Doesn't the big bang model rule out an infinite universe?

[CosmicVoyager]

Greetings,

#1 - If all matter energy, and space were concentrated at the big bang, and the universe has been expanding at a finite rate, then why is anything other than a finite universe considered?
http://en.wikipedia.org/wiki/Shape_of_the_Universe

#2 - Also, as discussed in another thread, matter/energy could not have been sitting in a larger space because that would be a black hole and could not expand. To be able to expand, the matter/energy had to fill all of space. Again, why is there any question about the universe being finite or infinite?
https://www.physicsforums.com/showthread.php?t=464438

Thanks

 

[edgepflow]

Inflation models and current measurements suggest the universe is geometrically flat. A flat universe should not have edges so it is considered likely the universe is infinite.

 

[CosmicVoyager]

Inflation models and current measurements suggest the universe is geometrically flat. A flat universe should not have edges so it is considered likely the universe is infinite.

I just gave two things that contradict that possibility. How do you reconcile them?

 

[Nabeshin]

Your point #1 suggests you seem to be under the impression that an infinite object cannot expand! This is not true. Just as well as you can imagine a finite object expanding, so too can the distance between any two points on an infinite manifold increase.

 

[CosmicVoyager]

Your point #1 suggests you seem to be under the impression that an infinite object cannot expand! This is not true. Just as well as you can imagine a finite object expanding, so too can the distance between any two points on an infinite manifold increase.

But according to this thread, space had to be finite to start or the universe would have been a black hole singularity that gravity prevented from spreading out.

How do you go from finite to infinite with a finite expansion rate in a finite amount of time.

I keep reading and seeing on science shows that there was no space outside the singularity.

https://www.physicsforums.com/showthread.php?p=3173090

 

[marcus]

#1 - If all matter energy, and space were concentrated at the big bang, ...
http://en.wikipedia.org/wiki/Shape_of_the_Universe
...

*If* :yuk:

You sound like you think that astronomers assume the U was concentrated "at a point" at the big bang. We have no evidence that all matter and space was concentrated in a finite volume at the BB, or at a point. Astronomers assume that at bang-time the volume was either infinite or finite---the two cases are treated separately.

So your premise is unjustified. You can't assume it started out finite.

 

[CosmicVoyager]

*If* :yuk:

You sound like you think that astronomers assume the U was concentrated "at a point" at the big bang. We have no evidence that all matter and space was concentrated in a finite volume at the BB, or at a point. Astronomers assume that at bang-time the volume was either infinite or finite---the two cases are treated separately.

So your premise is unjustified. You can't assume it started out finite.

If space was infinite, then how did anything escape the gravity of the singularity? All matter/energy was so concentrated nothing could escape it.

 

[marcus]

... matter/energy had to fill all of space...[/url]

Your reasoning about the black hole is wrong, but you get to a reasonable conclusion. It's fair to assume that matter/energy fills all space, and filled it (more or less uniformly) at BB time.

So if space was infinite volume at BB time, then there was an infinite amount of matter/energy.

And if space was finite at that time, then there was a finite amount.

That's all. There is no contradiction. Both cases are mathematically modeled and studied.

You have no argument to make against infinite universe (or at least haven't shown one so far.)

Personally I like to picture the U as finite volume, both now and at the start of expansion. But it is more a matter of personal taste than of logic and empirical evidence. Maybe some day we will know which case we are in, after more precise measurement of the largescale curvature.

So if you like the finite U picture, don't try to PROVE it's right, just hang in there until more data is collected and more numbers are crunched.

 

[marcus]

If space was infinite, then how did anything escape the gravity of the singularity? All matter/energy was so concentrated nothing could escape it.

Imagine you are in an infinite volume uniformly filled with matter--- uniform density in all directions. Which direction would you be pulled? See? You feel no force. There is no preferred place to collapse to. That's not completely airtight, perhaps, but put
that together with the INITIAL CONDITION of extreme expansion Think of inflation. Expansion so fast that BH dynamics can't get a grip. Expansion outracing any tendency to collapse.

Black hole collapse scenarios play out in NON-EXPANDING space, or only slowly expanding. That is definitely definitely not the case here! Collapse scenarios do not apply. We periodically get naives here who discuss the BB as if BH collapse dynamics would apply, but you can just ignore those threads.

You can either take the initial conditions for granted or you can delve into what might have caused them. Then you get into Quantum Gravity---the physics of gravity at very high densities. Empirically testable, but not tested so far. The bounce picture.

 

[CosmicVoyager]

Your reasoning about the black hole is wrong, but you get to a reasonable conclusion. It's fair to assume that matter/energy fills all space, and filled it (more or less uniformly) at BB time.

So if space was infinite volume at BB time, then there was an infinite amount of matter/energy.

And if space was finite at that time, then there was a finite amount.

That's all. There is no contradiction. Both cases are mathematically modeled and studied.

You have no argument to make against infinite universe (or at least haven't shown one so far.)

Personally I like to picture the U as finite volume, both now and at the start of expansion. But it is more a matter of personal taste than of logic and empirical evidence. Maybe some day we will know which case we are in, after more precise measurement of the largescale curvature.

So if you like the finite U picture, don't try to PROVE it's right, just hang in there until more data is collected and more numbers are crunched.

Okay, we are starting to get somewhere...

"So if space was infinite volume at BB time, then there was an infinite amount of matter/energy."

Isn't the leading theory that everything was concentrated in a tiny space at the big bang. Everything I see talks about the starting size and the sizes at different points in the expansion.

edgepflow said the leading theory is that the universe is flat and infinite. That is what didn't make sense to me.
"Inflation models and current measurements suggest the universe is geometrically flat. A flat universe should not have edges so it is considered likely the universe is infinite."

Is it generally considered that the universe had infinite volume and infinite matter/energy at the big bang, and that the universe is flat and infinite now?

Or that the universe had finite volume and finite space at the big bang, and that the universe is not flat and is finite now?

 

[CosmicVoyager]

Imagine you are in an infinite volume uniformly filled with matter--- uniform density in all directions. Which direction would you be pulled? See? There is no force of gravity.
Put that together with the INITIAL CONDITION of extreme expansion Think of inflation. Expansion so fast that BH dynamics can't get a grip. Expansion outracing any tendency to collapse.

Black hole collapse scenarios play out in NON-EXPANDING space, or only slowly expanding. That is definitely definitely not the case here! Collapse scenarios do not apply. We periodically get naives here who discuss the BB as if BH collapse dynamics would apply, but you can just ignore those threads.

You can either take the initial conditions for granted or you can delve into what might have caused them. Then you get into Quantum Gravity---the physics of gravity at very high densities. Empirically testable, but not tested so far.

"We periodically get naives here who discuss the BB as if BH collapse dynamics would apply, but you can just ignore those threads."

And there is my problem! :-) I am trying to piece together all the answers to all the important questions in so many threads. But there is no system to indicate what the consensus answers are. I need to know the consensus answers. They should all fit together.

This should be part of a FAQ system. Consensus answers indicated and alternative answers so indicated.

 

[marcus]

Isn't the leading theory that everything was concentrated in a tiny space at the big bang. Everything I see talks about the starting size and the sizes at different points in the expansion.
...

NO! THAT IS NOT THE LEADING THEORY!
Never trust popularizations, often they are just selling magazines or trying to wow people to boost their channel ratings.

You didn't read the fine print.
If the pop-author is any good he will say "the observable universe" was concentrated into a small space (like the size of an atom or a watermellon or whatever).
The observable universe is way way much less than the whole universe, which might even be infinite. The observable is finite and is a small part of the whole.

 

[marcus]

... But there is no system to indicate what the consensus answers are. I need to know the consensus answers. They should all fit together.
...

I call that the "mainstream" view. In cosmology things are pretty coherent. There is a mainstream model. (It can come in more than one version, but the basic equations are the same, just change the plug-in parameters)

One thing you can do is look for the S.A. badge under a poster's name. Like BA Powell is an S.A. Chalnoth is an S.A. It is not a perfect system but if you see a thread where nobody posting in it is a designated science advisor then the info COULD be all right but you might want to be a bit more cautious. Nabeshin is S.A. and George Jones, a mentor, is extremely well qualified so keep a look out for when he drops in. Those names come to mind but there are a bunch more as well.

Also it is a relaxed uncoercive system on the whole, and you are the one who is finally responsible. You have to decide for yourself who you can trust when they are talking about what. You have to patiently acquire that knowledge over time by watching.

So no guarantees! I could be an SA and yet I might make a mistake or be misinformed about something. You have to keep alert and compare critically etc.

BTW for a COSMOLOGY FAQ you should know about the Ned Wright website. He is a world-class cosmologist and interested in outreach, plus he teaches cosmo at UCLA. He has a widely googled FAQ and cosmo tutorial that has been translated into several languages etc etc.
Just google "ned wright". You will find it. Big website. Explore.
Check out "wright balloon model" for an animation.

 

[octelcogopod]

marcus, how do you define infinite and how does it change the theories about the universe or the origins of the universe? Is it a catch all term to state everything we don't know or is it a physical definition? It seems strange to me since by definition something which is infinite can never be understood fully (since there will always be a chance for something completely different from that which is already discovered), and since the observable universe has a definite volume and space it would mean that there would be infinite space and energy which would theoretically mean infinite scale and infinite time. Such a concept is almost incoherent by its very nature?

I can understand that it can't be ruled out but I'm curious about what implications it has /right now/ that are more than just a vague thought experiment.

 

[Chronos]

How did you deduce marcus was attempting to define 'infinity'? I must have missed that part.

 

[octelcogopod]

Well I could ask the same of you... How did you deduce that I deduced marcus was trying to define infinity? He said in an earlier post that the Op hadn't made an argument against an infinite universe, so I was wondering how one can argue for or against an infinite universe.

 

[hadsed]

NO! THAT IS NOT THE LEADING THEORY!
Never trust popularizations, often they are just selling magazines or trying to wow people to boost their channel ratings.

You didn't read the fine print.
If the pop-author is any good he will say "the observable universe" was concentrated into a small space (like the size of an atom or a watermellon or whatever).
The observable universe is way way much less than the whole universe, which might even be infinite. The observable is finite and is a small part of the whole.

This seems funny to me. When I picture the observable universe I'm thinking inflation or dark energy took over to expand the entire universe too fast for the speed of light to keep up. So when people talk observable universe, I'm thinking it's not as if the universe is really separate (from an omniscient perspective I guess) from our own little observable part. So how exactly could only the observable universe be compacted inside the big U? Or am I making some wrong assumptions here?

 

[TheTechNoir]

The rest of the universe was also compact. I think what he is saying is that it is believed that our observable universe used to be that small (watermellon, atom, etc. something small by a human scale) but that doesn't assign anything special to our oberservable universe (the horizon of which is constantly seeing less and less). Using the omnicient thought, if you could see another section of the universe that were spatially far removed from our OU but the same size, it too would have been the watermellon/atom size at the time of the BB. And so too would every other equivalantly sized section of the universe.

There are some lesser accepted theories that may indicate however that, that is not correct such as chaotic inflation. I think that would make what I said incorrect anyway, but I don't know much of chaotic inflation to say for sure, if you would like to read up on it go ahead. And that isn't to say what marcus said was in anyway wrong, it was not wrong at all.

EDIT: Also, while not popularily believed to be the shape of our universe, it is not impossible for the U to be finite and flat (and no it will not have a boundary - think of a pacman screen what happens when you keep going in one direction - or for a much more satisfying explanation search for toridial universe)

 

[marcus]

I often do not explain things this well. Each paragraph is clear and helpful. I was glad to see this when I got up and poured coffee this morning.

The rest of the universe was also compact. I think what he is saying is that it is believed that our observable universe used to be that small (watermellon, atom, etc. something small by a human scale) but that doesn't assign anything special to our oberservable universe (the horizon of which is constantly seeing less and less). Using the omnicient thought, if you could see another section of the universe that were spatially far removed from our OU but the same size, it too would have been the watermellon/atom size at the time of the BB. And so too would every other equivalantly sized section of the universe.

There are some lesser accepted theories that may indicate however that, that is not correct such as chaotic inflation. I think that would make what I said incorrect anyway, but I don't know much of chaotic inflation to say for sure, if you would like to read up on it go ahead. And that isn't to say what marcus said was in anyway wrong, it was not wrong at all.

EDIT: Also, while not popularily believed to be the shape of our universe, it is not impossible for the U to be finite and flat (and no it will not have a boundary - think of a pacman screen what happens when you keep going in one direction - or for a much more satisfying explanation search for toridial universe)

I think from your first paragraph the reader can see how the universe could have been infinite already at the start of expansion (an infinite number of finite watermellons?).
The fact that our OU began as a small finite vol does not imply that the entire U did.
We simply do not know at this stage which model is closer to the truth, spatial infinite or spatial finite ("closed", "compact") like for example hypersphere.

I think your second paragraph is a good point to make! Working cosmologists use a simple model U (called LCDM based on Friedmann equation) that does not involve a multiverse of different random inflations with possibly different laws of physics. The mainstream picture is just one U, governed by Einstein GR eqn., assumed approximately uniform, with no boundary. (and there are two main cases, spatial finite and spatial infinite, because our data is not yet good enough to determine the curvature accurately enough to distinguish.)
And like you said there ARE these other models like "chaotic inflation". They are not supported by data, they are speculations, they are not models that mainstream cosmologists use, but they interest people. As multiverse scenarios they appeal to some string theorists, for example. Your second paragraph strikes the right tone. It acknowledges these interesting scenarios which people think about but do not use for fitting observational data to.

Your third paragraph, the EDIT, also strikes just the right tone. There are these pacman geometries which the U could have. They look locally flat. As far as we can tell they would look like the flat infinite case. But if you could travel very very far (outracing expansion) you would eventually come round, like a pacman. Maybe life is too short to constantly be acknowledging the different possible geometries the U could have.

Pragmatically, you need a mathematical model to fit data to. Two or three cases of LCDM seem adequate based on measurements of largescale spatial curvature (pos, zero, neg).
So far signs of pacman (torus = donut type) have not appeared so it gets ignored or downplayed.

All I think I'm doing is amplifying what you said. It is either that or remain silent.
Thanks.

 

[Chronos]

A wrap around universe would a challenge to confirm. Only objects exactly half the distance to the wrap point would simultaneously appear the same when viewed in opposite directions. Spacetime curvature further complicates matters. It is unclear which direction would be 'opposite'. I suppose we could get lucky and spot simultaneous supernova or gamma bursters with identical characteristics. My point is a finite universe must eventually wrap back around on itself. Based on our current best evidence, it appears unlikely the universe is sufficiently curved and ancient enough for light to have completed a lap.

 

[marcus]

... hadn't made an argument against an infinite universe, so I was wondering how one can argue for or against an infinite universe.

I think the logical/observational context goes like this. Cosmology is a mathematical science---based on math models that you fit data to. You want the simplest model that gives a nice fit to the observations.

(like temperature fluctuations of the ancient light, like counts of galaxies at a given distance)

One thing these observations tell about is largescale average curvature, is it positive like a sphere surface, or zero like an infinite flat x-y plane, or negative like a saddle surface.

Cosmology is a math science so INFINITE means what it means in Freshman year college Calculus class, or in a good high school Pre-calculus. That's all we mean by "spatially infinite universe"---that it has infinite volume, similar to ordinary x-y-z space in math class.

You ask how do we argue for finite or infinite. We don't argue based on logic or philosophy, we measure curvature. Estimates of the largescale curvature have been getting better every two or three years.

The finite/infinite issue at present boils down to a 95% confidence interval for a number Omega-sub-k related to curvature.

In a mathematical science words have meanings special to the science, oftentimes.
They do not necessarily mean what they might in a religious or philosophical discussion. Or in poetry.

So with finite/infinite, if you want to talk cosmology here at cosmo forum then you use the simple math meaning of finite and infinite and it boils down to the current confidence interval [-0.02, 0.01]. If Omega-sub-k is zero or on the positive side we will say infinite. If, when we narrow the interval down further it turns out to be all on the negative side we will say finite. (And it isn't quite that simple but that's a first approximation.) We know the limits to more decimal places but I simplified the numbers.

 

[bcrowell]

And there is my problem! :-) I am trying to piece together all the answers to all the important questions in so many threads. But there is no system to indicate what the consensus answers are. I need to know the consensus answers. They should all fit together.

If you want a source of information you can trust, you're much better off reading a book than using an internet forum. An internet forum, especially one like PF that allows anonymity, is by its nature not a place to look for authoritative answers. It can, however, help you to understand what you read, or point you to books and papers you didn't know about. On cosmology questions that (like this one) are basically GR questions, you will also get more reliable answers in the relativity forum than in the cosmology forum. The cosmology forum is better for observational questions, e.g., how do CMB observations tell us about the universe's spatial curvature?