B Does the Big Bang model rule out an eternal universe?

[rootone]

But space is 3 dimensional right?

Well there are clever math arguments that it might be more than 3 at very microscopic scale,
but 3 dimensions is what is obvious. (plus time).
Asking why that is the way it is is pointless, you could ask the same question if there were 5 dimensions of observable space

 

[Varsha Verma]

Well there are clever math arguments that it might be more than 3 at very microscopic scale,
but 3 dimensions is what is obvious. (plus time).
Asking why that is the way it is is pointless, you could ask the same question if there were 5 dimensions of observable space

You are referring to sting theory, right?

 

[Varsha Verma]

Yes. And if you go back and read my post #24 again, carefully, you will see that I said a flat 3-torus. The picture you gave was of a 2-torus. A 3-torus is a 3-dimensional manifold, like the "space" that we perceive, and it has been suggested as a possible non-trivial topology for the space that we perceive, the space of the universe.
The general definition of a manifold, and its topology, works for any number of dimensions, not just 3. It isn't a "restriction", it's a particular mathematical concept, whose definition you would do well to learn properly before you post further in this discussion.

I have not done even high school physics. So I can't read GR stuff.

So, you are saying that the 3 dimensional universe we see is not like a 3 dimensional torus or Doughnut??

So, a flat 3-torus is not like a doughnut, its' a purely mathematical concept which we cannot actually visualize?
Then why is it called a torus?

 

[PeterDonis]

I have not done even high school physics. So I can't read GR stuff.

Understood. But in that case you should be more careful in the claims you make. Even if you are not quite ready to tackle GR yet, the fact remains that our current model of the universe uses GR, so you need to understand GR in order to really understand what the model says. And what the model says violates a number of the implicit assumptions you are making, so you need to realize that and stop talking as if those assumptions were obvious facts. They're not.

you are saying that the 3 dimensional universe we see is not like a 3 dimensional torus or Doughnut?

According to our best current model, our universe, spatially, is ordinary flat Euclidean 3-space. That is not like a 3-torus, no.

a flat 3-torus is not like a doughnut, its' a purely mathematical concept which we cannot actually visualize?

It is a 3-dimensional space which is the 3-dimensional analogue to the 2-dimensional torus (or doughnut), in the same way that ordinary Euclidean 3-space is the 3-dimensional analogue to the flat 2-dimensional Euclidean plane.

why is it called a torus?

Because "torus" in topology is a general term for manifolds of any dimension that have a particular set of properties. The 2-dimensional torus is just one of them.

 

[timmdeeg]

It equally supports a closed universe with a radius of curvature much larger than the Hubble scale.

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