Does the Big Bang model rule out an eternal universe?

[PeterDonis]

So, the big bang was the beginning of the observable universe, not the entire universe.

No, that's not correct, for two reasons.

First, the big bang was not the beginning; we don't know for sure what the beginning was, or even if there was one. The big bang is just the earliest state for which we have good evidence.

Second, the big bang involved the entire universe, not just the observable universe.

isn't 'space' the universe.

No. The universe is a four-dimensional spacetime, not a three-dimensional space.

are you saying that there was a universe without 'space' before the big bang and the big bang is where matter and space suddenly appeared and expanded rapidly inside that already existing universe without 'space'?

No. See above.

Is this the 'multiverse' you are referring to?

No. The "multiverse" is one hypothesis for what preceded (and caused) the big bang, but it is not the only one, and our current best model leaves the question open of what came before the big bang.

I think the general public have no clue about this big bang stuff.

It's quite possible that many people don't, yes.

 

[Varsha Verma]

No, that's not correct, for two reasons.

First, the big bang was not the beginning; we don't know for sure what the beginning was, or even if there was one. The big bang is just the earliest state for which we have good evidence.

Second, the big bang involved the entire universe, not just the observable universe.
No. The universe is a four-dimensional spacetime, not a three-dimensional space.
No. See above.
No. The "multiverse" is one hypothesis for what preceded (and caused) the big bang, but it is not the only one, and our current best model leaves the question open of what came before the big bang.
It's quite possible that many people don't, yes.

We know that 'space' is a thing. We live in it.

So, are you saying that the 'spacetime' is also a 'thing', a real 'thing'?

I thought 'spacetime' was just a mathematical concept which was used to explain GR.

Here is what's very difficult to comprehend even in layman terms. You say that that the big bang is the 'earliest state' of the 'entire universe', not just the observable universe. But you also say that 'space' was occupying a very small volume. You also say that the universe is not 'space'. So like, you are saying that the 'time' part was infinite at the 'big bang' or what exactly are you saying?

 

[rootone]

Things (like us) exist in time and space.
Time certainly is a dimension, GR found a way to describe how time has a physical relationship with the space dimensions.

 

[timmdeeg]

@Varsha Verma Perhaps you didn't get the point if we make a distinction between universe and observable universe. In this case the following might help:

Imagine a balloon with a tiny circle on its surface with us in the center. The circle marks our observable universe, we can't see any further. The total surface of the balloon "is" the universe. Now let the balloon shrink which means go back in time until the big bang era is reached. Then the balloon is tiny and the circle - the observable universe - is a tiny part of it. This is the point in time where the space - the universe and the observable part of it - will expand. The balloon analogy describes a spatially finite universe, which isn't really ruled out by the data, as @bapowell has pointed out in #19.

You can transfer this reasoning to a spatially infinite universe by simply not imagining a balloon but a infinite rubber sheet instead. When the universe started with the big bang the rubber sheet was infinite with a tiny circle on it, the observable universe. Then the infinite universe and the tiny observable part of it expanded to the size it has today. Note that the size of the universe then, now and in future is infinite in this case. "Infinite" is not a normal number, you can't say compared to the big bang time the universe now is so an so many times larger. You can say this for finite quantities like the observable universe.

 

[Varsha Verma]

@Varsha Verma Perhaps you didn't get the point if we make a distinction between universe and observable universe. In this case the following might help:

Imagine a balloon with a tiny circle on its surface with us in the center. The circle marks our observable universe, we can't see any further. The total surface of the balloon "is" the universe. Now let the balloon shrink which means go back in time until the big bang era is reached. Then the balloon is tiny and the circle - the observable universe - is a tiny part of it. This is the point in time where the space - the universe and the observable part of it - will expand. The balloon analogy describes a spatially finite universe, which isn't really ruled out by the data, as @bapowell has pointed out in #19.

You can transfer this reasoning to a spatial infinite universe by simply not imagining a balloon but a infinite rubber sheet instead. When the universe started with the big bang the rubber sheet was infinite with a tiny circle on it, the observable universe. Then the infinite universe and the tiny observable part of it expanded to the size it has today. Note that the size of the universe then, now and in future is infinite. "Infinite" is not a normal number, you can't say compared to the big bang time the universe now is so an so many times larger. You can say this for finite quantities like the observable universe.

Ok, this rubber sheet is a good analogy.

So, the universe was somehow (we don't know how) infinite at the moment of the big bang, the earliest observable state.

So, I suppose beyond the 'observable universe' there is no 'space'. It is some other stuff like exotic 'fields' or exotic matter. Is that what your saying?

 

[Ibix]

Ok, this rubber sheet is a good analogy.

It's not a bad analogy, but it has a lot of shortcomings. Don't assume you understand anything with it - that way lies an awful lot of silliness.

So, I suppose beyond the 'observable universe' is not space. Some other stuff like exotic 'fields'. Is that what your saying?

Beyond the observable universe is more of the same, we expect. Stars, galaxies, etc. That's what the cosmological principle says - on large scales, everything is the same everywhere.

 

[Varsha Verma]

It's not a bad analogy, but it has a lot of shortcomings. Don't assume you understand anything with it - that way lies an awful lot of silliness.
Beyond the observable universe is more of the same, we expect. Stars, galaxies, etc. That's what the cosmological principle says - on large scales, everything is the same everywhere.

Let me ask you this since you seem to know this stuff.

So you saying that if we take a spaceship which can travel at trillion times the speed of light (lets assume ok), the more you go out to the universe, the more galaxies you will encounter, right?

So, that means you encounter more 'space', right??

So, you are saying that the universe is just more and more space.

But the difficulty for us to understand is this: If the entire universe is just 'space' then is it the same space the point like thing occupied during at the big bang?
If not, then is it only the space of the observable universe that was compacted or there at the point of the big bang?

 

[Ibix]

So, you are saying that the universe is just more and more space.

It depends what you mean by "universe" really.

General relativity models everything as one four-dimensional whole called spacetime. You can imagine slicing that into a collection of three-dimensional things, each of which you would call "all of space at a given time". A simpler analogy - imagine drawing all the galaxies on a piece of paper. Then draw them again slightly further apart on another sheet, then again slightly further apart on another sheet. Stack them up. Each sheet of paper is "all of space at a given time" (and might need to be infinitely sized, or cover the whole surface of the Earth so it's actually a spherical sheet). The whole stack (or onion, I suppose, in the spherical version) is spacetime.

Which do you mean by "the universe"? A sheet? Or the stack? I don't know that there's a standard usage, which is why scientists use space and spacetime. Either way, in this model, "all of space now" is not the same as "all of space at the Big Bang" - they're different parts of spacetime (different sheets of paper in the stack-of-paper analogy).

If not, then is it only the space of the observable universe that was compacted or there at the point of the big bang?

Everything was closer together at the time of the Big Bang, but if the universe is infinite in extent then it always was. It was just more dense. The observable universe does definitely have a finite extent. So saying "everything was closer together" does imply that the observable universe was smaller.

 

[Varsha Verma]

Is it possible to create a 3-D model of the universe at the big bang?

Or is it not possible because of this 'spacetime' thing...

Spacetime is space with a time value to it, right??
Or is it something completely different thing?

 

[timmdeeg]

But the difficulty for us to understand is this: If the entire universe is just 'space' then is it the same space the point like thing occupied during at the big bang?
If not, then is it only the space of the observable universe that was compacted or there at the point of the big bang?

If the universe is infinite then the space is infinite at the big bang and at all times. Whereas an observable part of the universe isn't point like but very tiny at the big bang. In this case the entire universe consists of infinite observable universes, as you can imagine infinite circles on the infinite rubber sheet. One of those observable universes is ours. There may be others where the same discussion happens.

 

[Varsha Verma]

If the universe is infinite then the space is infinite at the big bang and at all times. Whereas an observable part of the universe isn't point like but very tiny at the big bang. In this case the entire universe consists of infinite observable universes, as you can imagine infinite circles on the infinite rubber sheet. One of those observable universes is ours. There may be others where the same discussion happens.

So this is the 'multiverse' you are describing right??

Also, why he "if"?

Because isn't the current model of the universe spatially infinite?? So, why use 'if'??

Does not this mean space was infinite at the big bang also??

If space was infinite at the big bang then even I can understand the stuff.

 

[Dale]

We know that 'space' is a thing. We live in it.

We don’t live in space, we live in spacetime. We have length, width, height, and duration.

Space is a convention regarding how we partition spacetime into space and time. The Big Bang Model makes the assumption that there is a way to partition spacetime into space and time such that at every moment in time the corresponding space is homogeneous, it is the same everywhere (at large scales).

Does not this mean space was infinite at the big bang also??

According to our best estimates, yes. Note that our best estimates do not go all the way back to a singularity, as is often portrayed. That is an extrapolation beyond our best estimates.

 

[Varsha Verma]

We don’t live in space, we live in spacetime. We have length, width, height, and duration.

Space is a convention regarding how we partition spacetime into space and time. The Big Bang Model makes the assumption that there is a way to partition spacetime into space and time such that at every moment in time the corresponding space is homogeneous, it is the same everywhere (at large scales).

According to our best estimates, yes. Note that our best estimates do not go all the way back to a singularity, as is often portrayed. That is an extrapolation beyond our best estimates.

Can you confirm this please. Because this will make my understanding of this big bang subject much clear.

That is, 'space' was 'infinite' at the Big Bang also?

This is what was confusing me all along.

If you scientists say that 'space' was also infinite at the big bang, then even lay people like us can kind of understand it much better.

Just look at any video on Youtube. Almost every on them say that space was created by the big bang. So, that is what is confusing understanding this.

 

[Dale]

Can you confirm this please. Because this will make my understanding of this big bang subject much clear.

That is, 'space' was 'infinite' at the Big Bang also?

Yes, but there are a couple of caveats. First, that is according to our best current model, which does not include a singularity as is often portrayed. Second, the data is right on the edge of a critical point in the model, so future data could easily change the best model from infinite to finite.

Just look at any video on Youtube. Almost every on them say that space was created by the big bang.

This is why the standard on this forum is the scientific literature, not you tube videos

 

[bapowell]

Wouldn't assuming a closed universe contradict the evidence that it expands accelerated which supports a open universe?

No. Why do you think this?

 

[phinds]

Just look at any video on Youtube. Almost every on them say that space was created by the big bang. So, that is what is confusing understanding this.

Varsha, in this and in other posts, you have made it clear that you are getting your "physics" from pop-science and consequently misunderstand almost everything . I empathize with this greatly because that's exactly the way I started out some years back. The problem is that, as you point out in this instance, they ALL get it wrong. They are completely consistent which makes you think they must have it right since they all say the same thing. As you are learning in your posts here, pop science presentations actually have most of it completely wrong (or at best, they state things in very misleading ways).

As has been pointed out, if you want to actually learn science, you have to avoid the pop-science presentations. The TV shows and videos in particular are enjoyable to watch (for simpletons like me anyway) because they have great pictures and graphics and are easy to understand. They just don't represent actual science and you need to get clear on that.

 

[timmdeeg]

So this is the 'multiverse' you are describing right??

No.

Also, why he "if"?

Because the data don't disprove the possibility that the universe is finite, see posts #19 and #64.

Because isn't the current model of the universe spatially infinite?? So, why use 'if'??

See said posts.

Does not this mean space was infinite at the big bang also??

Yes, if the universe is spatially infinite now, it can't be finite at earlier times including the big bang.

If space was infinite at the big bang then even I can understand the stuff.

OK

 

[Dale]

The TV shows and videos in particular are enjoyable to watch

Exactly right. They are entertainment, not education. The pop sci books are also, but not quite to the extreme as the videos

 

[timmdeeg]

No. Why do you think this?

I thought a universe which expands accelerated and thus eternally should be open. Unfortunately only after @PeterDonis corrected that in #38 I remembered Peacock stating in "Cosmological Physics" page 38: "If $\Omega_m > 1$, recollaps is only avoided if $\Omega_v$ exceeds a critical value." According to equation (3.55). I should have known it. And I will look at the Friedmann equations, they should reveal that.

 

[PeterDonis]

We know that 'space' is a thing. We live in it.

Not according to relativity. In relativity, "space" depends on your choice of coordinates.

Btw, do you remember that I said, back in post #34, that you need to be more careful of the claims you make? This is an example. The statement quoted above looks obvious to you--but it's false. Lots of other things that look obvious to you are also false. So you need to stop assuming that anything that looks obvious to you must be true, and stating it as such.

So, are you saying that the 'spacetime' is also a 'thing', a real 'thing'?

No, I'm saying that, in relativity, spacetime is the "real thing", the thing that plays the same logical role that "space" does in pre-relativity physics.

I thought 'spacetime' was just a mathematical concept which was used to explain GR.

Then that's another thing you thought was true that is false.

You say that that the big bang is the 'earliest state' of the 'entire universe', not just the observable universe.

The earliest state for which we have good evidence. Not the earliest state period; as I have already said, we don't know what the "earliest state" was or even if there was one.

But you also say that 'space' was occupying a very small volume.

No, that's not what I said. I did say that something was occupying a very small volume at the big bang, but that thing was not "space". Go back and read my previous posts again, carefully. I have already stated the key distinction you are missing here several times. I'm not going to keep repeating it. You need to read more carefully.

 

[PeterDonis]

the big bang, the earliest observable state

The earliest state we currently have good evidence for. But our ability to collect evidence could improve in the future; we could get evidence for states earlier than the one we currently call the "big bang".

 

[Varsha Verma]

In order to complete the picture I need to know this.

So, at t=0 of the Big Bang, not T=0, that is the start of the universe, we have
(1.) Infinite space - We don't know how this space came to be. Maybe it was 'created' or was there all along.
(2.) Very dense hot point the size of a atom. Since it is 'dense' and 'hot' that means it is 'matter'. Since there is already 'space', this dense point of 'matter' the size of an atom resides inside 'space'.

So now the big bang or more correctly the big expansion occurs.
At ^{10 -33} after t=0 inflation starts and expands that atom size dense point of 'matter' to ^{10 26} times the volume at ^{10 -33} . So now that 'point' of dense hot point of 'matter' is humongous.

At ^{10 -6} protons form and then normal expansion continuous.

What I want to know is, that point like dense hot 'thing' was not 'space', right?? It was 'matter' right?

So, what happened is that a very dense hot point like bit of 'matter' expanded rapidly inside the 'space' (infinite 'space') and this is what we call the 'observable universe' now.

Did I get it right?

PS: I think now that not only the point size dense hot point of matter, but the entire universe started expanding and inflating?

 

[PeterDonis]

In order to complete the picture I need to know this.

Stop looking at pop science articles. The picture you gave does not give times according to our actual best current model. It gives times according to an idealized model that nobody actually uses, which are correlated to the actual best current model by using temperature values. In short, the picture you are using is misleading you. That's what comes of not using valid sources to learn science.

To point out just one big difference: the term "big bang", in our actual best current model, refers to the event marked as "cosmic inflation ends" in the picture. The white dot at the far left of the line in the picture does not correspond to anything in our actual best current model.

at t=0 of the Big Bang, not T=0, that is the start of the universe

No. There is no "start of the universe" in our best current model. As has already been pointed out several times in this thread, our best current model does not include a "beginning" of the universe because we don't know what happened at any such "beginning", or even if there was one: it is possible that the universe has always existed, and that the "big bang" was simply the event that started off the expansion of one particular region of the whole universe, the one we live in. (Note that this region, all by itself, is still not just our observable universe--it's an entire spacetime that is spatially infinite.)

Infinite space

The spacetime that includes our observable universe--what we can see--is spatially infinite according to our best current model, yes.

We don't know how this space came to be. Maybe it was 'created' or was there all along.

Thinking about "how this space came to be" isn't really a good way of thinking about it. "Space" isn't something that has to be "created". A better way to put it is that the spacetime that includes our observable universe might be the only spacetime there is, or it might not; there might be a larger spacetime that includes ours as a portion.

Very dense hot point the size of a atom.

Our observable universe was that size. But the entire universe is still spatially infinite at this time, and all of it is very hot and dense, the same temperature and density as our observable universe was.

Since it is 'dense' and 'hot' that means it is 'matter'.

It depends on what you mean by "matter". It includes all of the fields of the Standard Model of particle physics: not just quarks and leptons, but gluons, W and Z bosons, and photons.

Since there is already 'space', this dense point of 'matter' the size of an atom resides inside 'space'.

Sort of. See my comment above about the entire universe all being very hot and dense.

At 10 -33 after t=0 inflation starts

No. Cosmic inflation comes before the big bang, not after. See my comment above on what the term "big bang" actually refers to. As I said, the picture is misleading you.

and expands that atom size dense point of 'matter' to 10 26 times the volume

No. All of that occurred before the big bang. Our observable universe being the size of an atom, and very hot and dense, occurs after inflation ends.

So, what happened is that a very dense hot point like bit of 'matter' expanded rapidly inside the 'space' (infinite 'space')

No. See above.

 

[Varsha Verma]

No. Cosmic inflation comes before the big bang, not after. See my comment above on what the term "big bang" actually refers to. As I said, the picture is misleading you.
No. All of that occurred before the big bang. Our observable universe being the size of an atom, and very hot and dense, occurs after inflation ends.
No. See above.

Inflation happened BEFORE the big bang?
I am completely flummoxed.

 

[phinds]

I am completely flummoxed.

reread post #76

 

[PeterDonis]

Inflation happened BEFORE the big bang?

Yes. Again, stop reading pop science sources and looking at pop science pictures and watching pop science videos. Any textbook on cosmology will explain our actual best current model properly and will give you the proper definition of the term "big bang".

I understand you might not feel ready to tackle a textbook on cosmology. But that doesn't change the fact that pop science sources are not good sources from which to learn the actual science. If you aren't ready to learn the actual science, then you just need to accept that you aren't yet ready to learn the right answers and be patient until you are. Going to the wrong sources and seeking out wrong answers won't help.

 

[Varsha Verma]

Please see this video of Alan Guth the 'father' of inflation:


I swear I hear Alan Guth say at the beginning that "inflation is a PREQUEL.. ", meaning that it happened 'before'.

But look at the diagram he shows after that. It clearly indicates that that inflation is happening AFTER the big bang.

Surely you can't call 'this' pop science because this is from the horses mouth, the guy who invented inflation.

What is going on here?

 

[nikkkom]

At ^{10 -33} after t=0 inflation starts and expands that atom size dense point of 'matter' to ^{10 26} times the volume at ^{10 -33} . So now that 'point' of dense hot point of 'matter' is humongous.

I think the point when matter was in a form of quark-gluon plasma is the earliest time for which our predictions are semi-accurate.

Before that, the conditions quickly become such that our experiments did not test them yet. Therefore, a depiction of what was happening before "~10^-12 s after BB", and for how long it was happening, is theory-dependent. For example, there could have been quite a bit more time than 10^-12 seconds. Eternal inflation theories, for example, have infinite amounts of time before that.

 

[bapowell]

That's not Alan Guth's diagram, but credits to NASA. He's using the diagram in a popular setting; no cosmologist takes it seriously. The words he uses are what you quoted: "prequel", which is what it is.

Your time would be better spent learning from an intro cosmology textbook rather than debating with experts on this forum.

 

[Varsha Verma]

That's not Alan Guth's diagram, but credits to NASA. He's using the diagram in a popular setting; no cosmologist takes it seriously. The words he uses are what you quoted: "prequel", which is what it is.

Your time would be better spent learning from an intro cosmology textbook rather than debating with experts on this forum.

I am not trying to debate. I am trying understand this stuff.

I am bit shocked that somebody like Alan Guth did not correct the diagram.