Big Bang Singularity: Was It As "Infinitely Dense" As We Think?

In summary: 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.So a singularity would not exist in our universe?In summary, according to this conversation, the big bang supposedly originated from an infinitely dense singularity of space/time, which is an artifact of our current perception of how immense our universe is. If you were present in the reference frame of that 'infinitely dense' singularity, you would see your universe as consisting of the compressed space/time that existed at that moment. There is no evidence to suggest that the entire universe gets smaller or just gets denser while being of infinite
  • #1
Ratman101
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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
Tom Rathz said:
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
Tom Rathz said:
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
Tom Rathz said:
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
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
Chronos said:
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).
 
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  • #7
Agreed.,Mea culpa, trivialization could be interred, but, not intended.
 
  • #8
Chronos said:
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
Paul Giandomenico said:
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
Paul Giandomenico said:
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
PeterDonis said:
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
Vanadium 50 said:
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
Paul Giandomenico said:
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
Paul Giandomenico said:
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
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
kimbyd said:
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
haushofer said:
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
Grinkle said:
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
bahamagreen said:
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
phinds said:
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
bahamagreen said:
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
bahamagreen said:
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:

bahamagreen said:
- 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.

bahamagreen said:
- 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

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
Infinity is a concept, It is not a quantity - something incomprehensible unless you are insane, like Cantor.
 
  • #26
bahamagreen said:
There is much here I do not understand.

Me too. To me, your questions are quite natural to wonder about.

I can't make a mental model of something infinite. I can easily (and only) picture finite pieces getting denser and less dense.

Also, I can't make a mental model of something that is a universe (and therefore it is everything that exists), is of finite size, and is not embedded in something else. My mind insists on picturing it from the outside, embedded in something from which I have a mental vantage point.

I accept that because I can't make visual images of such things does not imply that such things cannot be. Still, its very natural, imo, to wonder how they can possibly "look", and struggle with the concepts at that visualization level.
 
  • #27
Grinkle said:
I can't make a mental model of something infinite. I can easily (and only) picture finite pieces getting denser and less dense.
.
I think no one can‘t. One can accept that a plane has not necessarily a boundary. But I don‘t see why one should have a mental model of such a plane as a whole.
 
  • #28
bahamagreen said:
"taken as a whole"... or is that the operative concept that does not apply to a spatially infinite universe?

Yes.
 
  • #29
Grinkle said:
I can't make a mental model of something infinite.

Sure you can. You just can't visualize it mentally. But it's perfectly possible to make a consistent mathematical description of something infinite, like a spatially infinite universe, and that's a mental model.
 
  • #30
PeterDonis said:
You just can't visualize it mentally.

Yes, this is what I meant.

I agree that a mathematical model is as valid an example of a mental model as a visualization.
 
  • #31
Is there such a thing as pure, formless, energy ? If there is, could that have been what existed before the Big Bang ?
 
  • #32
Shane Kennedy said:
Is there such a thing as pure, formless, energy ?
No. Energy is a property, not something concrete that can exist by itself.
If there is, could that have been what existed before the Big Bang ?
No, because again, such a thing doesn't exist.
 
  • #33
phinds said:
No. Energy is a property, not something concrete that can exist by itself. No, because again, such a thing doesn't exist.
According to current theory.

I suggest that mass is a property too.
 
  • #34
Shane Kennedy said:
According to current theory.

As well as all current evidence.

Shane Kennedy said:
I suggest that mass is a property too.

For some definitions of the term "mass", yes, this is correct.

What does any of this have to do with the topic of this thread?
 
  • #35
PeterDonis said:
As well as all current evidence.
For some definitions of the term "mass", yes, this is correct.

What does any of this have to do with the topic of this thread?
What could have "seeded" the Big Bang
 
<h2>What is the Big Bang Singularity?</h2><p>The Big Bang Singularity refers to the initial state of the universe, where all matter and energy were concentrated in an infinitely dense and hot point. This is believed to be the starting point of the universe and the beginning of the expansion of space and time.</p><h2>Was the Big Bang Singularity actually infinitely dense?</h2><p>The term "infinitely dense" is used to describe the singularity because our current understanding of physics breaks down at this point. It is not possible to accurately measure the density of the singularity, as the laws of physics as we know them do not apply. However, it is believed that the singularity was incredibly dense, but not necessarily infinitely so.</p><h2>What evidence supports the existence of the Big Bang Singularity?</h2><p>There are several pieces of evidence that support the existence of the Big Bang Singularity. One of the main pieces of evidence is the cosmic microwave background radiation, which is leftover radiation from the early stages of the universe. Additionally, the observed expansion of the universe and the abundance of light elements also support the idea of a singularity at the beginning of the universe.</p><h2>What happened at the moment of the Big Bang Singularity?</h2><p>At the moment of the Big Bang Singularity, the universe is believed to have undergone a rapid expansion known as inflation. This expansion caused the universe to grow exponentially in size and eventually cooled down enough for particles to form. This led to the formation of the first atoms, and eventually, the formation of stars and galaxies.</p><h2>Is the Big Bang Singularity the only theory for the beginning of the universe?</h2><p>The Big Bang Singularity is currently the most widely accepted theory for the beginning of the universe. However, there are other theories, such as the Steady State theory and the Oscillating Universe theory, that propose alternative explanations. These theories are still being studied and debated among scientists.</p>

What is the Big Bang Singularity?

The Big Bang Singularity refers to the initial state of the universe, where all matter and energy were concentrated in an infinitely dense and hot point. This is believed to be the starting point of the universe and the beginning of the expansion of space and time.

Was the Big Bang Singularity actually infinitely dense?

The term "infinitely dense" is used to describe the singularity because our current understanding of physics breaks down at this point. It is not possible to accurately measure the density of the singularity, as the laws of physics as we know them do not apply. However, it is believed that the singularity was incredibly dense, but not necessarily infinitely so.

What evidence supports the existence of the Big Bang Singularity?

There are several pieces of evidence that support the existence of the Big Bang Singularity. One of the main pieces of evidence is the cosmic microwave background radiation, which is leftover radiation from the early stages of the universe. Additionally, the observed expansion of the universe and the abundance of light elements also support the idea of a singularity at the beginning of the universe.

What happened at the moment of the Big Bang Singularity?

At the moment of the Big Bang Singularity, the universe is believed to have undergone a rapid expansion known as inflation. This expansion caused the universe to grow exponentially in size and eventually cooled down enough for particles to form. This led to the formation of the first atoms, and eventually, the formation of stars and galaxies.

Is the Big Bang Singularity the only theory for the beginning of the universe?

The Big Bang Singularity is currently the most widely accepted theory for the beginning of the universe. However, there are other theories, such as the Steady State theory and the Oscillating Universe theory, that propose alternative explanations. These theories are still being studied and debated among scientists.

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