What Is Beyond The Observable Universe?

[sylas]

Hello. I've read through the last several pages again recently, in an attempt to resolve the issue I was wondering about earlier. I still haven't been able to resolve it, and could even say I'm even more confused and mystified as to what's been said by some.

I have a reasonable understanding of what homogenous and isotropic means. And I ask again, how can it be said that I am, (and we) are at the edge of the universe ? I can see for countless millions of miles in any direction, so it certainly doesn't seem that I'm on any edge.

Hi Tenny; I wrote this a while ago and then my network connection dropped out. Meanwhile Dave has answered. But with the network back I'll post mine as well...

There's no edge. The universe is (on large scales, as far as we can tell) homogeneous and isotropic, and that means there's no edge.

Blandrew was using some picturesque language which may have been misleading. The only sense in which there are edges is the sense of a limit to how far you can see from some position. Like a horizon on the ocean, there's a boundary beyond which you can't see; but at the horizon there's nothing special. You are at the center of your own field of view, and also on the horizon of certain other observers.

You can also think of a horizon in time. From here-and-now, we can see 2010; but you can't see 2011. You'll have to wait another six months. It's not that NOW is some special identifiable edge in time; merely the horizon of what we see from this point in time.

These kinds of "edges" are not something special about a location, but depend on an observer. They are better called "horizons".

Even better, just forget all that observer stuff and consider simply that the universe has no edge. (Unless the universe is really different from what we currently tend to think on the basis of available evidence.)

Cheers -- Sylas

 

[Sinker]

I can screw up a two car funeral, but here is my thoughts on subject:

I don't think we as a species have evolved enough yet to fathom infinity. However, I think we will get there eventually. We can guess, however: I think other big bangs occur from quantum fluctuations a great distance from our observable universe. It's got a whole new time frame and as such can never contact our observable universe for two reasons: i) expanding of space between observable universes, and ii) time differences.
Regards
Sinker

 

[ViewsofMars]

Orion1 deleted his/her contributions from this topic though I did quote (copy) Orion1 as noted in posts #296 and 297 prior to Orion1's action. Shame on you, Orion1! Beware people of Orion1's tricky nonsense. The person may do it again. You should know Orion1 that your message does appear in my mail box.

I can screw up a two car funeral, but here is my thoughts on subject:

I don't think we as a species have evolved enough yet to fathom infinity. However, I think we will get there eventually. We can guess, however: I think other big bangs occur from quantum fluctuations a great distance from our observable universe. It's got a whole new time frame and as such can never contact our observable universe for two reasons: i) expanding of space between observable universes, and ii) time differences.
Regards
Sinker

Hi Sinker. Welcome to Physics Forums. As a human being I like to deal with reality. Observations are based on evidence. There hasn't been any big bangs that have occurred outside our observable universe. Science isn't based on speculations but rather observations.

 

[Sinker]

Orion1 deleted his/her contributions from this topic though I did quote (copy) Orion1 as noted in posts #296 and 297 prior to Orion1's action. Shame on you, Orion1! Beware people of Orion1's tricky nonsense. The person may do it again. You should know Orion1 that your message does appear in my mail box.




Hi Sinker. Welcome to Physics Forums. As a human being I like to deal with reality. Observations are based on evidence. There hasn't been any big bangs that have occurred outside our observable universe. Science isn't based on speculations but rather observations.

Thanks for the welcome. Nice forum. Yep - observations are based on evidence. We can still use imagination to speculate and theorize to lay groundwork for observations.
Outside our observable universe is a lot of space. If it is expanding, we can never verify other bubble universes (observable universes to those inside). If we could understand dimensions beyond the known ones, it may be possible.
Humans some time will evolve to a state where we can understand the concept of more dimensions and their nature. Before the slow process of evolution, we may produce machines that will manufacture smarter machines that will do the job for us. (Singularity, I believe is the term.) Once we get a handle on quantum computers, I believe this will happen.
Regards
Sinker

 

[Flatland]

Blacks Space Until A Different Universe

That statement makes absolutely not sense whatsoever. If beyond the observable universe is "black space until a different universe" that "different universe" would still be part of our universe and not a different universe.

Get it?

 

[Sinker]

You don't get it. I said "observable universe"

Get it??

 

[marcus]

In conventional cosmology, do you know what "observable universe" means? It means the part of the whole universe that we have received light or some other signals from, so in principle we can see or have information about.

The observable portion of the universe is constantly increasing, as radiation comes in from farther and farther away.

There is no indication that anything is any different. So we assume that beyond the present range of observation there is just "more of same".

I never met anybody with as much as one introductory college course in Astro who thought that beyond the current observable range there was "black space". That wouldn't make dynamical (general rel.) sense and there is no evidence for it.
"Black space" surrounding the observable portion is a kind of extravagant fantasy without any observational or theoretical support.

Would suggest that anybody who thinks that beyond observable U is anything besides more of same should maybe just take a beginning Astro course.
The conventional model everybody uses could of course be wrong! there could be a large plexiglass sphere just outside the range of what we currently can see. But no reason to think that. So, by Occam's razor (the "keep it simple" principle) we assume more of same until evidence to contrary. If there is ever any evidence of something else you will hear about it! The pros would love a surprise like that and would immediately jump on it!
===================

Edit to reply to the post by NUTGEB that comes next:

Nutgeb, notice that I just said "more of same", not "infinite".

Space could be for example a very large 3D hypersphere, with matter more or less uniformly scattered all over, and a small patch of it what we can now observe. To do the usual analysis and modeling they don't have to assume that the "finite or infinite" question is answered.

I interpret the question of this thread in a modest limited way: what is just out beyond the limit of the observable portion?

I wouldn't encourage anybody to speculate about what the whole space looks like, what topology etc etc. We just don't know enough yet! (In my opinion.) Infinite is a logical possibility, so are various versions of finite. But what is gained by speculating?

But I think it is reasonable to assume "more of same" out past the present bound of what we are observing. (which of course we are observing not how it is at present but how it was way back in the past.)

So I will leave the speculating about infinite or not to you, for the time being.

 

[nutgeb]

There's no doubt that the idea of a finite universe surrounded by empty space is not accepted by cosmologists. Applying Occam's Razor makes sense. And the balloon analogy works very well if used properly. It makes sense to study cosmology before critisizing it.

Nevertheless, isn't it accurate to say that the infinite nature of the universe is a strong assumption, based on the Copernican Principle and CMB observations, rather than a demonstrated fact? It might be helpful for someone to describe specifically how the existence of an outer edge would contradict general relativity or current observations.

I am not aware of anything specific in GR or in the FRW metric that prohibits the existence of 'empty' space outside of a finite universe. GR math seems to work fine with a model with finite space and time, as long as everything external to that universe is ignored.

Consider a model universe that begins with an 'explosion' of mass energy away from a single point in otherwise empty space. It seems possible that, depending on the nature of the explosion, the fluid mass-energy distribution would arrange itself homogeneously (within the total expanding envelope of the exploding mass-energy), and all individual comoving observers (except those in a position to look beyond the outer edge) would observe the same isotropic Hubble's Law that we observe. For example, a homogeneous distribution might arise if the explosion resulted from a brief period of strong mutual repulsion among particles that previously were tightly packed together. Particles that originally were packed further from the 'center' would gain a proportionally higher recession rate relative to the center.

The original center of the explosion would immediately lose any local uniqueness and as a practical matter would become locally indistinguishable from any other location within the expanding envelope. Once the explosion occurred, gravity and later dark energy would begin to affect the expansion rate of the comoving stream.

One complication is that the positive pressure of free radiation, particularly in the dense, radiation-dominated early universe, would contribute a factor of acceleration to the expansion rate, because a finite universe would have a pressure gradient at its outer edge, giving the pressure something to push into. I don't think that would cause a problem for the FRW metric, but it would need to be factored into the expansion parameters. Perhaps that factor would be found to contradict observations in some way.

If our arbitrary location happened to be very, very far from the supposed edge of a very large but finite universe, the edge would be too far away for us to see, and could also be too far away to affect the homogeneity and isotropy of the CMB radiation we are currently receiving. The non-isotropic distribution of photons at the largest scale just wouldn't have arrived here yet. Then someday, perhaps billions of years in the future, we might detect a slowly growing cold spot in the CMB signalling the first indication of an outer edge.

 

[Tenny]

Keep in mind this is merely a spurious analogy, designed to show that sometimes "what we see" is not the only way to see things.

OK - I understand. One could end up a little nonplussed about the spurious nature of the analogy though.

An ant is crawling on the surface of an balloon. Every direction it looks it sees more balloon surface - there are no boundaries, no edges. If the ant is not at the centre of this universe, it is certainly not near any edge.

Yes, and we could extrapolate the analogy to apply to a human being on the surface of the earth.

I look down upon the ant from my vantage point. I can see that it is at the surface of the balloon. In fact, it is at the boundary of a sphere that is defined by the balloon's skin. Moreso, as I inflate this balloon, I can even say that the ant is on the leading edge of an expanding sphere that is defined by the balloon's skin.

OK ..

The 2D surface of the balloon is analagous to our 3D universe (with one dminesion ignored). The expansion of the balloon in a radial direction is analagous to the movement of our 3D universe through the 4th dimension of time. The ant cannot go "forward" any more than the balloon expands. Outward is the future, inward is the past.

OK. I thought at the outset, we were talking about an edge to the three spatial dimensions. If the universe is homogenous and isotropic as we say it is, then it should be the same for me here, as if I was gazzillion miles away. And I can see no end or edge in sight (or out of sight) to the three spatial dimensions, so I guess it's infinite in all directions - spatially.

Thanks for taking the time to explain this.

 

[Tenny]

Hi Tenny; I wrote this a while ago and then my network connection dropped out. Meanwhile Dave has answered. But with the network back I'll post mine as well...

There's no edge. The universe is (on large scales, as far as we can tell) homogeneous and isotropic, and that means there's no edge.

Blandrew was using some picturesque language which may have been misleading. The only sense in which there are edges is the sense of a limit to how far you can see from some position. Like a horizon on the ocean, there's a boundary beyond which you can't see; but at the horizon there's nothing special. You are at the center of your own field of view, and also on the horizon of certain other observers.

You can also think of a horizon in time. From here-and-now, we can see 2010; but you can't see 2011. You'll have to wait another six months. It's not that NOW is some special identifiable edge in time; merely the horizon of what we see from this point in time.

These kinds of "edges" are not something special about a location, but depend on an observer. They are better called "horizons".

Even better, just forget all that observer stuff and consider simply that the universe has no edge. (Unless the universe is really different from what we currently tend to think on the basis of available evidence.)

Cheers -- Sylas

Hi Sylas. Thanks. Your explanation is very clear and helpful.

The universe has no edge, but we can consider our present location in time, this moment, as a horizon to our limits of perception.

Can I then make the assumption that the universe is spatially infinite in all directions ? Given that;

- it is homogenous and isotropic
- I see no spatial end / edge to it for millions of miles - light years even
- if I were in a location a billion miles away, I would probably see the same as I do now


EDIT; I just saw marcus has answered this somewhat, but I would still appreciate your response.

 

[DaveC426913]

- I see no spatial end / edge to it for millions of miles - light years even
- if I were in a location a billion miles away, I would probably see the same as I do now

Not to put too fine a point on it but:
- "millions of miles" doesn't even get you to Mars
- "a billion miles away" doesn't even get you to Pluto

You want to be talking about billions of light years.

 

[Chiclayo guy]

I never met anybody with as much as one introductory college course in Astro who thought that beyond the current observable range there was "black space". That wouldn't make dynamical (general rel.) sense and there is no evidence for it.
"Black space" surrounding the observable portion is a kind of extravagant fantasy without any observational or theoretical support.

But wasn’t empty, black space the dominant condition prior to the big bang, and if so why couldn’t it exist now outside of the observable universe?

Tom

 

[DaveC426913]

But wasn’t empty, black space the dominant condition prior to the big bang, and if so why couldn’t it exist now outside of the observable universe?

Tom

No, the dominant condition pre-BB was no space at all.

The BB did not occur in space, the BB created space.

 

[nismaratwork]

No, the dominant condition pre-BB was no space at all.

The BB did not occur in space, the BB created space.

*Pipes up* and time!

 

[justwondering]

The 'edge' of our observable universe is receeding faster than the speed of light: which means you can't get there from here.

But NOT any Mass that may be near the 'edge', that can NOT be traveling away from us that fast?

 

[DaveC426913]

But NOT any Mass that may be near the 'edge', that can NOT be traveling away from us that fast?

Yes, galaxies are receding from us at speeds greater than the speed of light.

No, that does not violate SR.

 

[Max™]

They aren't traveling away from us through space, they're being dragged along with the expansion of the Universe.

Locally they aren't moving that rapidly, it is only when you examine scales approaching that of the observable horizon that you see this apparently forbidden motion.

Nothing can move THROUGH spacetime faster than light, spacetime itself is not moving through spacetime, it has different constraints.

 

[sylas]

They aren't traveling away from us through space, they're being dragged along with the expansion of the Universe.

This depends, I suggest, on what analogies or similes you like. But IMO the technical descriptions do not really correspond to anything "dragging" things. "Dragging" suggests some kind of force, which pulls objects along in some way. But there is no "force" corresponding to a drag that is forcing things apart. There's dark energy of course, which is a bit like a pressure that accelerates expansion; but you still get all the same superluminal proper recession velocities in a universe where there is no dark energy and the expansion is slowing down. You get superluminal expansions in a model where the expansion rates are linear, without any acceleration or deceleration. That is, no forces at all, just expansion of things with recession velocities for co-moving objects remaining constant.

That is -- you STILL get superluminal recessions whether the expansion is being pushed to increase or dragged to slow down, or if it is just coasting.

So for this reason, I feel that speaking of space "dragging" things is really misleading.

Personally, I think might be better to say that space is being dragged along by all the stuff that is expanding, if we speaking of anything being "dragged". Any sufficiently large volume withing boundaries that are locally not superluminal will be increasing in total volume. That is, there's more space between things as time goes by.

Locally they aren't moving that rapidly, it is only when you examine scales approaching that of the observable horizon that you see this apparently forbidden motion.

That's definitely incorrect on two counts.

The most unambiguous problem is with the suggestion that superluminal recession velocities have anything to do with being at boundaries of the observable universe. That just isn't true. You get superluminal proper recession velocities for galaxies that are "now" something like 14 billion light years away in proper distance, or only about a third of the way to the edge of the observable universe. Roughly speaking.

More subtly, I don't think it is really meaningful to say "locally they aren't moving that rapidly".

Of course, everything out at some distant location is all moving locally with respect to each other at small velocities, much like in our own neighbourhood. But that's simply not bothering to look at expansion of the universe at all. The expansion of the universe is not a local effect. It really means that density of things is falling as everything moves apart from everything else (on sufficiently large non-local scales).

The proper distance between things IS increasing that rapidly. Whether you speak of this as things moving apart, or as the space between things increasing; it still isn't any object being dragged.

The precise way to describe expansion of the universe is GR, of course. The rest is analogies or similes, and all too often they end up being actively misleading if you are not careful to remain consistent with the underlying GR account.

Cheers -- Sylas

 

[Max™]

I didn't mean to imply that it had to do with being at the edge of the observable Universe, as symmetrically we would expect our current position to appear the same from the perspective of an arbitrarily distant point.

I use the term dragging out of habit, as the space in which you are located itself is moving, you aren't going to observe forces relative to it, you would only notice red shift from everything else moving away from you.

I'm only speaking of the GR description, I take that for granted in such discussions.

 

[Mihael@@/&]

Things have changed since Mar8-06, 06:08 AM :)

http://en.wikipedia.org/wiki/Observable_universe

"No evidence exists to suggest that the boundary of the observable universe corresponds precisely to the physical boundary of the universe (if such a boundary exists); this is exceedingly unlikely in that it would imply that Earth is exactly at the center of the Universe, in violation of the Copernican principle. It is likely that the galaxies within our visible universe represent only a minuscule fraction of the galaxies in the Universe. According to the theory of cosmic inflation and its founder, Alan Guth, the lower bound for the diameter of the entire Universe could be at least in the range of 10^23 to 10^26 times as large as the observable universe."

 

[heusdens]

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?

You didn't mention this option in the poll, but the most serious candidate for the universe outside the horizon, since the horizon is just an arbitray limit, is that the universe outside of the horizon is simply more of the same, and in fact not anything different then we see here.

All other assumptions are speculations. Why would it be that the universe outside of the horizon (which used to be bigger in the past and will be smaller in the future) would be any different then the part we see?

That needs an explenation.

So the only serious option is that the universe outside of the horizon is the same (same homogeneity and isotropy) as the part we see.

 

[heusdens]

.. On the other hand, the total universe is is still finite, but there is no such thing as outside.

How do you know that?

 

[heusdens]

But most evidence points to the universe being flat. In that case a what would happen if you reach the end of the universe? If it were a spherical universe then we would just reach the other side. But what would happen if you reach the end of the universe? Would their be like a giant hard barrier that you can hit but not explode? Sounds to mythical to me. The universe seems to complex for it to be just finite.

The universe isn't infinitely big and infinitely small?

The current assumption or basic premise is that the universe has no edge or boundary.

You will never see a sign in space "end of universe, turn back now" there will simply be "more universe" after such a (fictional) sign.

For practical measurements, the universe we can observe only exists in finite parts and measurements, but this does not imply the universe must be finite and spatially closed (like a 2d surface on a 3D sphere, but then for the 3D analogous situation).

But from a more philosophical point of view I would think that by necessity the university (the totality of it) must be infinite, yet at the same time this infinity only exists in the form of finitudes. Indespite that this seems a contradiction, it is not as can be seen from this analogy: you can pick arbitraily any natural number, and you will always come up with a definite finite (and countable) quantity, yet the number set itself is infinite, you can always draw more numbers and larger numbers, no matter how long you draw a number.

Our measurements are like the numbers we can draw, the universe is like the complete set.

Notice also that the finitude and/or the infinitude of the world form a dialectical unity, you can not determine one without the other, they are like the two sides of the same coin, or the two poles of a magnet.

 

[heusdens]

...

Both those regions are at the edge of our observable universe... and we are the edge of their observable universe as measured now. But those two regions at opposite sides of the sky are OUTSIDE the observable universe for each other.

Cheers -- sylas

Good argument, and when we invoke the cosmological principle, that the universe looks (at sufficient large scale) the same in all parts of the universe, we arrive at the conclusion that beyond the horizon of observation there is just more universe that looks about the same as the stuff and structure we see.

 

[zahero_2007]

there is a multiverse

 

[TheTechNoir]

there is a multiverse

Care to elaborate?

And my answer is that what exists beyond the visible universe is the exact same as what exists here. I find it an unlikely possibility that somewhere far off the laws of physics change.

I believe that the universe has no boundary and therefore I also believe that outside the observable universe you see the same thing there as we see here (speaking on a large, far away view that is - obviously locally there are lots of differences but broadly speaking the universe is quite uniform). Because of no boundary, this means that a spatially flat universe with a finite size, a spatially flat universe with infinite size (seems to currently be a lot of people's preferred belief whereas flat/finite seems to be more fringe), the closed universe or 2d surface of sphere anology universe, the saddle, and all other known possibilities would all lead me to believe that in any given region of space outside of our observable universe you would observe roughly the same thing as we do here.

EDIT: Question: I understand it, but am I the only one who finds it a bit difficult to mentally visualize the step up from 2d analogue to full 3d closed universe?

EDIT 2: Lastly, I find it EXCEEDINGLY unlikely that just past the exact tip of our observable universe at any given time all matter disappears and space becomes utterly empty. Heck if from ANYONE'S perspective this were to be the case I would be beyond shocked. But for it to happen to be exactly/perfectly around us, no less? Me thinks the poll options are both completely crackpot.

 

[cristo]

EDIT: Question: I understand it, but am I the only one who finds it a bit difficult to mentally visualize the step up from 2d analogue to full 3d closed universe?

I think it's perfectly normal not to be able to visualise in 4D

 

[mistergrinch]

Beyond the observable universe it's turtles all the way down -- aka the "turtleverse". Turtleverse Theory is about to revolutionize the field of cosmology!

 

[harcel]

Beyond the observable universe there is lots and lots of unobservable universe,

 

[Radrook]

Beyond the observable universe there is lots and lots of unobservable universe,

Where is the universe located?