What Is Beyond The Observable Universe?

[sylas]

So how are we able to observe the CMB at z ~ 1090? I perceive an ATM reply.

What is the problem? At that redshift, the wavelength is in the microwave region, at about 2mm wavelength. You observe it with a microwave receiver; nothing ATM about it.

 

[marcus]

That numbers are just about right according to calculator. Does it mean that it tends to even at C at some distant future?

Let me ask you another question about calculator. I obviously lack math behind it, but for any redshift (z), you can put any value for H(t) above ~35 and speed away is not affected, just the distance is changing proportionally. Why is that? Can you even "go trough time" with changing just H(t), or you must change Omega too?

Calimero I just this moment saw your post it is after midnight here and I need to get some sleep. Other wise I'd answer!
I found a source with exact estimates of the cosmic event horizon.
A paper by Egan and Lineweaver (excellent guy)
I was saying something like 15 Gly and in fact that was close---actually 15.7 Gly. But it is shifting slightly and in the longterm limit it will be 16.4 Gly
Yes. Asymptotically the recession speed will be c at that distance. Your intuition was correct.

I can't answer your question about the calculator right now because sleep is more important. In case anyone wants to look at Egan Lineweaver I'll get the link.
http://lanl.arxiv.org/abs/0909.3983
It is a technical paper but it has some useful numbers in an appendix at the end.

So on Lineweaver's excellent authority, if a galaxy is 15.6 Gly from us right now, then we could send them a message today and it would eventually get there! But it would take a hell of a long time.
And if the galaxy is now 15.8 Gly from us there is no way, the message would never get to them. Even if we set off a supernova they would never see it.

 

[lambdaorionis]

May I try and simplify...

Observable = Baryonic matter
Outside this is the Vaccum -

The Big bang did not cause the baryonic matter to "spread out" but it is the reaction of the vacuum which is responsible.
The big bang did not start from a singularity.

Anyone like to add?

 

[qwe]

Is it just black space extending forever? Or perhaps black space for a finite distance until another universe?

I find it hard to believe our universe is just the only universe. I don't see how it wouldn't extend for eternity instead. What is so special about our universe and the space we are in?

What do you think?

By logical necessity, there is literally 'nothing' beyond the observable universe. It is impossible to apply falsifiable predictions to something that is inherently unobservable.

In other words it is more in the realms of philosophy right now. But don't you think in the future we may be able to see beyond what is now considered the observable universe and find other universes? What do you think?

It just doesn't make any sense how this universe could be the only one. I always thought of the universe/multiverse/omniverse as infinite.

If there is 'nothing' outside the observable universe it would be just black space for eternity, right? Unless the universe is round (which all current evidence points to it being flat) then you can't arrive back in the universe in the other side. Plus most likelly space, like "time" is infinite.

don't you all mean "universe"? the observable universe extends to the Hubble sphere right, but there's no evidence that everything would just stop exactly at the line of Earth's observable universe. that'd place us at the center, which is silly. things seem like they extend for quite some space beyond the sphere we can observe right?

 

[marcus]

... that'd place us at the center, which is silly. ...

You have the right idea. It would be quite silly to limit our idea of the U to an observable sphere around us. Because models which extend beyond that can be tested by what they predict that we can observe.

Restricting our models to the immediately observable range would make them unnecessarily and unreasonably complex. Nobody does it and there would be no justification for it. So you are right. Silly is the correct word.

But you need to check the definition of the Hubble sphere, and be sure what the different horizons mean.

the observable universe extends to the Hubble sphere right,

Actually not Qwe, the observable extends way way beyond the Hubble sphere. The Hubble radius corresponds to redshift z = 1.4, ( BTW that is a proper distance of 13.7 Gly. Proper means freeze-frame, it would be what you would measure by conventional means if you could freeze the expansion process at this moment, and depends on estimated values of cosmo parameters).Anyway we see way beyond z = 1.4. We see millions of galaxies with redshifts larger than 1.4, out to more than z = 7! And the Background has redshift about z = 1090!

When we observe the microwave background we are looking at matter which is now at a proper distance of over 45 Gly. If you could freeze expansion it would take 45 billion years for a flash of light to get from here to the matter that emitted the background radiation we are now receiving. Of course that matter used to be a thousand times closer but that is how far it is now.

By definition the Hubble radius is merely the distance that is expanding at rate exactly c. If a galaxy is right at z = 1.4 then (if it is stationary relative to background) the distance to it is increasing at rate c. If it is farther, with larger redshift, then of course the distance is increasing at more than c.

The Hubble law v = Hd is defined in terms of proper distances and the current rate they are increasing.

But anyway, just to be clear, the observable is way bigger than the Hubble.

but there's no evidence that everything would just stop exactly at the line of Earth's observable universe.

That's right! It's an important point. It would be foolish to pretend the universe "stops" at a proper distance of 45 Gly (the distance today of the most distant matter we are observing.) In cosmology we work with a mathematical model and our view of the universe is inferred. The standard model certainly does not stop at 45 Gly!
It wouldn't work if it did.

The game in a math science like cosmo is to get the simplest model that fits the data. And nobody I know of uses a bounded model where the U "cuts off" at some observation limit like 45 Gly (proper).

A few (pretty clearly misguided) people might say we OUGHT to only consider the universe what we can see in real time----mathematical inference based on model-fitting should not be allowed, they imply.
Heh heh, but almost NOTHING is being eyeballed real-time. A dragon could have eaten everything out beyond redshift z = 0.1 yesterday and we wouldn't know it for a long time!
Almost everything we can say is based on inference using a model (which has been tested every way we can think using all the data we can scrape up!)

We infer how things are nowadays from light we are getting now that was mostly emitted long ago. Strictly speaking that is mathematical model based inference, and relies on the simplicity of the model. You could always postulate a dragon who has moved things around or eaten a bunch of stuff, so that the real U is different from what we infer, but putting in the dragon would complicate the model.

Same way with putting in a boundary with a cutoff at some distance like 45. It would just make things more complicated and there is no reason to do it.

 

[Flatland]

In other words it is more in the realms of philosophy right now. But don't you think in the future we may be able to see beyond what is now considered the observable universe and find other universes? What do you think?

It just doesn't make any sense how this universe could be the only one. I always thought of the universe/multiverse/omniverse as infinite.

If there is 'nothing' outside the observable universe it would be just black space for eternity, right? Unless the universe is round (which all current evidence points to it being flat) then you can't arrive back in the universe in the other side. Plus most likelly space, like "time" is infinite.

If the universe is infinite then there will just be more stars and galaxies beyond the observable universe.

 

[marcus]

If the universe is infinite then there will just be more stars and galaxies beyond the observable universe.

I agree, Flatland. The basic premise in cosmology is matter distributed uniformly. The so-called "Cosmological Principle" that Einstein stated in a 1917 paper on cosmology. Then restated in a paper in 1931. It was first called that in 1935, I believe

As long as we have no reason to think otherwise, we assume the Universe is just more of the same in every direction. And that makes the models simple, because no edges to worry about.

It's kind of a Copernican idea---that we aren't special and our location isn't special. But it's more important than just ideas, it makes the math simple and makes the model work right. And it continues to check out pretty well, at least to first order approximation. Can never be sure of course.
=========================
A good exercise in this connection is to use one of the online calculators to find the distance to the surface of last scattering. This is basically the location of the matter that emitted the radiation we detect as microwave background (the socalled CMB).

The CMB has an estimated redshift z = 1090. So for instance you google "cosmo calculator" and get Ned Wright's online calculator and put 1090 in the z box.
You get that the proper distance (the kind that goes into Hubble law) is 45.6 billion light years. The light travel time (a time, not a distance) comes out to be 13.7 billion years. That is sometimes called the "look-back time". Because of expansion, the lookback time is not readily convertible into the distance and shouldn't be confused with it.
Here's a quote from the calculator output:

=====quote Wright's cosmo calculator==

For H_o = 71, Omega_M = 0.270, Omega_vac = 0.730, z = 1090.000

It is now 13.666 Gyr since the Big Bang.
The age at redshift z was 0.377 Myr.
The light travel time was 13.665 Gyr.
The comoving radial distance, which goes into Hubble's law, is 13995.7 Mpc or 45.648 Gly.
===endquote===

 

[Chronos]

Most modern cosmological models limit the extent of the observable universe to one that has an observed [and model dependent] limit of about 13.7 billion light years from earth. At that point your hit a wall called the surface of last scattering. We are currently unable to observe more ancient/distant entities. There are theories suggesting there may be more than we can currently observe, but, none have observational support to date. Marcus is better acquainted and frequently posts on this very interesting topic.

 

[jreelawg]

Let's rephrase the question. What is beyond the observable everything? That doesn't even make sense. There is no observable universe. The universe includes both observable and the unobservable. Taking all we see and calling it the universe is dumb. Lot's of cosmologists do this and it is annoying. Some of them have the decency to at least say observable universe which as I pointed out makes no sense also, but at least I can understand what they intend to say.

 

[marcus]

Let's rephrase the question. What is beyond the observable everything? That doesn't even make sense. There is no observable universe. The universe includes both observable and the unobservable. Taking all we see and calling it the universe is dumb. Lot's of cosmologists do this and it is annoying. Some of them have the decency to at least say observable universe which as I pointed out makes no sense also, but at least I can understand what they intend to say.

I agree that it is silly--or as you say, "dumb"--to say universe when what you mean is just the observable portion. My favorite cosmology prof at Berkeley was very careful about this---always said "observable universe" when he meant just the that limited portion. My impression is that most others, professionals, are equally careful but I couldn't swear to it.

It strikes me as either misleading or crackpot to insist on a nonstandard terminology, where only the directly visible portion is treated as the whole. Except when talking to journalists or to a lay audience I can't think of any working cosmologist who makes that mistake.
I could be wrong though, you may have encountered that more than I have.

We do get a small amount of it here at PF though. Most people are not confused, I hope.
A kind of mild ineffectual trolling---best thing could be to simply ignore.

 

[MotoH]

Wouldn't the non-observable universe be everything and nothing? Since electrons can take all paths to a given point, and only chooses a specific path once it is observed, shouldn't anything beyond our observations be either here nor their?

Am I out of line in believing this?

 

[marcus]

Wouldn't the non-observable universe be everything and nothing? Since electrons can take all paths to a given point, and only chooses a specific path once it is observed, shouldn't anything beyond our observations be either here nor their?

Am I out of line in believing this?

Well you could be getting quantum mechanics mixed up with classical concepts here

Words have different meanings in different contexts. The observable universe in cosmology is a CLASSICAL idea (not quantum mechanical). It is a big spherical region a ball with us at the center. Full of all the matter from which we could be currently getting light and other signals (neutrinos, gravity waves...)

Or in 4D you can think of it as contained in our backwards lightcone.

=======================

In Quantum Mechanics, what passes for observation does not need to involve humans, or even consciousness. It is a very general idea. A particle can be forced to choose simply by interacting with other particles---with its environment.

For us, the most distant matter that we get light from is at the socalled "surface of last scattering"---currently about 45 billion light years away.
That's the distance you would measure if you could stop the expansion process now and use conventional means. Sometimes called the now distance, or the current proper distance.

Don't think of the stuff that is out beyond 45 Gly as somehow "fuzzy" just because WE are not out there to see it We aren't that important.

 

[hurk4]

existence, whatever that is!
hurk4

 

[marcus]

existence, whatever that is!
hurk4

Heh heh, good to hear from you, Hurk!
Sounds like you hit the nail on the head.

Or how about this: "what is beyond the observable universe?"
"The unobservable universe."

 

[iflexit_1]

Perhaps an easier explanation is available, premised in GR. In General Relativity, space is dynamic. It can expand, shrink and curve without being embedded in a higher-dimensional space. In this sense, the universe is self-contained. It needs neither a center to expand away from nor empty space on the outside (whatever that is) to expand into. When it expands, it does not claim previously unoccupied space from its surroundings.

 

[Chronos]

If you see it, it is by definition part of the observable universe. The 'unseeable' parts of the universe are, and will always be observer dependent.

 

[aashay]

I think it makes no sense in thinking what is there beyond the observable universe as we cannot visualize it. It is like an ant on a surface of an expanding balloon who cannot imagine anything outside the surface of the balloon.

 

[DarkStar7]

The edge of the Universe is where all matter is expanding away from us at the speed of light. Any energies lost over this universes 'event horizon' is returned back into the quantum realm to drive microbangs that produces dark energy inflation along with all the other properties of the excepted standard
model.


I think if this were true dark energy and an excess cosmic microwave radiation should be more concentrated near large black holes such as our Milkyways super massive one. Is there such evidence?

 

[bapowell]

Most of the accepted hypotheses regarding the origin of the universe (U) refer to it as if we could take a God's Eye view and see it from the perspective of an outsider. For instance, Alan Guth's Inflation postulates an inflaton particle that came into existence as a statistical necessity because it could do so in an extremely high energy state which could then decay into the U that we see. For this to happen, there must have been some sort of meta space and meta time, a metaspacetime.

What do you mean by metaspacetime? How does that follow from your previous point? Low energy inflation can certainly be implemented in the early universe, although many would argue that it's not as natural as high energy inflation.

 

[bapowell]

Where is (or was) the Inflaton particle? We are speaking of it as if it had an existence of its own before it decayed into our universe. If so, there was and is a whole set of dimensions beyond this universe that I call metaspacetime. don't let semantics get in the way. It doesn't matter what we call it. Pythagoras said "All is Number." If we can represent it mathematically, it is real. Somewhere, somewhen.

I think you're misunderstanding the premise of the inflationary universe. The inflaton didn't decay into our universe, rather, it is a field that lived in our universe. It is typically modeled as a quantum field, just like any other. Linde's early chaotic inflation model takes the stage soon after the standard hot big bang. It postulates that the inflaton field existed everywhere in space in the early universe (along with all the other matter and energy from the big bang), with an energy density stochastically distributed in space. Regions of high relative inflaton energy density underwent inflation. The small inflationary patch grew in size to a an empty, homogeneous, essentially flat region. The inflaton field then decays into radiation. This is now the hot big bang from the perspective of the inflated region. In other words, the inflationary patch that eventually grew to form our observable piece of the universe was itself just an ordinary region of space already existing in the universe soon after the big bang.

 

[bapowell]

Lots of physicists still resist the quantum concept as it may apply in the macroscopic world,

Isn't this what decoherence addresses?

 

[bapowell]

Decoherenc is another word for collapse of the quantum waveform upon observation. But, decoherence is just an interpretation according to the Copehagen convention. Wheeler and his students would opt for superposition and divergence of some of the "many worlds" upon any such observation. But is any of this testable?

I don't know if it's testable, that's not the point I was refuting. You claimed earlier that physicists resist the application of QM to the macroscopic world. I was pointing out the decoherence as it applies to the 'qauntum-to-classical' transition is an example of how quantum mechanics is applied to macroscopic objects. I agree that there are several competing interpretation of exactly what all this means, but that's a different matter entirely from whether or not QM, at least phenomenologically, is applied to the macroscopic world.

 

[hurk4]

Heh heh, good to hear from you, Hurk!
Sounds like you hit the nail on the head.

Or how about this: "what is beyond the observable universe?"
"The unobservable universe."

Dear Marcus, thank you for your kind remark.
My answer "Existence" was the most general I could imagine, but what can we do we this "non practical" remark, I guess almost nothing.
I could eventually fill in more details at the price being speculative.
So I could for instance add "coherence with our observable universe".
Glad to see that Martin Bojowald came with a book "before the big bang" in german language also translated in dutch language year 2009. This of course is his view on what is beyond.
I must also remark to you that the (our) unobservable universe is partly observable by a virtual observer near the edge of our observable universe and so on until there might eventally/(inevitably?!) be a real event horizon for our domein of the universe .
kind regards,
Hurk4

 

[ChrisHG23]

Hi,
This will be my first ever post. and I apologise of its wrong but,...

It seems to me the universe is only flat because like fish in water we are only able to perceive our realm, and not the whole universe in which our watery realm exists. Outside of the observable universe is the rest of time, either left behind, or yet to come. I see the universe as an infinite singularity in which our experiences are relative points formed from the balance between the infinite expansion and contraction of time, space and the universe as a whole. The Big Bang was also a Big Crunch and it's is still happening beyond the boundary of the 'Observable Universe'.

To me 'we' are still shrinking, why else would galaxies spin so fast and Black Holes get so small. This is a product of The BigBangBigCrunch. If THE infinite nothing that has no scale, then the primordial universe could not be limited to form or scale as it would have the infinite spectrum of form and scale to 'evolve', 'grow', 'expand' or 'contract', and it is this relative part of time and space we call home, our universe.

 

[bapowell]

Hi,
This will be my first ever post. and I apologise of its wrong but,...

It seems to me the universe is only flat because like fish in water we are only able to perceive our realm, and not the whole universe in which our watery realm exists. Outside of the observable universe is the rest of time, either left behind, or yet to come. I see the universe as an infinite singularity in which our experiences are relative points formed from the balance between the infinite expansion and contraction of time, space and the universe as a whole. The Big Bang was also a Big Crunch and it's is still happening beyond the boundary of the 'Observable Universe'.

To me 'we' are still shrinking, why else would galaxies spin so fast and Black Holes get so small. This is a product of The BigBangBigCrunch. If THE infinite nothing that has no scale, then the primordial universe could not be limited to form or scale as it would have the infinite spectrum of form and scale to 'evolve', 'grow', 'expand' or 'contract', and it is this relative part of time and space we call home, our universe.

I'm going to assume that this is a joke and that you aren't really looking for an intelligent response to this...

 

[DaveC426913]

I'm going to assume that this is a joke and that you aren't really looking for an intelligent response to this...

I do not think it is a joke (that's not very nice - he did say it was his first ever post), but Chris should review the PF rules on posting overly-speculative ideas.

Overly Speculative Posts:
One of the main goals of PF is to help students learn the current status of physics as practiced by the scientific community; accordingly, Physicsforums.com strives to maintain high standards of academic integrity. There are many open questions in physics, and we welcome discussion on those subjects provided the discussion remains intellectually sound. It is against our Posting Guidelines to discuss, in most of the PF forums or in blogs, new or non-mainstream theories or ideas that have not been published in professional peer-reviewed journals or are not part of current professional mainstream scientific discussion. Personal theories/Independent Research may be submitted to our Independent Research Forum, provided they meet our Independent Research Guidelines; Personal theories posted elsewhere will be deleted. Poorly formulated personal theories, unfounded challenges of mainstream science, and overt crackpottery will not be tolerated anywhere on the site. Linking to obviously "crank" or "crackpot" sites is prohibited.

https://www.physicsforums.com/showthread.php?t=5374

 

[Ray Palmer]

New to forum - what lies beyond the stuff that lies beyond the edge of the universe? The recent "dark flow" observations are perhaps some circumstantial evidence for other branes. However, if we are just holographic projections then a richer reality lies beyond.

 

[hurk4]

Observable?
Observations on systems near the edge of the (our) observable universe could give information about the observable universes of those systems. Their OU's partly overlap with our unobservable universe (UOU). So IMO our UOU can be indirectly be observed?
regards,
hurk4

 

[SheldonCooper]

This is a fascinating question. I think the answer to the poll at the beginning of this thread has to be "C - Other", since we cannot possibly hypothesize what we have never observed, not just with our senses, but also with our math. Garrett Lisi, Steven Hawking, LHC, and the rest are all doing important work, but the real interesting questions may still remain even after Higgs is explained. How did this singularity with such enormous potential just appear in this infinite void in space/time? Think about it; really think about it. Things are going to get very interesting very soon I think. Right Rhody boy?

 

[marcus]

...since we cannot possibly hypothesize what we have never observed...

?

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

We use logical and mathematical models all the time to reconstruct the past, and to infer conditions in regions where we have no direct sensory knowledge.

There are no absolute certainties, but people can weigh the evidence in good faith and search for consistent economical explanations.

 

[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.