Does the Big Bang model rule out an eternal universe?

[Dale]

Actually, I think they imply the absence of a boundary even with a non-simple topology. A flat 3-torus, for example, still has no boundary.

Hmm, yes. In fact now that you mention it, doesn’t homogeneity by itself imply no boundary?

 

[Grinkle]

doesn’t homogeneity by itself imply no boundary?

I don't understand why this isn't related to scale. The observable universe was once the size of something small. I read grapefruit-sized somewhere, in this thread there is a link saying it was the size of an atom; anyway, it was small in comparison to what it is today.

If it were 1cm across, and the entire universe was at that time the size of today's earth, the odds of our observable universe being anywhere close to the boundary of the entire universe are very small. I don't 'get' the argument that Occam's razor clearly favors an infinite universe.

 

[PeterDonis]

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

No. With a cosmological constant you can have accelerated expansion even if the universe is spatially closed.

 

[PeterDonis]

the odds of our observable universe being anywhere close to the boundary of the entire universe are very small. I don't 'get' the argument that Occam's razor clearly favors an infinite universe.

"No boundary" is not the same as "spatially infinite". A spatially closed universe with topology $S^3$ (a 3-sphere) is not spatially infinite, but it has no boundary.

 

[PeterDonis]

doesn’t homogeneity by itself imply no boundary?

Yes.

 

[Dale]

I don't 'get' the argument that Occam's razor clearly favors an infinite universe.

The point is that zero curvature is a reasonable model to propose, and with no free parameters it makes fairly specific predictions. The alternative model essentially has the curvature as a free parameter which can be tuned to fit a wider range of data, so its predictions are less specific. That is Occam’s razor in a nutshell: a model without a free parameter is preferred over a model with a free parameter if they both fit the data well.

 

[rootone]

You are referring to sting theory, right?

Yes those, but so far they don't predict anything about the world everyone can see.
I am not saying these ideas are useless, but then they are not useful either, until they do predict something

 

[Varsha Verma]

I think most people who come to this forum are 'laymen', meaning we don't have any knowledge of technical physics stuff.

So, regarding this topic, I am willing to believe what 'scientists' say about the current status of the

universe: That is, that the universe at present is 'infinite'.

Come to think about this, I suppose EVERYTHING about human experience is BELIEF isn't it. Even when a Nobel prize winning scientist says that the evidence shows that the universe is infinite, then we actually 'believe' him/it isn't it. I mean we don't do any experiment ourselves. We take the word of the scientists as true.

I suppose when it comes to 'my parents', it is also a 'belief' isn't it. I mean I don't know whether my parents are my biological parents. I just 'believe' what they say isn't it.

So, this also ends with a very awkward situation. That is, what is the difference between 'knowing' and 'belief'. When can we say that we KNOW something, as opposed to ‘believe’ something?

So, for example, I KNOW that my house exists because I can feel it with 2 of my sense organs, namely the eye and touch.

I actually don't KNOW whether the moon is actually there, because although I can see it with eyes, I cannot touch it. I have not been there.

So, if we really analyse, only the people who landed on the moon KNOW it is real. Everybody else 'believes' what the moon landers say, isn't it. You know there are millions of people even now who give ‘evidence’ to show that the moon landing never happened.

How do we actually KNOW anything about things which we cannot feel with our sense organs?

I think this could be a different topic altogether.

So, coming back to the discussion, I will believe that the universe at present is 'infinite'.
But what I cannot even 'believe' is this:
(1.) You say that the universe started as a very small point of matter. That is, all matter in the current universe was compacted into this tiny point.

(2.) You also say that there was no SPACE. THIS is what laymen like us find it difficult to even 'believe'. So, if there was no SPACE then how do you say that the universe was infinite even then?? Because it's SPACE that extends to infinity, right?? So, space extends to infinitely NOW. BBT says that there was no SPACE at the big bang. So, then how can the universe even at the time of the big bang be 'infinite'??

If you give an answer in layman’s terms it will be really great.

 

[Drakkith]

(2.) You also say that there was no SPACE. THIS is what laymen like us find it difficult to even 'believe'. So, if there was no SPACE then how do you say that the universe was infinite even then?? Because it's SPACE that extends to infinity, right?? So, space extends to infinitely NOW. BBT says that there was no SPACE at the big bang. So, then how can the universe even at the time of the big bang be 'infinite'??

The big bang theory does not deal with the status of the universe before the universe existed. It only describes the evolution of the universe starting from a point in time where the universe (and space) already existed.

 

[rootone]

I suppose EVERYTHING about human experience is BELIEF isn't it.

No, if say you are out running just for fun, and a dog gets in your way and then you fall over. that is not a belief , it's physics.

 

[Varsha Verma]

No, if say you are out running just for fun, and a dog gets in your way and then you fall over. that is not a belief , it's physics.

Your right here. Not EVERYTHING is belief. But a lot of it is isn't it.

 

[PeterDonis]

We take the word of the scientists as true.

No. We look at how accurately the predictions of their models match the data.

I don't know whether my parents are my biological parents. I just 'believe' what they say

You can get DNA tests if you want to test this hypothesis with data.

I actually don't KNOW whether the moon is actually there, because although I can see it with eyes, I cannot touch it.

Why doesn't seeing it count as evidence that it's there?

How do we actually KNOW anything about things which we cannot feel with our sense organs?

We look at the evidence.

You say that the universe started as a very small point of matter.

No, we say that the observable universe started occupying a very small volume. The observable universe is not the entire universe. This has already been pointed out in this discussion.

You also say that there was no SPACE.

Who is saying that? Where are you getting this from?

BBT says that there was no SPACE at the big bang.

It says no such thing.

 

[Varsha Verma]

No. We look at how accurately the predictions of their models match the data.

Here what I meant is the public. The public does not do experiments. They BELIEVE what the scientists say. That is what I was saying.

 

[Varsha Verma]

No, we say that the observable universe started occupying a very small volume. The observable universe is not the entire universe. This has already been pointed out in this discussion.
Who is saying that? Where are you getting this from?
It says no such thing.

So the BIG BANG is where there was some matter and space and it expanded rapidly. I think I get it now.

So, if there was SPACE at the big bang, then that SPACE was occupying a small volume like a small sphere I am guessing??

So, isn't that SPACE finite having a boundary.??

 

[rootone]

The public does not do experiments. They BELIEVE what the scientists say.

It is an important thing that publication is an essential part of scientific method.
Exactly so that anyone willing and able to do experiments can in fact do that and compare/dicuss results.

 

[Varsha Verma]

It is an important thing that publication is an essential part of scientific method.
Exactly so that anyone willing and able to do experiments can in fact do that and compare/dicuss results.

Well, the general public cannot look at science publications. We can't read Nature.

So, for example, one person in the general public A asks another person in the general public B "How old is the universe?", then if B has read about this in Google or some article, he will say "it's 13.8 billion years old", right?

You know like that. So, general public 'trust' the scientists.

 

[rootone]

The public does not do experiments. They BELIEVE what the scientists say.

In the same way that most people would trust their doctor as being probably accurate if the doctor said that they had 'flu. and there are medications which are known to effective. so you should be OK in about a week.

 

[PeterDonis]

The public does not do experiments.

You don't have to do the experiments yourself to check whether a model's predictions match the data.

You don't even have to believe the data; you can test the hypothesis that the data itself is flawed, or mis-reported, or otherwise not reliable.

Of course you can't check every single thing yourself. That's true of everything in life. You operate on a daily basis with many beliefs that you did not personally verify. But you are also constantly checking to see if they work.

 

[Varsha Verma]

In the same way that most people would trust their doctor as being probably accurate if the doctor said that they had 'flu. and there are medications which are known to effective. so you should be OK in about a week.

True. I have no problem with it.

Although I have read where reputed scientists like Brian Greene mind you that say that we MIGHT be in a giant computer simulation. He explores this possibility this in his book The Hidden Reality which I have read.

So, in THIS case even if we are "running just for fun, and a dog gets in your way and then you fall over" then we are not sure isn't it??

That is, our reality could be not a 'real' reality but a simulated one.

I don't how to get about that problem though.

 

[PeterDonis]

if there was SPACE at the big bang, then that SPACE was occupying a small volume like a small sphere I am guessing?

Our observable universe was occupying a small volume (which was spherical, yes) just after the big bang. Not "space".

isn't that SPACE finite having a boundary

The space occupied by the observable universe is finite and has a boundary, yes. But, as I've said several times now, the observable universe is not the entire universe. Please stop and think very carefully about what that means. You are spending a lot of time asking questions that have already been answered.

Also, please stop using all caps and too many question marks. That is the equivalent of shouting at people. It's not polite, and it's also against PF rules.

 

[Varsha Verma]

You don't have to do the experiments yourself to check whether a model's predictions match the data.

You don't even have to believe the data; you can test the hypothesis that the data itself is flawed, or mis-reported, or otherwise not reliable.

Of course you can't check every single thing yourself. That's true of everything in life. You operate on a daily basis with many beliefs that you did not personally verify. But you are also constantly checking to see if they work.

Yes, I have no issue with believing things. We do that all the time.

 

[PeterDonis]

our reality could be not a 'real' reality but a simulated one

That is off topic for this discussion (and this forum).

 

[PeterDonis]

I have no issue with believing things. We do that all the time.

Then why do you keep bringing it up? What point are you trying to make?

 

[Varsha Verma]

Our observable universe was occupying a small volume (which was spherical, yes) just after the big bang. Not "space".
The space occupied by the observable universe is finite and has a boundary, yes. But, as I've said several times now, the observable universe is not the entire universe. Please stop and think very carefully about what that means. You are spending a lot of time asking questions that have already been answered.

Also, please stop using all caps and too many question marks. That is the equivalent of shouting at people. It's not polite, and it's also against PF rules.

Thanks, this is a good explanation.

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

But isn't 'space' the universe. Everything is inside space isn't it.

So, 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'?

Is this the 'multiverse' you are referring to?

PS: I think the general public have no clue about this big bang stuff. What we think we know is completely different to what scientists know isn't it?

 

[Varsha Verma]

Because when we the general public mean the universe, we mean the entire universe, everything, not just the observable 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.