B Is the Universe Truly Finite According to the Big Bang Theory?

[stuart100]

TL;DR Summary

Everything passed out of single point over a finite time into universe from Big Bang, etc. Is content
therefor finite?

My first question. I was taught in school about Big Bang as a theory of physics, so then if the entire content of our universe came out from one tiny point boundary over a finite span of time, would that
mean the content was finite?

 

[Ibix]

Although that's a common description of Big Bang theory, it's wildly incorrect, confusing multiple different concepts.

If the universe is infinite in extent, as our current models suggest, then it was always infinite in extent, and always fairly uniformly filled with matter and radiation (or more exotic things early on). The observable universe, the finite-sized bit we can see, was at some point very small. A lot of sources conflate "what we can see" with "everything".

 

[PeroK]

Summary:: Everything passed out of single point over a finite time into universe from Big Bang, etc. Is content
therefor finite?

My first question. I was taught in school about Big Bang as a theory of physics, so then if the entire content of our universe came out from one tiny point boundary over a finite span of time, would that
mean the content was finite?

Mathematically you cannot map a single point to anything other than a single point. The Big Bang theory, therefore, cannot start from a single point.

 

[Drakkith]

One thing to keep in mind is that the Big Bang is an extrapolation of what we can see now backwards using a specific set of theories and models. When we take General Relativity, use its rules to make a model, and run that model backwards, we find that at a certain point the math starts churning out infinities as the answers to the calculations. This point in the model, this place where the math 'breaks down', we call a singularity. It is from this mathematical singularity that the idea of a physical singularity is derived.

Note that if we take a finite volume of space, such as the observable universe, and watch as its volume shrinks as time is moved backwards in our model, the volume is still non-zero when we reach our singularity. So in the actual model used by cosmologists the universe is never a single point in size.

 

[PeterDonis]

the volume is still non-zero when we reach our singularity

This is somewhat misleading. The singularity itself is not part of the manifold at all, so there is no way to "reach" it. Anywhere that is part of the manifold, the volume will be non-zero (more precisely, it will be non-zero on any spacelike 3-surface that is part of the manifold).

 

[Drakkith]

This is somewhat misleading. The singularity itself is not part of the manifold at all, so there is no way to "reach" it. Anywhere that is part of the manifold, the volume will be non-zero (more precisely, it will be non-zero on any spacelike 3-surface that is part of the manifold).

So would it be better to say "as we approach the singularity" or something similar?

 

[PeterDonis]

would it be better to say "as we approach the singularity" or something similar?

Yes, or you could just say "on every spacelike 3-surface", since in this respect (the spatial volume of the 3-surfaces being nonzero) there is nothing special about approaching the singularity.

 

[stuart100]

One thing to keep in mind is that the Big Bang is an extrapolation of what we can see now backwards using a specific set of theories and models. When we take General Relativity, use its rules to make a model, and run that model backwards, we find that at a certain point the math starts churning out infinities as the answers to the calculations. This point in the model, this place where the math 'breaks down', we call a singularity. It is from this mathematical singularity that the idea of a physical singularity is derived.

Note that if we take a finite volume of space, such as the observable universe, and watch as its volume shrinks as time is moved backwards in our model, the volume is still non-zero when we reach our singularity. So in the actual model used by cosmologists the universe is never a single point in size.

I like how the math breaks down near infinities. I never liked them anyway. They could have gone deeper in year twelve to give us more hints. Thanks for that. Stuart100.

 

[phinds]

@stuart100 I recommend the link in my signature

 

[valenumr]

Try to reconcile this concept with the universe starting from a zero-dimensional point: in every direction we look, we see evidence of the "big bang".
This is hard to articulate I suppose, but if it started as a zero-dimensional point, it, I would think, would imply an actual center to the universe (not the observable universe). How could the center of something be visible in all possible directions?
As I understand (by no means an expert), the big bang happened "everywhere", and was an exponential increase of the scale factor of the universe (inflation theory?). The extent of "everywhere", is a bit if unknown, but is assumed to still be infinite, I believe. At least large enough that our observable universe is much, much smaller than the entire universe.

 

[weirdoguy]

but if it started as a zero-dimensional point

It didn't. At least there is no such thing in the big bang theory.

 

[valenumr]

It didn't. At least there is no such thing in the big bang theory.

Perhaps not formally, but I was trying to demonstrate that by accepting a pointlike beginning postulate creates a dilemma with observation. I did not say the universe actually has a point like beginning...

 

[PeroK]

Perhaps not formally, but I was trying to demonstrate that by accepting a pointlike beginning postulate creates a dilemma with observation. I did not say the universe actually has a point like beginning...

Exactly, otherwise the theme song to The Big Bang Theory, which starts "The whole universe was in a hot dense state ..." would instead start "The whole universe was in a zero point state ...".

 

[PeterDonis]

if it started as a zero-dimensional point, it, I would think, would imply an actual center to the universe

No, that's not the problem. The problem, as was already pointed out in this thread, is that you can't map a single point to anything other than a single point, at least not with the kind of mapping you need to use in a model in General Relativity.

But if it were possible to map a single point to a 3-space, which is what a model in which the universe started as a single point would have to do, there would still be nothing picking out any particular point in the 3-space as "the original point". The entire 3-space would correspond to the original point.

 

[valenumr]

Okay, fair enough. Let me try and illustrate what I mean though, and let's not be pedantic. I think there is a common notion that somehow a point somehow ballooned into a giant sphere that is the universe. I am just trying to illustrate that it is illogical to find yourself somewhere within that sphere, look in every direction, and see the "center", or "origin", without being extremely technical. The evidence for the big bang is observable in every direction. To me, that is sufficient to me to discredit the notion that the universe was ever point-like, given the above argument... Is that incorrect?

 

[Vanadium 50]

Is that incorrect?

Yes.

I think there is a common notion that somehow a point somehow ballooned into a giant sphere that is the universe.

That "common notion" is not what the Big Bang Cosmology says, so why discuss it?

 

[valenumr]

Yes.
That "common notion" is not what the Big Bang Cosmology says, so why discuss it?

Why is that analogy wrong? And maybe to illustrate why the concept of a point-like singularity doesn't even fit observation? See the OP's original question.

 

[PeterDonis]

Is that incorrect?

Yes.

Why is that analogy wrong? And maybe to illustrate why the concept of a point-like singularity doesn't even fit observation?

These questions have already been answered in previous posts.

See the OP's original question.

The OP's original question was already answered more than a month ago.

Thread closed.