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

[Oink Honey]

Does it?

 

[phinds]

Does it?

No. The Big Bang Theory predicts an infinite future (and heat death) for the universe but says nothing about what might have come before inflation, so an eternal universe is possible although I believe it is considered unlikely. It would require some kind of fundamental state change, since the universe before the singularity would have to have been different in some significant way than the current universe, otherwise the singularity would not have happened (we see no singularity in the future of the current universe). I'm not widely read in this but I have never encountered a theory of an eternal universe that seemed to be anything other than pop-sci blather.

 

[Drakkith]

I believe the historical idea of an eternal universe is one that is static, unchanging, and has no beginning and no end. The big bang theory absolutely rules this specific type of eternal universe out, as it says that at every scale the universe is neither static nor unchanging but has a very dynamic existence. At its largest scales it expands and at smaller scales it experiences huge changes from the formation and evolution of galaxies, stars, planets, and other bodies. What the BBT doesn't say is whether or not the universe has a beginning or an end. The fact that the BBT predicts a singularity doesn't mean that it has a beginning, it means that the theory can no longer be used at that point. Another one must be found. As for an ending, there isn't one predicted by the BBT, but who's to say our current understanding of physics is sufficient to predict what will happen hundreds of billions of years from now?

 

[rootone]

It means that before approx 14bn years ago, nothing of our present observable Universe existed.
This does not rule out the existence of some kind of Universe beyond the observable.
However if something does exist beyond that it probably never can be observable, thus is not something which science can address.
There are all manners of metaphysical speculations, but they really don't explain anything.

 

[berkeman]

I believe the historical idea of an eternal universe is one that is static, unchanging, and has no beginning and no end. The big bang theory absolutely rules this specific type of eternal universe out, as it says that at every scale the universe is neither static nor unchanging but has a very dynamic existence

Back when I was about 2nd year in high school, my English/Spanish/History teacher knew of my interest in science, and he gave me a paperback book by Fred Hoyle on the Steady-State Universe. He said, "This is an old theory that has now been proven wrong, but it is still an interesting read". I really enjoyed the book, and it motivated me to go on and learn more about the BBT and more modern understandings.

 

[windy miller]

Hawking and Penrose showed that given certain assumption the big bang is associated with a spacetime singularity which marks the beginning of the universe. However these assumptions are no longer considered realistic and so its quite possible that the universe existed eternally into the past. there is no reason to either confirm nor deny that possibility. What most cosmologists agree is necessary is quantum theory of gravity to be able to probe this further. There are no theories of quantum gravity that have passed experimental verification. But there are some that theorists think have a lot going for them. When applied to the big bang these theories seem to suggest the universe existed before the big bang. Perhaps eternally into the past. I think these are the best bets we have at the moment, but they are not more than that. good bets but not verified experimentally. It is not impossible that we will be able to probe this experimentally and there are suggestions for how to do this. but it hasn't been done yet. So we don't know.
The universe will expand forever into the future assuming dark energy is a cosmological constant. There is always the possibility that dark energy is not a constant in which case the future of the universe is more uncertain.

 

[Varsha Verma]

Ok, so I have read now that the current model of the universe has no boundary. That is an assumption.

But, what I am trying to understand is this: Big Bang says that the the Big Bang itself created space.
We know that matter was compacted to a tiny point.

My question is, how can that tiny compaction of matter have no boundary?

When this tiny point expands, then surely it expands WITH a boundary, right?

 

[PeterDonis]

I have read

Where? Please give a reference.

the current model of the universe has no boundary

To be precise, our best current model of the universe has the universe being spatially infinite.

That is an assumption.

No, it's a conclusion based on evidence.

Big Bang says that the the Big Bang itself created space

No, it doesn't.

We know that matter was compacted to a tiny point.

No, we know that our observable universe, right after the Big Bang, occupied much less spatial volume than it does now. But our observable universe is not the entire universe.

 

[Varsha Verma]

Here is the reference for space been created by the Big Bang: https://www.space.com/52-the-expanding-universe-from-the-big-bang-to-today.html

"The universe did not expand into space, as space did not exist before the universe, according to NASA Instead, it is better to think of the Big Bang as the simultaneous appearance of space everywhere in the universe. "

 

[jbriggs444]

Here is the reference for space been created by the Big Bang: https://www.space.com/52-the-expanding-universe-from-the-big-bang-to-today.html

That is a pop-sci presentation authored by a science reporter. It falls well short of the Physics Forums standards. However, the language used is closer to the mark than most popularizations.

 

[Grinkle]

@Varsha Verma

The first line from your link -

"The universe was born with the Big Bang as an unimaginably hot, dense point."

Replace "universe" with "observable universe" and imagine that the hot, dense stuff is of infinite extent. The "point" being discussed is the tiny amount of the dense stuff that we can today see as our observable universe, but that point was not all that existed - the young universe was very dense, but it wasn't more or less bounded than today's universe. We have no model or theory to suggest that physical things can transition from finite extent to infinite extent or vice versa, so the thinking is that if the universe is infinite in extent today, then it always was, even if it was was much more dense in the past than it is today.

Its hard to describe with words something that is of infinite extent expanding and becoming less dense. I am told it is easier to describe with mathematics than words, or at least it is possible to be more precise about what one is talking about with math than words.

I suggest you don't read too much into someones claim that it is better to think the big bang as the simultaneous appearance of space everywhere in the universe. Whether it is really better to think of things that way or not is a very judgement driven conclusion - reasonable people can disagree. In most discussions I have read, participants have trouble agreeing on what descriptions like that really mean or are trying to get across.

 

[PeterDonis]

Here is the reference for space been created by the Big Bang:

As @jbriggs444 has pointed out, this is not a valid reference. Please consult a cosmology textbook.

 

[Chronos]

How best to proceed when words become ambiguous? The short answer is mathematics. Mathematics accommodates infinities [both countable and uncountable] with relative ease whereas ambivalence towards the principles of cause and effect is a Gordian Knot for logic.

 

[Varsha Verma]

Where? Please give a reference.
To be precise, our best current model of the universe has the universe being spatially infinite.
No, it's a conclusion based on evidence.
No, it doesn't.
No, we know that our observable universe, right after the Big Bang, occupied much less spatial volume than it does now. But our observable universe is not the entire universe.

What evidence is there to show that the the current model of the universe is specially infinite??

Is it observable evidence??

How can we 'observe' that something is 'infinite'? I don't understand this.

 

[Grinkle]

What evidence is there to show that the the current model of the universe is specially infinite??

Read -

https://www.physicsforums.com/threads/the-universe-vs-observable-universe.938274/page-2

Post #23 for some digestible discussion on that.

 

[PeterDonis]

What evidence is there to show that the the current model of the universe is specially infinite??

You mean, what evidence is there to show that our best current model of the universe, which says that it is spatially infinite, is correct? The fact that the universe, according to our best current measurement, is spatially flat, and that we see no evidence of non-trivial topology (e.g., we don't see multiple images of the same distant object in different directions). Spatially flat + no non-trivial topology = spatially infinite.

[Edit: corrected to "no non-trivial topology" above.]

 

[jbriggs444]

Spatially flat + non-trivial topology = spatially infinite.

@PeterDonis, I think you mean "no non-trivial topology" there.

 

[PeterDonis]

I think you mean "no non-trivial topology" there.

Yes. Post corrected.

 

[bapowell]

I think people are being too heavy handed with claims that data support an infinite universe. It equally supports a closed universe with a radius of curvature much larger than the Hubble scale.

 

[rootone]

I think people are being too heavy handed with claims that data support an infinite universe. It equally supports a closed universe with a radius of curvature much larger than the Hubble scale.

I wonder what William of Occam would make of this dilemma.

 

[bapowell]

I wonder what William of Occam would make of this dilemma.

Good point. What does Bayesian model selection have to say? Do you have a good prescription for a prior on the model space?

 

[PeterDonis]

a prior on the model space?

The prior that appears to be implicitly adopted by our best current model is that spatial flatness is more likely, other things being equal. But I don't think anyone has explicitly thought that out or advanced an argument for such a prior.

Of course part of the problem is that it's not even clear what, exactly, the model space is or how to parameterize it.

 

[Dale]

To be precise, our best current model of the universe has the universe being spatially infinite.

No, it's a conclusion based on evidence.

The assumption referenced by @Varsha Verma is the assumptions of homogeneity and isotropy. Together (with simple topology) they also imply the absence of a boundary, regardless of whether the universe is finite or infinite. So the lack of a boundary is indeed an assumption, but as you say the spatial infinite-ness is a conclusion based on the evidence.

 

[PeterDonis]

Together (with simple topology) they also imply the absence of a boundary

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.

 

[Varsha Verma]

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.

Here is a picture of a torus. I can see boundaries here. Especially the 'hole' in the middle is a boundary right.

 

[PeterDonis]

Here is a picture of a torus. I can see boundaries here.

No, you can't. What you see are artifacts of embedding the torus in a higher dimensional space. The torus itself has no boundary. Someone restricted to moving just on the torus could cover its entire surface and never reach an edge.

Especially the 'hole' in the middle is a boundary right.

No. See above.

 

[Varsha Verma]

No, you can't. What you see are artifacts of embedding the torus in a higher dimensional space. The torus itself has no boundary. Someone restricted to moving just on the torus could cover its entire surface and never reach an edge.
No. See above.

But space is 3 dimensional right? Why have that restriction??

 

[PeterDonis]

space is 3 dimensional right? Why have that restriction??

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.

Why have that restriction?

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.

 

[PeterDonis]

@Varsha Verma before posting further, I strongly suggest that you take the time to work through a basic textbook on GR and the fundamental geometric and topological concepts that it uses. Sean Carroll's online lecture notes would be a good place to start:

https://arxiv.org/abs/gr-qc/9712019

These notes have an entire chapter on cosmology, but I would suggest working through the first few chapters on manifolds and curvature first. You really need to properly understand these basic concepts in order to understand how our best current model of the universe works.

 

[PeterDonis]

space is 3 dimensional right? Why have that restriction??

One further suggestion: there is no need to over-use question marks. One per question is enough.

 

[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.