Does the Universe have a Boundary ?

[Aquafire]

Subject: Does the Universe have a Boundary ?


Since nothing can exist outside of the Universe, how then can the Universe have a boundary in any conventional sense?

Surely, if time before time is considered potentially unfathomable; in similar vein to speak of a boundary to the Universe, seems illogical.

Then again, if we accept the premise, then we strike the first of quandaries; for if the Universe is limited, ( re the extent of expansion since the 'Big Bang') then that implies an outer edge along which the Universe is expanding into nothingness.

No matter how I dissect this matter, it only seems to raise more problems than answers.

So, I put it into the hands of other posters.

Does the Universe have a boundary ?

Can anyone answer this definitively ?

Aquafire

 

[marcus]

Then again, if we accept the premise, then we strike the first of quandaries; for if the Universe is limited, ( re the extent of expansion since the 'Big Bang') then that implies an outer edge along which the Universe is expanding into nothingness.

this again! It seems to be Public Confusiuon Number One.

We mostly discuss standard bigbang cosmology at this forum, which does NOT imply that space has a boundary.

the part of space that we are receiving light from is limited by a shifting horizon----beyond that horizon light hasn't had time to reach us yet---but as more light has time to reach us this "observable" part includes more and more galaxies. the whole of space is way bigger than the portion we have so far gotten light from and are able to see

you shouldn't think of horizons (temporary limits to observation) as physical boundaries.

as far as we know what is currently outside our sight is the same average picture we see already---same old universe just more of it.

the whole shebang might have finite spatial volume (but no boundary, analogous to a ring) or it might have infinite volume (and no boundary, analogous to a infinite straight line)

there are exotic cosmologies (which some String thinkers dream up) where there is some kind of boundary separating us from regions of space with different versions of physics. this is more a fantasy. it is not mainstream cosmology.

let's forget exotica and focus on the mainstream picture---ordinary bigbang cosmology.
this does NOT assume the whole universe started off from a point (THAT IS A LIE TOLD BY POPULARIZERS WHO WANT TO SELL BOOKS, get it out of your head)

in ordinary professional cosmologist's picture the universe during bigbang could have had infinite spatial extent and no boundary (analogous to a line), or it could have had finite extent but no boundary (analogous to a ring). either version works.

if it was infinite then, it is infinite now
if it was finite then, it is finite now
in either case space is so large (compared with the currently observable part) and so challenging to measure that WE CANNOT YET TELL if it is finite or not.

in neither case does the standard picture have a boundary

 

[S1nG]

I've seen a similar question somewhere before. Just that i'll have to dig it up. Well as what Marcus said space itself has no boundaries, even before the bigbang, space had already existed. Just that its only after the big bang that time and matter started to come in place.

In fact, when you keep thinking that space is finite. Then that's the reason why that would raise more problems than answers in your question.

 

[Ratzinger]

What I don't get is the infinite volume version. I thought there is finite number of particles in the universe and also that spacetime came into excistence with the big bang and space expanded thereafter.

How does the infinite version fit in.

(I have no problem with a closed, curved universe with the balloon analogy, but infinite volume, don't get it.)

 

[marcus]

What I don't get is the infinite volume version. I thought there is finite number of particles in the universe and also that spacetime came into excistence with the big bang and space expanded thereafter.

How does the infinite version fit in.

(I have no problem with a closed, curved universe with the balloon analogy, but infinite volume, don't get it.)

Your Holiness, the mainstream majority of working cosmologists have never committed to the idea that there are only a finite number of particles

the standard LCDM model essentially comes in two flavors, the flat (normally assumed to be spatial infinite and beginning with an infinite bigbang)
and the positive curved (the finite one you understand). They didn't decide yet which is closer to Nature.

a certain number of science journalists and popularizers have LIED to the public and imprinted people with the idea that cosmologists say the bigbang occurred at a point and involving only finite number of particles. they have to lie because they want to SELL and they can't sell if the public can't picture what they are saying.

anyway that's all done now and no use crying about it. We still don't know which is right. MAYBE IT IS FINITE.
The lie is when some journalist gives people the idea that cosmologists have made up their mind in favor of finite. They have not yet made up their mind.

finite could turn out to be right, eventually, so in some sense the journalists and the public would be exonorated.

 

[zankaon]

does the universe have a boundary?

does a hypercube (4-cube) have 3-faces and 2 hearts!

 

[Ratzinger]

many thanks, Marcus! I completely didn't know this.

 

[Ratzinger]

Also this thread with EL remarks has just clarified my thinking of infinite space.

I believe one great but often unnoted aspect of PF are the archives and the search function. There so many great explanations and thoughts accumulated here!

 

[Bob Elston]

No.. Nothing never ends

As simpel as this sounds it is the answer... How can nothing end... what would be on the other side... of the boundry? :!)

 

[Chronos]

Pardon the interruption, this is nonsense. Put your facts on the line before asserting conclusions . . . Oh, I see you have no undisputed facts . . . science does not work that way.

 

[marcus]

As simpel as this sounds it is the answer... How can nothing end... what would be on the other side... of the boundry? :!)

Hi Bob, welcome to PF! I see this is your first post. I think you may be essentially right---or at least in harmony with the mainstream scientists who work in cosmology.

I'm a "science-watcher", not an authority. So I just report what's happening in the lines of research I watch and speak my opinion. But when I think over all the mathematical models that quantitative cosmologists have tried to fit to the data and match the universe, they NONE of them had any boundary except at places where they broke down.

In other words, as far as I know, nobody wants to have a boundary on their model of space or spacetime either. the only time they get a boundary-like limit is where something blows up

some models, if you evolve them back in time they keep on fitting the data reasonably well for a while and then at a certain point they go haywire and fail to compute sensible answers. that is not a clean boundary, but it does represent a LIMIT TO THE MODEL. so it is in a way like a boundary.

a breakdown of a mathematical model is called a singularity. like the function f(x) = 1/x has a singularity at x = 0 because it goes off to infinity when you try to divide by zero. the usual model of spacetime around a black hole has a singularity at the pit of the black hole-----so people are trying to fix that by developing models of a black hole which don't break down and might correspond better to what nature actually does. but so far it is an unresolved problem.

So I think you are RIGHT in the sense of being in accord with conventional quantitative working cosmologists. they don't think of spacetime as having boundaries------in any case not clean well-defined ones.
And I think the REASON they don't put a clean well-defined boundary in their models is essentially the reason that you gave!
they simply cannot imagine what could be on the other side!

but I must hasten to stress that I might be wrong about this. this is simply my opinion. We have lots of differences of opinion here, for the most part courteous and civil.

 

[castlegates]

this again! It seems to be Public Confusiuon Number One.

This is because it hasn't been answered to the satisfaction of the many members here. If you are a no boundry proponent, then it behooves you to ask ... what have I missed that makes this so hard for others to understand? Obviously the explanations aren't cutting it, and for good reason IMO. When a big Bang advocate uses phrases like "when the universe was the size of a grapefruit", others cannot accept these advocates wish to say that the universe has no boundry, when they just use one to describe it.

We mostly discuss standard bigbang cosmology at this forum, which does NOT imply that space has a boundary.

Actually it is all we discuss. All other threads are locked.

the part of space that we are receiving light from is limited by a shifting horizon----beyond that horizon light hasn't had time to reach us yet---but as more light has time to reach us this "observable" part includes more and more galaxies. the whole of space is way bigger than the portion we have so far gotten light from and are able to see

The CMB is the end of the line ... is it not? Although we don't know if or when the last microwave enters our picture. We simply don't know.

you shouldn't think of horizons (temporary limits to observation) as physical boundaries.

Have to agree here, but that's not what's being discussed.

as far as we know what is currently outside our sight is the same average picture we see already---same old universe just more of it.

You don't know that, it is inferred, which in turn begs the question.

the whole shebang might have finite spatial volume (but no boundary, analogous to a ring) or it might have infinite volume (and no boundary, analogous to a infinite straight line)

And the whole shebang may have a boundry. Why did you leave that possibility out?

there are exotic cosmologies (which some String thinkers dream up) where there is some kind of boundary separating us from regions of space with different versions of physics. this is more a fantasy. it is not mainstream cosmology.

This does not exclude the possibility.

let's forget exotica and focus on the mainstream picture---ordinary bigbang cosmology.
this does NOT assume the whole universe started off from a point (THAT IS A LIE TOLD BY POPULARIZERS WHO WANT TO SELL BOOKS, get it out of your head)

These popularizers are not telling lies. They just don't know of any way to explain a no boundry universe ... no one does. Which once again begs the question.

in ordinary professional cosmologist's picture the universe during bigbang could have had infinite spatial extent and no boundary (analogous to a line), or it could have had finite extent but no boundary (analogous to a ring). either version works.

Could have had? Well which is it? Or could it be none of the above?

if it was infinite then, it is infinite now
if it was finite then, it is finite now
in either case space is so large (compared with the currently observable part) and so challenging to measure that WE CANNOT YET TELL if it is finite or not.

Exactly! We just don't know.

in neither case does the standard picture have a boundary

In this case you choose to ignore the obvious, by putting up boundries that clearly are not there. Pun intended

 

[Bob Elston]

More questions

Wow! (Words Over Wires) Ok, here is my formula 0=0 or =o= ...lighten up. The bigger question, to me, is how small can something be? What if you cut a string (as in String theory) and what is a brane (M-theory) made of? What is Atomic Expansion vs. Gravity? The biggest thing in the universe may be the ego.:!)

 

[marcus]

... When a big Bang advocate uses phrases like "when the universe was the size of a grapefruit", others cannot accept these advocates wish to say that the universe has no boundry, when they just use one to describe it.

That is where the popularizers mislead you. they should never have said "when the universe was the size of a grapefruit"

Personally I don't like grapefruit, but it would be OK if someone says "when the OBSERVABLE universe was the size of a grapefruit". Then there is no boundary, only an horizon. That means when all the stuff which we have so far gotten light from, which is just a part of the whole, could have been contained in a volume the size of a grapefruit.

Or, if they are not talking about the observable but about the whole thing---which we can only make inferences about by measuring curvature and by modeling what we are able to see----if they are talking about the whole thing then they shouldn't say grapefruit

because spatially the whole thing is normally assumed to be either infinite (and a grapefruit is not infinite) or else something like S³ which we don't yet have a non-techie word for in English. The technical word is "three-sphere"-----or something else but those are the two leading pictures

The CMB is the end of the line ... is it not? Although we don't know if or when the last microwave enters our picture. We simply don't know.

Observationally the CMB is not the end of the line. Primordial neutrino background is almost certainly there and waiting to be observed when people get neutrino detectors that work in the right range.

The CNB neutrinos come from the first second. The CMB comes from when the universe was 380,000 years old.

If CNB neutrinos are NOT observed when detectors are built that should see them, then there will be a huge uproar in cosmology . Not seeing them would have to cause a revolution.

If they are seen, then people can study the CNB spectrum and map it etc just like they study the CMB now and learn more stuff.
================

In your post you oftentimes say "We simply don't know."

That is par for the course. Think about it. NO SCIENTIFIC THEORY IS EVER PROVEN TO BE TRUE. All that happens is that theories get continually tested and eventually they predict something that isn't observed and get proven false.

then an improvement replacement is devised which can withstand all the other tests plus that additional one.

it is a kind of darwinian thing---or a "last man standing" except it never ends.

survival of the best fit.

When I talk at Cosmology forum, I do the best I can to report the STANDARD COSMOLOGY CONSENSUS which is based on 1915 GENERAL RELATIVITY.
And I try to leave room for some improvements to General Relativity which are under development----quantum gravity.

Do I BELIEVE 1915 general relativity? Of course not! It has singularities, places where it breaks down. Therefore it can't be right and will eventually be replaced. (I guess if I have a belief it is in this process of testing and replacement where the laws and models are gradually improved.)

I don't have time to continually be telling you my philosophy of science, which includes a deep sense of the limitations of human knowledge. I'm very interested in what are the best models so far and I want to talk about that, not philosophy.

So when people come and say "what can we know?" :zzz: "how can we be sure, we are mere finite animals on a little planet?" I just find it boring and mostly ignore it. Skepticism doesn't tell me anything I don't know already.

The amazing thing is how well the models work.

What we get a lot of at PF is INCOMPETENT skeptics who are unable to see how well the standard LCDM model works because of a FAILURE OF IMAGINATION. Either they can't imagine a big bang with infinite spatial extent, or they can't imagine a three-sphere, or some mathematical thing just boggles them.

My attitude is I don't care if someone rejects the LCDM----I don't believe in the standard model myself. What I don't accept is when someone rejects the LCDM because of a damn GRAPEFRUIT.

We have an amazingly good provisional model that matches all kinds of data better than we had any right to expect, so I don't like to see it rejected for the wrong reasons or out of simple ignorance.

If you understand the conventional mainstream model and then go off on your own and find some other model you like better that's fine. And about saying "we don't know" all the time---OF COURSE we don't know, just don't preach us sermons about it
======================

For me, the hottest thing in cosmology right now is the fact that Reuter has presented a model that gets just the right amount of inflation in the early universe without having to concoct any exotic "inflaton" field or arbitrary "slow-roll" potential. he gets the amount that people decided was needed to fit the data and he gets it without adding any JUNK.

he is also able to explain some other things. It is a minimalist approach that somehow gets a lot out of meager assumptions. He goes beyond 1915 GR, but only in a way that was well-established by people like Feynman in other branches of physics a long time ago. So only in a very careful conservative way. If you want to learn about cosmology beyond the conventional consensus LCDM, then my suggestion would be to forget this boring boundary business and look at stuff like Reuter that is happening in the field.

 

[marcus]

Wow! (Words Over Wires) Ok, here is my formula 0=0 or =o= ...lighten up. The bigger question, to me, is how small can something be? What if you cut a string (as in String theory) and what is a brane (M-theory) made of? What is Atomic Expansion vs. Gravity? The biggest thing in the universe may be the ego.:!)

Well you sound cheerful this morning! Hello.
The usual answer is the Planck length.
(this is not about string or M----it goes back to a paper of max Planck in 1899)

the estimate is still standing after more than 100 years.

essentially the argument is something like this (maybe someone else will provide an alternative) the idea of length has the idea of being able
to MEASURE a length in it. in a practical operational sense it doesn't mean anything unless you can compare two lengths and say bigger and smaller.

the Planck length is 1.6 x 10^-35 meters and there are quantum mechanical (1926) reasons why nobody has been able to propose a method for comparing lengths smaller than that. even in a thought-experiment it doesn't work.

it has to do with the inherent fuzziness of the world and if you try to fight fuzziness by making things more massive or energetic (like accelerators use higher energy to probe microscopic structure) then you get to a point where you are concentrating too much mass (or energy) into too small a space and the fabric of space will not support it and it goes *glup*.

measuring smaller than Planck length gets caught between the rock and the hard place----between the fuzzy indeterminacy of the one hand and the black hole rupture strength on the other.

Planck is interesting because he calculated the Planck length back in 1899 many years before there was any proper sense of quantum mechanics or Heisenfuzz uncertainty. he had an intuition about it.

 

[jimgraber]

even earlier natural units

Google “Natural units before Planck” and you will find George Stoney used similar units as early as 1874. This was mentioned last week by one of the speakers at the IGC Inaugural conference at Penn State, I forget who.
They differ from Planck units by powers of alpha, the fine structure constant, approximately equal to 137, and were deduced from the electron charge even before Planck's constant was known.

Jim Graber

 

[amitrajkhowa]

Finitude or infinity of space is a thing of the mind. The human mind is not capable of thinking in a different way and that is why we tend to set a mental boundary to the universe and try to visualize what lies 'outside'.
But for all we know, there need not be an outside to everything we see.

 

[setAI]

That is where the popularizers mislead you. they should never have said "when the universe was the size of a grapefruit"

exactly- they should emphasize that they are only talking about the matter we can currently observe within the Hubble Volume being within a small volume-

I think a lot of this confusion would go away if the popular literature would simply say that the universe was aprobably already infinite at the time of the big bang- and was in an infinitely dense state that expanded- this gif illustrates it perfectly:

http://www.atlasoftheuniverse.com/bigbang.gif

 

[Mentz114]

setAI said :

I think a lot of this confusion would go away if the popular literature would simply say that the universe was aprobably already infinite at the time of the big bang

This is not the case with the 'standard' FLRW model, where a(0) = 0 gives an infinite energy density and no spatial extent at all. Have you got a reference for the model you are drawing on ?

 

[Wallace]

setAI said :

This is not the case with the 'standard' FLRW model, where a(0) = 0 gives an infinite energy density and no spatial extent at all. Have you got a reference for the model you are drawing on ?

Misconception upon misconception...

The 'standard' Hot Big Bang model says nothing at all about t=0 and certainly nothing about a=0. The standard Hot Big Bang model starts with a hot dense infinite universe that undergoes a brief period of inflation, followed by many other interesting phases of evolution.

The greatest misconception about the Big Bang that people have is that the theory is something that is focused on explaining some moment of creation. In fact it is a theory that describes the evolution of the Universe from a dense hot state to cooler less dense states as it expands and says nothing about how and why the moment of creation occurred or indeed whether there was a particular moment when it happened. This seems somewhat inadequate which is why people are now looking at extensions to the present theory that deals with the thorny pre-inflation issues, but this is not standard Big Bang theory, these are extensions to it.

The Big Bang is a terrible sign for the theory it signifies but history has stuck us with it.

 

[Haelfix]

Well, there is a singularity at t=0 in the FRW universe, as the curvature goes infinite, but yes it was never meant to be used as a model anywhere near that regime, and instead is simply a description of things at some finite time afterwards when things have settled down.

I don't know what's so shocking about this statement. Who really expects the universe to be homogenous and isotropic in a decidedly quantum regime anyway? I'd say the reverse is true. Its rather shocking that the FRW model does in fact do *so* well at predicting the correct experimentally verified observables (big bang nucleosynthesis, relic CMB backgrounds etc).

I'd say that's the primary virtue of inflation, in that it explains the unreasonable effectiveness of the FRW solution.

 

[Wallace]

But the problem is that for those not fully versed in the details of modern cosmology, the name 'the Big Bang' makes it seem like the 'meat' of the theory is in explaining the first instant, the explosion, the 'Bang' that started it all. It's kind of hard to explain that the Hot Big Bang model explains everything reasonably well apart from the Bang part!

 

[marcus]

the name 'the Big Bang' makes it seem like the 'meat' of the theory is in explaining the first instant,

good point. standard cosmology is not a 'Big Bang' theory---it is an expanding universe model called LCDM

... model explains everything reasonably well apart from the Bang part!

exactly. It is a bad semantic situation, but it's hard to reform popular language. (Lineweaver has been one of the boldest about trying to get the public misconceptions straightened out----but for every Lineweaver you have two or three slop-mongers.)

Maybe we just have to accept the status quo popular language.
I expect eventually some version of quantum cosmology will take hold (there are already signs of a convergence of various approaches) and that will deal with the cosmo singularity and various other puzzles. So there finally WILL be a "big bang" theory----the missing detail in LCDM will be resolved.

 

[moon77]

universe has boundary?

at time time of bigband space of universe converged to point and time to infinite; coz according to special theory of relativity, space and time are interchangable as mass and energy. now the universe is expanding and time is converting into space. so boundry is meanigless concept. universe will expand till time becomes zero and then it might contract to point. space or time are just way for velocity to travel. we measure space in terms of time and velcocity of light and vice versa. space and time was apart from matter. and matter cannt exit without space-time and vice versa. therefore universe has no boundary. where matter and light can't reach there is no universe or space-time.

ram chand
+92 333 2533175

 

[Wallace]

I wonder if moon77 would pass the Turing test?

 

[Mentz114]

Moon77 says:

now the universe is expanding and time is converting into space

Everlasting Lorentz boost. That makes no sense to me.

 

[bungydiver]

While I'm very impressed with some of the replies to this thread and the obvious knowledge of math and physics that some readers possess, I remain a hopeless layman who has long pondered the question of "what is there on the other side of the "boundary?"

Whether thinking of the universe in a linear mode...starting at A and proceeding to infinity, or considering it as a ring or balloon, these concepts all seem to have to exist within a framework and therefore, would have some area that also exists outside that framework.

When considering the physical universe as opposed to a mathematical concept, I cannot comprehend the value of infinityas there must always be a point that would exist beyond the physical limit.

The only (perhaps silly) postulate I have been able to make that would satisfy the idea of no boundaries or "other side" would be that there is no such thing as a physical universe, but instead what "is" is an amorphic "state" similar to an idea or thought...those can in fact be limitless because they occupy no physical space nor respond to any rules of time limits.

Of course, this then raises all kinds of questions regarding how we can commonly perceive the same object...possibly explained by a collective consciousness that functions from some single root or essence. I'm not speaking in a religious or spiritual way here. There appear to be certain "givens" in our perception of "things." For instance, an average apple would be commonly agreed to be red, roughly round, somewhat hard to the touch, etc. However, what we don't know is how exactly each consciousness truly perceives this apple...what shades of red, what the "taste" is, what it feels like, it's exact size relative to other things and so on. These would probably be somewhat different in each cnscious experience...once again, in my mind, making room for the postulate that the apple doesn't really exist in any absolute form of its own but rather in our perception of what it is...an idea.

Any thoughts or takers on this thread?

 

[marcus]

You are making too much work for yourself.
The balloon surface universe is a purely 2D analogy for 3D space.

All existence is concentrated on the surface, there is no rubber. There is no air or space inside the balloon. There is no room outside the balloon. You have to concentrate your mind so that you can imagine all existence on the surface.

The beings there must be flat 2D animals, to exist there. Then you should imagine experiencing that 2D space, which contains all of existence, as one of them. Move around. Explore it. Measure angles and distances if you want, as a creature in that 2d world.

Of course there is no boundary to space, in that world. Imagine going looking for it. You will not find it. Point in any direction, it is not there.

Where you mess up is you don't use the analogy itself, you put extra junk into the picture. The space inside. The space outside etc etc. All that is extra. The rubber itself is extra. Don't clutter up your mind with extra stuff that doesn't belong to the sphere surface universe. All that belongs in the model is the 2D surface. Think of an abstract sphere surface, if that is easier for you.

 

[Naty1]

Interesting discussions...I completely agree with the post about the big bang not including time = 0...no theory works there (yet) and as I believe Marcus commented, that's a sign relativity and QM as currently understand can't be completely accurate...In fact that is likely one form of "boundary" problem.

If you are a no boundry proponent, then it behooves you to ask ... what have I missed that makes this so hard for others to understand? Obviously the explanations aren't cutting it, and for good reason IMO

The explanations don't cut it because I don't believe there is a good theory for cosmological boundaries nor what's on the other side of any. Why should we understand a "boundary" when nobody can explain time,mass,energy, etc...Nobody even knows fundamentally what space is, so if there is some sort of spatial boundary why could we possibly understand the interface when we don't even understand what's on our own side??

Further, the mixing of space and time in relativity further reduces our understanding of each in some sense...sure we know they are intertwined and can calculate and predict many experimental outcomes, but at the same time such an understanding raises as many issues as it answers... When space morphs into time inside a black hole that's enough to flabbergast me...but it at least it suggests a type of transformation that is possible at a boundary.

The only difference between an 'understanding' of, say mass and a boundary is that most people THINK they know what mass is because they believe they can see and feel it...like my computer keys I am tapping...they seem "real" enough...How can they be a form of energy?...

yet so does the immutable flow of time SEEM real and was believed to be so for thousands of years until Einstein THOUGHT about it carefully...we are subjective animals and physics begins to peel away some of that but our ability is limited to feeble senses constrained by three dimensional experience...we did not evolve needing experience in four dimensionmal spacetime to survive...

Who would have thought that mass and energy, for example, were two sides of the same basic phenomena? If such a "crazy" equivalence can be true, who can even imagine what might occur at a boundary...What is the "boundary" between energy and mass??

When I saw the following quote it seemed perfect to reflect our feeble understanding of our environment:

 

[apeiron]

Since nothing can exist outside of the Universe, how then can the Universe have a boundary in any conventional sense?

I don't think it is necessary to presume that nothing can exist outside the universe. You could instead have a situation where "everythingness" exists outside. And that our universe exists as a constraint on this wider potential.

Linde's inflationary cosmology would have this kind of logic. We would be a bubble of locally cooler order - a dimunition of dimensionality - within a wider raging realm of "energy".

We would in this case have a boundary that in some sense shields us. Not one that we would step across and find "nothing".

A lot of this is about being blinded by the chosen metaphors. So to present a different kind of image or our situation, let's remember that the "expansion" from the Planckscale is about both a growth from the Planck smallness and a cooling from the Planck heat. It is a falling as much as an expansion.

So think about falling down a well. There was a Planckscale hole in a Lindean super-hot everythingness (an infinoverse). We, as our universe, are now plummeting down that hole, growing larger and colder. We can look back and see the tiny bit of hot sunlight that was the mouth of the well. But we will fall forever towards a colder and emptier depth. Time's thermodynamic arrow allows us to go in only the downwards direction.

This is not meant to be a more accurate mental image. Just illustrate how we can swap the strange notion of expanding freely - the swelling image - with an equivalent notion of falling freely towards some ever darker cooler realm.

Too often the head-scratching focuses on attempts to imagine the expansion of the universe. But the cooling should present people with at least as hard a conundrum.

There is the puzzle of lightspeed, but the equal puzzle of absolute rest. There is an upper bound on Planck heat, Planck energy density, but also the equal puzzle of the lower bound of absolute K, the coldness of the zero point energy. There is the puzzle of a small hot beginning and the equal puzzle of the large cold end.

So two points on the boundary issue.

1) We could as easily be inside everything (a constraint of a realm of higher symmetry: ie a phase transition) as be a something with nothing beyond. And this is a story that makes even more sense if you treat everything as a "vagueness".

2) The boundaries are always dual in nature. Which should be a big clue. It is both heat and spatiotemporal scale that get symmetry broken in the big bang. The transition is from a hot point to a cold void.

 

[Fluxman]

This is true for our time.

 

[samssf]

While reading this thread (and some others discussing the balloon analogy), I thought of an idea/analogy of my own that I'm hoping someone can either quickly destroy or perhaps elaborate on...

Lets use a basketball rather than a balloon... Now let's assume the basketball is expanding... is it possible that the expansion of the basketball is time / creates time, and that as the basketball expands that more information is wrapped up within the dimensions along the surface of the basketball, and therefore our expanding universe is our universe progressing through time? Every state would be a specific size basketball with the previous state being a smaller basketball?

Light would move along the surface of the basketball while the basketball simultaneously expands. Traveling faster than the speed of light wouldn't allow you to "go back in time" since the basketball has already expanded.

Now imagine the basketball actually is a balloon and is not a perfect sphere... some parts might be expanding faster than other parts, warping it, and making it appear as though the universe is expanding toward the "great attractor"?

Regardless of my thoughts above, one question I have is this: If the universe is boundless... can an EM wave be thought of as a wave moving out in two dimensions along the surface of a balloon? And if so, would that not cause problems with measurements from LIGO, etc? Or is that not the case because not enough time has elapsed since the big bang?

I also read that the universe is expanding faster than the speed of light. If that is true, would that explain why the above situation would never occur?

 

[typhis]

One thing I would like to add is that as humans some things to us are just not comprehend able such as believing in an infinite boundary to our universe.

Also here is one idea i came up with just while reading this thread. I do not think much of it but ill say it anyway. According to multi-verse universes can be with another. Let's say for example that a universe within our own is in an empty water balloon. As you fill the balloon up with water(That what we could call light or matter) the boundaries expand
until they cannot anymore. Since in different universes time can be different a millisecond to the one filling up the balloon could be a trillion years to us. Thats all I thought of so far.

 

[dotapin]

Your Holiness, the mainstream majority of working cosmologists have never committed to the idea that there are only a finite number of particles

the standard LCDM model essentially comes in two flavors, the flat (normally assumed to be spatial infinite and beginning with an infinite bigbang)
and the positive curved (the finite one you understand). They didn't decide yet which is closer to Nature.

a certain number of science journalists and popularizers have LIED to the public and imprinted people with the idea that cosmologists say the bigbang occurred at a point and involving only finite number of particles. they have to lie because they want to SELL and they can't sell if the public can't picture what they are saying.

anyway that's all done now and no use crying about it. We still don't know which is right. MAYBE IT IS FINITE.
The lie is when some journalist gives people the idea that cosmologists have made up their mind in favor of finite. They have not yet made up their mind.

finite could turn out to be right, eventually, so in some sense the journalists and the public would be exonorated.

THats u mean using balloon to describe the universe big bang therory is not appropriate??