Big Bang "singularity"

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The big bang supposedly originated from an infinitely dense singularity of space/time. The concept of infinitely dense is an artifact from our current perception of how immense our universe is today and how spread-out space/time is. However, if I were present in the reference frame of that 'infinitely dense' singularity, would I not possibly see my 'universe' as consisting of the compressed space/time that existed at that moment? All things being relative that singularity might have been huge in that reference frame.
 

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  • #2
Grinkle
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an infinitely dense singularity
Dense does not mean small, it just means dense. As you roll back the clock, the universe gets hotter and denser.

The observable universe gets smaller.

Whether the entire universe gets smaller or just gets denser while being of infinite extent is a question that expansion / inflation theories (as far as I know or have seen discussed here) don't address.

I haven't seen anyone here argue against the idea that the entire universe is either finite or infinite, and whichever it is today, it has always been that way.

So no one argues for an idea that the universe started as a very dense but finite point and then later expanded to something that became of infinite extent.
 
  • #3
kimbyd
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The big bang supposedly originated from an infinitely dense singularity of space/time. The concept of infinitely dense is an artifact from our current perception of how immense our universe is today and how spread-out space/time is. However, if I were present in the reference frame of that 'infinitely dense' singularity, would I not possibly see my 'universe' as consisting of the compressed space/time that existed at that moment? All things being relative that singularity might have been huge in that reference frame.
The singularity is an artifact of General Relativity, which doesn't take quantum mechanics into account. The correct quantum theory of gravity (which we do not yet know) cannot have a singularity.
 
  • #4
phinds
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The big bang supposedly originated from an infinitely dense singularity of space/time.
No, the Big Bang Theory says that at one point in TIME (not space) in the distant past (about 13.7 Billion years ago) the universe was a hot dense plasma and it expanded from there to where we are now. The BB Theory is silent as to any origin story. The word "singularity" is just a place-holder for the phrase "if you extend the math back to t=0 you get a non-physical result which means we know that the math model is invalid in that domain and we call the place where our knowledge breaks down a singularity"
 
  • #5
Chronos
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It would probably be unwise to dismiss singularities as merely an artifact, or mathematical curiosity. Misner is claimed to have said 'the singularities in solutions to EFE are a source from which we can derive much valuable understanding of cosmology'. Which likely came from his half century old paper "The Absolute Zero of Time", https://journals.aps.org/pr/abstract/10.1103/PhysRev.186.1328. I was unable to unearth a free version of the original paper, but, this snippet should be sufficient to convey the gist of it. Misner went on to flesh out 'misner space' as a hint of the properties that might be reasonably expected of a pre BB space time. The idea still pops up from time to time in the literature as evidenced by papers such as https://arxiv.org/abs/1406.4552, Topology of the Misner Space and its g-boundary and https://arxiv.org/abs/1710.06317, Bouncing and emergent cosmologies from ADM RG flows. It has also served as a major inspiration for the popular big bounce model. Misner's quip that singularities should be taken seriously has left its mark.
 
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  • #6
PeterDonis
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It would probably be unwise to dismiss singularities as merely an artifact, or mathematical curiosity.
Just to be clear, I don't think the viewpoint @kimbyd and @phinds were describing says that singularities are "merely" artifacts or mathematical curiosities. It just says that the presence of singularities in particular solutions of the EFE indicates that those solutions cannot be complete descriptions as they stand. Either they will end up being effective descriptions on top of some more fundamental theory (such as a theory of quantum gravity), or the interpretation of what those solutions are describing will need to be modified (which seems to be the approach Misner is taking in the 1969 paper).
 
  • #7
Chronos
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Agreed.,Mea culpa, trivialization could be interred, but, not intended.
 
  • #8
phinds
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Agreed.,Mea culpa, trivialization could be interred, but, not intended.
Glad to hear you are burying the trivialization o0)

(I know ... damn type-ahead)
 
  • #9
Might be helpful to think of a singularity as not existing in our universe, rather a pre-universe phenomena?
 
  • #10
PeterDonis
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Might be helpful to think of a singularity as not existing in our universe, rather a pre-universe phenomena?
What would "pre-universe" mean?
 
  • #11
Vanadium 50
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Might be helpful to think of a singularity as not existing in our universe, rather a pre-universe phenomena?
Also, a singularity is not an object.

Looking at your recent posts, you might want to take a look at the rules on personal theories.
 
  • #12
What would "pre-universe" mean?
Good question. But seriously if the known universe existed solely in the form of a singularity and that singularity had a S=0 point in time then a "pre-universe' would be an S-minus timeline.
 
  • #13
Also, a singularity is not an object.

Looking at your recent posts, you might want to take a look at the rules on personal theories.
Hence the term "pre" which was referring to a point on a time line?
 
  • #14
Orodruin
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Good question. But seriously if the known universe existed solely in the form of a singularity and that singularity had a S=0 point in time then a "pre-universe' would be an S-minus timeline.
I strongly suggest that you stop speculating based on what you have read in and understood from popular literature. It is not a good proxy for understanding the real science.
 
  • #15
PeterDonis
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if the known universe existed solely in the form of a singularity
It didn't. Please take some time to look at a cosmology textbook and learn what our best current model actually says.
 
  • #16
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Singularity is not a physical state of something.
It refers to conditions that cannot be described mathematically by our current best theories.
Attempting to do so produces unreasonable infinities.
 
  • #17
haushofer
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The singularity is an artifact of General Relativity, which doesn't take quantum mechanics into account. The correct quantum theory of gravity (which we do not yet know) cannot have a singularity.
Well, from an EFT point of view, we expect that. But I don't know any law which forbids singularities in quantum gravity.
 
  • #18
Grinkle
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I don't know any law which forbids singularities in quantum gravity.
My own hope is that a quantum gravity theory won't have singularities. If it does, we will need a new buzz-word for the next family of theories to explain whatever mysteries lie behind the singularities in the otherwise successful new quantum gravity theory.
 
  • #19
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Whether the entire universe gets smaller or just gets denser while being of infinite extent is a question that expansion / inflation theories (as far as I know or have seen discussed here) don't address.

I haven't seen anyone here argue against the idea that the entire universe is either finite or infinite, and whichever it is today, it has always been that way.

So no one argues for an idea that the universe started as a very dense but finite point and then later expanded to something that became of infinite extent.
There is much here I do not understand. I think no one argues for an idea that the universe started as a very dense but finite point and then later expanded to something that became of infinite extent because a non-infinite extent (including an originating point) cannot become an infinite extent... and an infinite extent cannot become a non-infinite extent.

But what I don't understand is this; if that is true, why does that same line of thinking not seem to impair the suggestions that:

- a universe of infinite extent may become larger or smaller?
This seems counter to the definition and distinction between infinite and non-infinite, suggesting an infinite extent may be subject to changing size.

- a universe of infinite extent may become more or less dense?
This is more subtle, but any measure of density must be an average for some volume, so a density of the universe might be approximated by using arbitrarily large volumes... but the point of the question is that taking as given some nominal value... changes in this density value in a universe of infinite extent would mean that either the numerator or the denominator (or both) were changing... but change in the denominator would be assuming a change in the size of an infinite extent, and change in the numerator would be assuming a change in the "total mass-energy?" or whatever is the substance of the density measure, which means some of it is appearing or disappearing from or into...where?, in an infinite extent?

I can't be the only one who has wondered about this...
 
  • #20
phinds
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a universe of infinite extent may become larger or smaller
That is a correct statement. In essence it's infinity + 1 = infinity and infinity - 1 = infinity.

Google Hilbert Hotel.
 
  • #21
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That is a correct statement. In essence it's infinity + 1 = infinity and infinity - 1 = infinity.

Google Hilbert Hotel.
How so?
If infinity +/- 1 = infinity, that clearly indicates that infinity does not change size, therefore a universe of infinite size does not become larger or smaller.
Hilbert's hotel is infinite, and it certainly does not change size, does it?
 
  • #22
phinds
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How so?
If infinity +/- 1 = infinity, that clearly indicates that infinity does not change size, therefore a universe of infinite size does not become larger or smaller.
Hilbert's hotel is infinite, and it certainly does not change size, does it?
"size" is a vague term in this context. "Things get farther apart" in an infinite universe, is a better way to say it. Translated into English vernacular, this sounds a lot like "an infinite universe increases in size".
 
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  • #23
PeterDonis
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I can't be the only one who has wondered about this...
You aren't; there are plenty of previous PF threads on this same topic. But to take your two specific issues:

- a universe of infinite extent may become larger or smaller?
It can't and it doesn't. More precisely, the concepts "larger" and "smaller" as you are intuitively using them here do not apply to a spatially infinite universe.

- a universe of infinite extent may become more or less dense?
The concept of "density" also does not apply to a spatially infinite universe, taken as a whole. However, it does apply to any finite volume of such a universe, and the statement that "the density of the universe is decreasing with time" just means that the amount of matter in any given finite volume of space in the universe decreases with time according to any observer inside that volume. Nothing about a spatially infinite universe prevents this from working.
 
  • #24
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Thanks

larger or smaller... ("no") - OK, that's what I was thinking.

more or less dense... ("yes") - I can see it for "any given volume", but not so well for "all large enough volumes", which is the cosmological assumption... how can it happen "everywhere", meaning "taken as a whole"... or is that the operative concept that does not apply to a spatially infinite universe?
 
  • #25
Chronos
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Infinity is a concept, It is not a quantity - something incomprehensible unless you are insane, like Cantor.
 

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