What Is Beyond The Observable Universe?

[DaveC426913]

?

Like living dinosaurs? Like the formation of the solar system?

We use logical and mathematical models all the time to reconstruct the past...

We have more than logical and mathematical models of dinosaurs and newborn solar systems. We have direct observation.


I think that the connection you are trying to draw between [living crocodiles + bones and soft-tissue samples of extinct animals, etc.] and [a part of the universe we can never observe, or any indirect evidence of it - even in principle] is very weak.

 

[marcus]

I think that the connection you are trying to draw

What connection do you imagine I am trying to draw?
In what I just said I was not talking about the early universe or whatever, I was responding to a general statement which the poster made:

"we cannot hypothesize about anything we have not directly observed"

I want to contradict that general proposition, and there are a million and one different examples. I chose living dinosaurs.

Fairies may have manufactured fossils and buried them where we would find them, to fool us. Or leprechauns. But it is simpler to assume that they lived and that the fossils are their remains.

My point there is simply that we CAN hypothesize and infer about (at least some things) that we have not observed!

You are putting words in my mouth if you think this is being equated to cosmology, or put on the same level of certainty, or something. That is your, Dave's, idea.

However we do use inference in cosmology, and pick simple economical explanations. What the level of certainty is, is another question that needs a separate discussion.

 

[bapowell]

Fairies may have manufactured fossils and buried them where we would find them, to fool us. Or leprechauns.

This is what I've been saying all along! At last, someone who shares my view!

 

[marcus]

I guess the point should be made, in case anyone hasn't followed the discussion, that from a standard cosmo viewpoint the correct "OTHER" in the poll was "more of same". Essentially because that is the simplest thing to assume.

The simplest mathematical model that fits the data (and conforms with fundamental physics like GR) does not have any boundary or discontinuity at our observational horizon. The most distant matter we see is what we see as it was 13.7 billion years ago as it was hot gas radiating the CMB light we now detect and use to map its temperature and density variations. We have no reason to suppose that that matter has not evolved just like ours has---cooling and condensing to form stars and galaxies.

It would complicate the model physically to try to introduce some sort of boundary out there, or some drop-off of density. So we assume that things look, out there, pretty much the same as they do here, in all directions.

This was the choice which the original poster Silverback, who set up the poll, should have included explicitly. But did not, causing many of us to either not vote, or choose "other".

 

[ComfortNumb]

Gonna be flippant on my very first post.

I reckon there is a big brick wall made out of dark matter encircling our universe. Then when a galaxy hits the wall the energy passes through it to create a big bang on the other side and therefore creating another universe!

Then I woke-up!

 

[DaveC426913]

What connection do you imagine I am trying to draw?
In what I just said I was not talking about the early universe or whatever, I was responding to a general statement which the poster made:

"we cannot hypothesize about anything we have not directly observed"

...which is a direct comment on hypothesizing about what is outside the observable universe.


Either you are suggesting that
- we can hypothesize about what is beyond our observable universe
analagous to the way
- we can hypothesize about dinosaurs and our own solar system
or you are not paying attention.

What I am saying is that yours is a faulty analogy. I refute that there are a million and one examples. I suggest the examples could be counted on one hand at the most. There are few, if any, things in all creation that are analagous to hypothesizing about outside the universe. It's just not the same thing as extrapolating from known bones to unknown dinosaurs.

 

[marcus]

*sigh*
We aren't communicating Dave.
I made no reference to the observable universe (in that statement).

What we have not directly observed (like live T. Rex walking around) is not the same as what a cosmologist means by "outside the observable universe."

You misinterpreted what I said, and you made a false imputation. I don't see how it benefits you to continue insisting. I don't see why I should discuss it with you until you acknowledge your mistake and stop telling me I said something that hadn't even crossed my mind

 

[marcus]

.
...Either you are suggesting that
- we can hypothesize about what is beyond our observable universe
analagous to the way
- we can hypothesize about dinosaurs and our own solar system
or you are not paying attention.
...

Dave, instead of your telling me all about what you think I said and what you think it implies, let's move the discussion in a more productive direction! Do you have anything substantive to say about the topic?

Would you like to summarize what you've said already? (about our knowledge/reasonable assumptions about what is beyond the limits of observation.)

Would you like to say briefly what you think those limits are?

 

[DaveC426913]

Dave, instead of your telling me all about what you think I said and what you think it implies, let's move the discussion in a more productive direction! Do you have anything substantive to say about the topic?

Would you like to summarize what you've said already? (about our knowledge/reasonable assumptions about what is beyond the limits of observation.)

Would you like to say briefly what you think those limits are?

No. I am unnecessarily interrupting the discussion. Carry on.

 

[Dav333]

So what could "other" be? lol something our primitive minds could never comprehend?

This topic makes me go a little crazy thinking about it.

 

[friend]

Dave, instead of your telling me all about what you think I said and what you think it implies, let's move the discussion in a more productive direction! Do you have anything substantive to say about the topic?

Would you like to summarize what you've said already? (about our knowledge/reasonable assumptions about what is beyond the limits of observation.)

Would you like to say briefly what you think those limits are?

I'd like to venture a possibility, maybe it's already been said in this thread. But each observer has a different observable horizon than any other observer by the distance that separates the two observers. The fact that both observers see the same interactions at the edge of their observability would suggest that what is unobservable to us is essentially the same as what we do observe.

 

[marcus]

So what could "other" be? lol something our primitive minds could never comprehend?
...

Not necessarily. For many of those who answered "other", the preferred choice would have been "more of the same". I gather this from several comments made already. Also it is the standard view of professional cosmologists.

It's just that the person who set up the poll overlooked that possibility and neglected to include "more of same" as one of the choices!

So those of us who go with the conventional view (as the simplest and most likely working hypothesis) had no choice but to select "other".

... would suggest that what is unobservable to us is essentially the same as what we do observe.

Friend, it sounds like you are searching for an argument to justify the "more of same" answer (which in this rather imperfect poll we have had to represent by choosing "other").

In other words if you and I could magically transport ourselves out 45 billion lightyears from here, to the matter which 13.7 billion years ago as a glowing hot gas emitted the background radiation which we are now receiving with the WMAP spacecraft , we would see that that matter had condensed into stars and galaxies similar to ours.

We would see that from that point at the edge of Earth's observable, the universe looks pretty much the same in all directions as it does viewed from earth. Same types of galaxies, with the same types of stars in them, distributed in the same more or less random way. Without a detailed map one wouldn't expect to be able to tell the difference.

This is what is generally assumed because it is the simplest, and because nothing has been observed that suggests anything else. And probably also, I suppose, because it leads to a nice mathematically simple, manageable model.

 

[atyy]

This is what is generally assumed because it is the simplest, and because nothing has been observed that suggests anything else. And probably also, I suppose, because it leads to a nice mathematically simple, manageable model.

There's interesting commentary about this http://books.google.com/books?id=uG...&resnum=7&ved=0CCAQ6AEwBg#v=onepage&q&f=false , p236.

"Today the cosmological principle still has no direct observational verification, while models not obeying the principle ... are known .... inertia in thinking and of emotional attachment to the, mathematically elegant ... However, natural sciences, ... are said to use the criterion of consistency with observation ... At the very least in order to verify the cosmological principle, alternatives have to be considered ..."

 

[friend]

We would see that from that point at the edge of Earth's observable, the universe looks pretty much the same in all directions as it does viewed from earth. Same types of galaxies, with the same types of stars in them, distributed in the same more or less random way. Without a detailed map one wouldn't expect to be able to tell the difference.

The alternative would be that the constants of nature are not constant. If they don't change sharply at the edge of observability, then we would see them changing gradually. But we don't see them changing gradually, and we don't expect to see them change abruptly. So we conclude that they don't change from one patch of observability to the next.

 

[DevilsAvocado]

Since this question can’t be answered by current theories... I guess it’s 'Carte blanche' for some personal speculations...

Many have expressed a similar view – but I would take it to the next step and say: There must be more (observable) universe, much much more. I would claim the universe must (probably) be infinite.

Why!?

To start with "much more", as marcus mentioned – it’s the only logical path. Imagine a lot of observers at the edge of our observable universe, 45 billion lightyears from the Earth. They clearly must see the same as we do, in all directions. And so on, and so forth, etc...

Any objections against this would put us back to the Middle Ages, where the Earth was the center of everything, and was flat. And the obvious question would remain – "Fall down into what?"

And WMAP has shown that our local universe is almost flat, meaning we are not looping around our local universe.

The "infinite question" is answered by the same logic. Why would we be in the center of something that looks like the cosmological principle? But in other very remote parts of the universe we would experience chaotic collisions with other (explosive) "stuff", or a strange "sharp edge", or "wall"??

The only problem with an infinite universe is the propagation of the cosmological principle (and physical laws). Some say inflation can fix this. But, as to my understanding, it must then be an "infinite inflation"...??

Well, that's my personal speculations, and nothing more.

 

[pixchips]

If I imagine that I am the center of the universe (my wife suggests that this is often the case), and everything extends only to the 45billion lyr radius, then density calculations of the universe make some sense. The total matter/energy in that sphere is pretty big, but finite. So the original singularity (or whatever) that started the BB may have been of infinite density, but finite mass. If, on the other hand, the universe is infinite, and no matter where I transport myself, I see the same thing out to 45billion lyrs, then the original singularity had infinite mass. And was doubly infinitely dense (Aleph1 ... or what?) I don't know how to think about this. Homogeneous and infinite sounds good from a modelling standpoint, but how can we ever imagine that it all started with an infinitely small spec of infinite mass? Or am I making a bogus assumption? Was the 'primordial egg' really infinitely big, and just really really dense, and just started expanding?

According to recent popular culture: "The whole universe was in a hot, dense state, when nearly 14billion years ago expansion started ..." So maybe it wasn't all scrunched into a point?? It would be great if real physical insight came from the lyrics of a silly sitcom.

Dave

 

[bapowell]

Right. Noboby thinks that singularities are physical. It's most like a breakdown of classical gravity where quantum gravitational effects are likely to become important. The Big Bang model of cosmology is just what you said: it's a model of how the universe evolved from a nascent hot, dense state.

 

[pixchips]

Okay, so the notion that there was an infinitely dense, infinite mass singularity that effectively exploded is just a holdover from the popular notion that the BB was just a really big bomb. The actual model says that the whole extent of the universe was extremely hot and dense, and once the BB started, it was expanding and cooling, and transistioning through its various epochs. This prompts the question: what was 'the whole extent' of the universe as t ->0.

Is it accurate to say that the universe started with infinite extent (even as t->0), and then started expanding? This changes the view (in my head anyway) that when radiation decoupled from matter (universe become transparent) it: a) happened within a finite sphere the represents the extent of a finite universe at about 100k years, or, b) happened everywhere at once in an infinitely extended universe whose density had fallen sufficiently. If (a), then the universe is not infinite, but if (b) it has always been infinite.

How would we tell the difference?



Dave

 

[DevilsAvocado]

... that effectively exploded ... the BB was just a really big bomb ...

pixchips, this is a BIG question for a small event...

Some 'facts':

1) No, absolutely no "big bomb". It’s a very common erroneous assumption that the Big Bang clearly must derive from the BIG BOMB! BOOOM! But that’s totally wrong. Expansion yes, bomb no.

2) We can’t tell if the (global) universe is finite or infinite (yet), and maybe we never can.

3) When cosmologists are talking about the "BB singularity" in the size of a few centimeters, they are talking about our local universe (the observable universe). (Don’t ask me where one 'buys' a ruler at BB, because I don’t know! )

4) If the universe turns out to be infinite, then (as far as I understand) the "BB singularity" must also be infinite in size. (Please, don’t ask how this 'works' because I have absolutely no idea... )

Besides that, why don’t you start a new thread on this topic, this is a little "Beyond The Original Question"...

 

[bapowell]

Is it accurate to say that the universe started with infinite extent (even as t->0), and then started expanding?

As DevilsAvocado (do I really need to call you that...can I just call you Dave or something?) says, we don't know what the global extent of the universe is. However, it's perfectly OK to imagine things as you state, operationally anyway. If the observable universe is sufficiently small relative to the actual size of the universe (if it's closed then small relative to the radius of curvature).

This changes the view (in my head anyway) that when radiation decoupled from matter (universe become transparent) it: a) happened within a finite sphere the represents the extent of a finite universe at about 100k years, or, b) happened everywhere at once in an infinitely extended universe whose density had fallen sufficiently. If (a), then the universe is not infinite, but if (b) it has always been infinite.

How would we tell the difference?

I think the main difference here is not whether the universe is finite vs. infinite, but whether the whole universe is larger than our observable universe. Decoupling happened everywhere at once (roughly) in the universe, whether it was finite or infinite. If our observable universe is 'all there is', then yes, this would coincide with having occurred within a finite sphere. However, the fact that we still see the CMB means that the CMB photons that we observe today originated outside of our horizon at decoupling. That's one way we know the difference. But, there's no way at present to determine whether the universe is truly infinite or just really #*$&@ big.

 

[DevilsAvocado]

DevilsAvocado (do I really need to call you that...can I just call you Dave or something?)

bapowell, you can call me anything you like, as long I as understand you’re addressing me (and that it’s not the 'usual' name-calling ). Dave is OK!

Anyhow, this is getting real interesting, so I think I’ll pop in for a sec...

Decoupling happened everywhere at once (roughly) in the universe, whether it was finite or infinite.

This is the magic (at least to me): happened + everywhere + infinite

I’ve been pondering 'some' over this. It’s not only a question of decoupling, it’s question of; how do we propagate the cosmological principle + the laws of physics + everything else, that’s needed for a "working universe" – (almost) simultaneously in an infinite universe – when we know that the word 'NOW' is already completely useless in our 'little' local universe...?

I’m complete and utterly lost here. Some say a really #*$&@ big inflation could fix this... but I doubt it...

Any ideas...


Edit: Maybe DAve or DA’ve is better... ;)

 

[pixchips]

Beyond The Original Question[/B]"...

Okay, I was thinking of that anyway. But it does seem that 'what's beyond the observable universe' has to have a lot to do with what actually happened when BB started. On the other hand, the eventual evolution of bunny rabbits hopping around pastural fields also has a lot to do with the BB, and I don't think you'd let me pursue that topic right here. So I'm off to a new thread as soon as I get my afternoon chores done.

...

I think the main difference here is not whether the universe is finite vs. infinite, but whether the whole universe is larger than our observable universe. Decoupling happened everywhere at once (roughly) in the universe, whether it was finite or infinite. If our observable universe is 'all there is', then yes, this would coincide with having occurred within a finite sphere. However, the fact that we still see the CMB means that the CMB photons that we observe today originated outside of our horizon at decoupling. That's one way we know the difference. But, there's no way at present to determine whether the universe is truly infinite or just really #*$&@ big.

Okay, I was afraid of this. The right answer is probably in the middle. If it is, then the universe isn't infinite, it's just really really big. And for us, that's practically the same as infinite. But when we ask, 'What's beyond the observable universe?', I still think there's a difference between 'more of the same forever' and 'more of the same, up to a point'. Seems like the latter is the best answer we can give, but may not actually be a scientific question if there's no way to test it.

Thanks much for your thoughts,
I'm off to other threads ...

Dave

 

[bapowell]

DA've,
Sorry to take so long in replying. Couple things:

Sure, there's always conceptual trouble with an infinite space. However, nothing prohibits us from defining a time slicing such that all comoving observers are inertial. Then, there's a common time that all such observers can use, and decoupling could in principle occur simultaneously everywhere (that this slicing is defined) simultaneously. In the absence of perturbations, one can argue that this decoupling could occur simultaneously in any patch that is sufficiently uniform. Inflation could be invoked to prepare such a patch. Of course, the real universe isn't that simple, which is why I said "roughly" in my original post. Perturbations cause some regions to decouple sooner than others, both for microphysical reasons and because of the good old time dilation associated with the overdensities.

 

[pixchips]

Thanks for the clarification, Bapowell. I'm interested in understanding more about the time slicing you mention. This may not be the place for such a discussion, but do you have a good reference or pointer? I will try some more googling.

Dave

 

[bapowell]

Sure pixchips. This just different language for choosing a coordinate system. It is also referred to as choosing a spacetime foliation, or foliating spacetime. There are several good reviews on the arXiv: I'm not sure what level of detail you're interested in. There have also been some discussions about foliation here on the forum -- I participated in one a while back: https://www.physicsforums.com/showthread.php?t=381521

 

[DevilsAvocado]

... But it does seem that 'what's beyond the observable universe' has to have a lot to do with what actually happened when BB started. On the other hand, the eventual evolution of bunny rabbits hopping around pastural fields also has a lot to do with the BB, and I don't think you'd let me pursue that topic right here. So I'm off to a new thread as soon as I get my afternoon chores done.

pixchips, don’t take this too seriously, it was more of a 'joke' than anything else. I for one, have done this 'hijacking' numerous times... sorry if I gave the wrong 'impression'. Let’s hope the mentors don’t notice us now...

 

[DevilsAvocado]

... I'm interested in understanding more about the time slicing you mention.

Maybe this will work:
http://en.wikipedia.org/wiki/Inertial_frames"
http://en.wikipedia.org/wiki/Comoving_observer"

 

[DevilsAvocado]

DA've,
Sorry to take so long in replying.

No worries mate!

In the absence of perturbations, one can argue that this decoupling could occur simultaneously in any patch that is sufficiently uniform. Inflation could be invoked to prepare such a patch.

Okay, that’s what’s been running around in my head for awhile... to get the almost smooth initial conditions, we need inflation to "stretch the sheet out" almost perfectly, leaving just a few "wrinkles"... but then some argue that the pre-conditions needed for this inflation to ever occur, is even more 'remarkable' than the resulting smoothness...

And if we add 'infinite-factor' to this – I do get some kind of 'wooziness'...

Some "personal speculations" to maybe get this into my head:
Could we think of the 'propagation' as some kind of a 'symmetry breaking', in the quantum-fluctuations of the 'primordial soup'...?? Like a "giant infinite lake"... the temperature is getting lower, and lower, and lower, and suddenly the whole lake is frozen, roughly at once...? (= 'symmetry breaking') ...or...??

 

[DevilsAvocado]

This just different language for choosing a coordinate system. It is also referred to as choosing a spacetime foliation, or foliating spacetime.

Excellent explanation in your post #2!

bapowell https://www.physicsforums.com/showpost.php?p=2597749&postcount=2"
As a simple analogy, allow a book to represent a 3D spacetime. Then, the 2D pages of the book are your hypersurfaces, and the page numbers define how you go from one to the next: the pages and page numbers make up your foliation.

I looked around for Foliation and found http://en.wikipedia.org/wiki/Foliation" , but it doesn’t explain it as well as you did.

Could this be considered as a picture of Foliation?

An open surface with X-, Y-, and Z-contours shown.
(Looks like two eyes or a bra... )


Reeb foliation (foliation of the 3-sphere) is illustrated as this:

Looks nice for a 'torus universe'!

 

[qwe]

What Is Beyond The Observable Universe?

the unobservable universe, aka the rest of our universe, aka the galaxies we don't see but are right outside of our sight range so they are next to galaxies we do see

What is beyond the Universe Altogether?

the bulk, because the universe is a brane :P

What is beyond the bulk?

not sure if this question has any meaning yet