universe and that the "bubble" produced by the Big Bang is itself spatially infinite. $\endgroup$
    – Sten
    Commented Jan 4 at 21:57

2 Answers 2

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Could it be that the Universe is truly infinite

It looks like it is infinite in space in all directions, and infinite in time in one direction. Going back in time one reaches a singularity.

It is possible that the (apparent) singularity exists only in our models, and it is possible that time extends infinitely backwards. There are philosophical problems with this, namely what was the universe doing for an infinitely long time, and why, after an infinite period of time did it change?

Bubble universes are possible, perhaps a "bubbles of "True vacuum" in a universe that is elsewhere inflating (and has always been inflating). The problem with all this is that such a theory makes no predictions, and as such is untestable. Any other bubble universes would be beyond a cosmic horizon, and could have no effect on us. It's possible to model such universes, but it's not clear if these models relate to reality in any way, or if there is any way to investigate their truth.

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You're asking about something that's beyond the realm of well-understood physics, so a lot of it is speculation. However, people work on the idea, so it's not impossible. The key words you want to look for are eternal inflation and multiverse.

The gist is that in our universe, shortly after the Big Bang, there was a phase where the universe expanded at an exponential rate (known as "inflation").* It turns out that one way to model inflation is that the true universe is always inflating, and our universe is just a bubble within that eternally-inflating universe. Eternal inflation has some fun consequences, e.g. if it's true, then there are multiple universes out there each with their own Big Bang and laws of physics. The universe is probably not stable as a result (but don't start buying insurance, because if the universe really is not stable, then when the false vacuum decay happens, the insurance company will also cease to exist).

If eternal inflation turns out to be true, then the answer to your question is 'yes'. The problem of course is that eternal inflation itself is a speculative model which is very far from being experimentally verified. Some people even argue it's not science since it doesn't make verifiable predictions. Other people think it's still worth working on, because if string theory turns out to be true, then string theory could naturally lead to eternal inflation. People who take this view argue that in the same way that we trust General Relativity to predict what happens inside black holes (even though we cannot observe that directly), we should also trust eternal inflation if it's predicted by string theory. Yet other people pour cold water on that, since in this picture we are one of the estimated $10^{500}$ universes that is predicted in string theory, and if there actually are $10^{500}$ universes then almost anything is possible and the theory loses predictive power (this number is many hundreds of orders of magnitude larger than the number of atoms in the observable universe).

Finally, if your question is not whether it happened but whether it's plausible, then the question is unambiguously 'yes', since eternal inflation hasn't been rejected yet - but again, we are talking about things that's beyond the realm of well-understood physics, and a lot of it is speculation.

*It's worth pointing out that there's no compelling evidence for inflation yet. Many cosmologists believe it's true, but the evidence is not at the level where it is compelling.

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  • $\begingroup$ A very nicely worded answer. But, technically did not answer the question. Eternal Inflation requires one Big Bang and QP asked about multiple Big Bangs. This occurs in other speculative theories. $\endgroup$
    – eshaya
    Commented Jan 5 at 14:36
  • $\begingroup$ @eshaya I'm pretty sure that in eternal inflation, there are multiple universes, each of which began with their own Big Bang. $\endgroup$
    – Allure
    Commented Jan 5 at 15:11
  • $\begingroup$ In the sense of "the universe expands from a hot dense phase", the Big Bang is after inflation (and so each bubble would indeed be said to undergo its own Big Bang). $\endgroup$
    – Sten
    Commented Jan 5 at 18:57
  • $\begingroup$ This is a semantics discussion now. Each sub-universe would be Big Bang like, but if inflation follows a Big Bang, it would be confusing to call each patch of inflation the same thing as the singularity Big Bang. And further confusing theories where multiple Big Bangs occur in a higher dimensional metaverse, forming universes each with different physical constants. The Eternal Inflation has many disconnected sections arising from the same singularity event. $\endgroup$
    – eshaya
    Commented Jan 6 at 16:22
  • $\begingroup$ It was semantics from the outset. But I find it problematic to define "Big Bang" as a singularity. What if there was no singularity? $\endgroup$
    – Sten
    Commented Jan 6 at 23:26

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