Big bang occuring everywhere vs everything "compressed"

In summary, the concept of the "big bang occurring everywhere" suggests that the universe's expansion and creation of space happened uniformly throughout, rather than from a single point. In contrast, the idea of "everything compressed" implies that all matter and energy were once concentrated in a dense state before the big bang, leading to a subsequent explosion and expansion. Both perspectives explore the origins of the universe but highlight different aspects of how space and matter came into existence.
  • #1
Martyn Arthur
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TL;DR Summary
I am trying to reconcile the concept of the Big Bang of occurring "everywhere" (an infinite Universe) - v - the concept of the origin of the Big Bang occurring in a particular compressed state and then expanding?
Hi
I am trying to reconcile the concept of the Big Bang of occurring "everywhere" (an infinite Universe) - v - the concept of the origin of the Big Bang occurring in a particular compressed state and then expanding?
Thanks
Martyn Arthur
 
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  • #2
What needs reconciling? In an infinite universe the density is the same everywhere (ideally) and simply increases towards infinity as you run time backwards towards the initial singularity.
 
  • #3
It depends on whether you embed the Universe in more dimensions. Consider a balloon being expanded. If only the rubber surface exists, there is no dimension that includes a "center". That is the image that a bug on the rubber surface would have. If you want to include another dimension, as we see it, there is a center point. But in the currently favored physics of the Universe, there is no justification for adding dimensions that would include a "center".

CORRECTION: The above is only valid for a finite, curved Universe. Current theory is that the Universe is infinite and flat. (thanks, @PeterDonis )
 
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  • #5
Martyn Arthur said:
TL;DR Summary: I am trying to reconcile the concept of the Big Bang of occurring "everywhere" (an infinite Universe) - v - the concept of the origin of the Big Bang occurring in a particular compressed state and then expanding?

... the Big Bang of occurring "everywhere" (an infinite Universe) ...
Just to clarify that the Big Bang occurred everywhere regardless of the universe being infinite or not.
 
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  • #6
FactChecker said:
Consider a balloon being expanded.
This analogy only works for a universe that is spatially finite. Our best current model of our universe says it is spatially infinite and spatially flat. For such a universe there is no way of embedding it in a higher dimensional space such that there is a "center" in that higher dimensional space (and physically, we have no evidence of any such embedding anyway).
 
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  • #7
Thank you; I think that there is not a subject that I can deal with at my current level of "understanding" lack of "physics" understanding.
This requires me to progress to my next leveL of my degree and look at stuff from that context.
As such my question is fully satisfied!
Thanks to all as always!
Martyn Arthur
 
  • #8
Assume the Universe is infinite and isotropic (the same density everywhere). Then it has an infinite amount of mass. The density is then an infinite amount of mass divided by an infinite amount of space. No number can be calculated in this way. However we can measure the density directly. If the density decreases then we say the Universe is expanding.

The only way a finite universe can become infinite is to expand at an infinitely fast rate. This seems nonphysical. So if the Universe is infinite now then it was always so. No matter how dense the Universe may have been it was still infinite in extent.
 
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  • #9
Hornbein said:
The only way a finite universe can become infinite is to expand at an infinitely fast rate. This seems nonphysical.
I believe the same thing, however, I seem to understand that, in the link below, Alan Guth says that it is possible that the universe started out finite and became infinite. Isn't that what he says?

 
  • #10
Jaime Rudas said:
I believe the same thing, however, I seem to understand that, in the link below, Alan Guth says that it is possible that the universe started out finite and became infinite. Isn't that what he says?



He's saying that from the "outside" it's finite in space but on the inside appears to be infinite in space.

Such things are beyond my pay grade. But I would guess that on the inside -- where I suppose we are -- it always appears infinite.
 
  • #11
I believe the model Guth is describing in the video is the basic eternal inflation model that he also describes in Section 2.1 of this paper:

https://arxiv.org/abs/hep-th/0702178

The paper appears to show the individual "pocket universes" as spatially finite, but I'm not sure that's actually required by the model. I think we have had other PF threads on this general topic.

It is worth noting, also, that our current evidence does not rule out the possibility that our universe is spatially finite, just much larger than our observable universe. I believe some inflation proponents, at least, view inflation as predicting that we will eventually find that our universe is spatially finite.
 
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  • #12
PeterDonis said:
I believe some inflation proponents, at least, view inflation as predicting that we will eventually find that our universe is spatially finite.
Finite and closed, like textbook FLRW? Or finite with a boundary?
 
  • #13
Ibix said:
Finite and closed, like textbook FLRW? Or finite with a boundary?
It's hard to tell. Many discussions of inflation involve bubble nucleation, where a bubble of post-inflation spacetime forms and expands, which, at least to me, implies an expanding boundary between the bubble and the inflating region. However, I've also seen discussions that appear to be saying that an individual bubble is spatially closed, with 3-sphere topology and no boundary. I have not found a definitive description of a model that resolves this, but there is a lot of literature in this field that I have not read.
 
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  • #14
Thanks for continuing the dialogue; I hope my maths etc level gets to what you guys know. Cliff Richards is 82 and going strong, at my age 72 I so hope to get to your levels by 82.
thanks
martyn
 
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  • #15
PeterDonis said:
I believe some inflation proponents, at least, view inflation as predicting that we will eventually find that our universe is spatially finite.
I think the point here is that for the universe to be infinite requires spatial flatness before inflation. But that seems to be a matter of speculation, perhaps forever.
 
  • #16
timmdeeg said:
I think the point here is that for the universe to be infinite requires spatial flatness before inflation. But that seems to be a matter of speculation, perhaps forever.
Why couldn't it be hyperbolic?
 
  • #17
martinbn said:
Why couldn't it be hyperbolic?
As far as I understand it inflation flattens any curvature to a high extent.
 

FAQ: Big bang occuring everywhere vs everything "compressed"

Did the Big Bang happen at a single point in space?

No, the Big Bang did not happen at a single point in space. Instead, it occurred everywhere in the universe simultaneously. The Big Bang was an expansion of space itself, so every point in the universe was once condensed into a very hot, dense state.

How can the Big Bang occur everywhere at once?

The Big Bang represents the rapid expansion of space itself. At the moment of the Big Bang, all of space was concentrated in an extremely dense and hot state. As space expanded, it did so uniformly everywhere, meaning every point in the universe was part of this expansion.

What does it mean that the universe was "compressed" before the Big Bang?

Before the Big Bang, the universe was in an extremely hot and dense state, often referred to as a singularity. This means that all the matter and energy in the universe were compressed into a very small volume. The concept of "compressed" here refers to the high density and temperature of the universe at that time.

Is the center of the universe where the Big Bang happened?

There is no center of the universe in the context of the Big Bang. Because the Big Bang occurred everywhere in space simultaneously, every point in the universe can be considered as having originated from the Big Bang. The universe has no edge or center; it is uniformly expanding in all directions.

How does the expansion of the universe relate to the Big Bang happening everywhere?

The expansion of the universe is a direct consequence of the Big Bang. Since the Big Bang happened everywhere, the expansion is also happening everywhere. This means that galaxies are moving away from each other as space itself expands. This expansion can be observed through the redshift of light from distant galaxies.

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