How vast is the univese?

[Dylab]

By vast I don't mean if space is open or closed but the amount of space that is out there that is similar in density to ours.

Sorry for the ignorance but I don't know where to start loking.

[Eos of the Eons]

I have some questions too since I don't know much on the subject either.

Is space "dense"?

What is space? I know there is space in between us and the moon once you are no longer in the atmosphere of earth anymore.

What is that space? There's no air. The particles floating around aren't "space", but are in space.

Space seems to be nothing. Nothingness, that planets and suns and gasses, and particles, and asteroids, etc. exist in.

Nothing can't end. Thus space never ends.

I don't know if I believe that, but I can't seem to come up with anything else.

I know Einstein went on about "space time", but isn't that a concept like time, gravity, etc. and not really a "thing", like energy, air, carbon?

So is the universe/space never ending?

[Cecil]

Well, since the universe has only been around for about 13 billion years, we can't see anything farther away than 13 billion light years (because the light hasn't had time to reach us). We therefore see ourselves at the center of a "bubble" 26 billion light-years in diameter, the edge of which is expanding at the speed of light. There may very well be matter outside this bubble that we can't see yet, but we cannot know for sure until it comes into view.

So the answer to your question is

4/3 * pi * r^3 = ~265 cubic gigaparsecs, or 9.2*10^30 cubic ly.

[Cecil]

Here's a great site I just found about this very topic.

http://www.anzwers.org/free/universe/universe.html

[rockoon]

Originally posted by Cecil
We therefore see ourselves at the center of a "bubble" 26 billion light-years in diameter, the edge of which is expanding at the speed of light.


Umm... ?? I'm pretty sure the velocity of the stuff we see 13 billion light years away is not moving away from us at the speed of light.

[Cecil]

Originally posted by rockoon


Umm... ?? I'm pretty sure the velocity of the stuff we see 13 billion light years away is not moving away from us at the speed of light. The matter itself isn't, but the "horizon" is. That is, every year we can see 1ly farther into space.

[DrChinese]

Actually, with the age of the universe a lttle less than 14 billion years (13.8 let's say), it actually appears that the observable universe is quite a bit larger than might be imagined.

As I recently learned, courtesy of some papers by Tamara Davis and Charles Lineweaver, the ongoing expansion of the universe causes some portions of the universe to recede from us faster than the speed of light. As a result:

1. The observable universe is now 46 billion LY in diameter. This is the size of the light cone to the past.
2. There are observed galaxies receding from us with a red shift indicating their speed relative to us exceeds the speed of light.
3. Every year that passes, the universe expands even faster.

See Common misconceptions... (25 pages PDF) (http://arxiv.org/PS_cache/astro-ph/pdf/0310/0310808.pdf). Personally, I am still trying to understand the implications of the paper, but looks pretty strong overall.

[Eos of the Eons]

What is the universe expanding in/to?

[fishbob]

What is the universe expanding in/to? Well, pretty soon it will become an even bigger universe.

Oh, you mean what was there before the universe expanded into it? We don't know. We can't know. So consider it nothing.

[TillEulenspiegel]

This is one of those non-intuitive concepts one must address from time to time ( 9 to 5 for some folks) in the study of the Universe.

The Hubble volume is all that exists in this universe ( worded as to not exclude multi-verse ideas ).

The Universe IS expanding but not into "nothing" as nothing is something. The expansion makes it's own space ( or reality ) as it expands.

That might sound like a dodge or fanciful conjecture , but it is accepted in the "The Standard Model" of our Universe.


edit: syntax

[Eos of the Eons]

Originally posted by TillEulenspiegel
This is one of those non-intuitive concepts one must address from time to time ( 9 to 5 for some folks) in the study of the Universe.

The Hubble volume is all that exists in this universe ( worded as to not exclude multi-verse ideas ).

The Universe IS expanding but not into "nothing" as nothing is something. The expansion makes it's own space ( or reality ) as it expands.

That might sound like a dodge or fanciful conjecture , but it is accepted in the "The Standard Model" of our Universe.


edit: syntax


Thanks for trying to explain it everyone. It still just does not make sense.

Nothing can't be something, or light would bounce off of it.

Sighs. I just wish we really knew more.

Thus, I don't believe the universe is expanding, but all the matter in the universe is in motion. The universe has always been here, just the matter is in a constant flux. The universe can't expand because it is already unending in every direction. We haven't seen the end to the galaxies yet. We just see more and more the farther we can see out.

No, I don't think the world is flat :D

Yes, I'm weird.

No, I don't profess to be right, it's just the only thing that makes sense to this layperson :)

[rockoon]

Originally posted by Eos of the Eons

Thus, I don't believe the universe is expanding, but all the matter in the universe is in motion. The universe has always been here, just the matter is in a constant flux. The universe can't expand because it is already unending in every direction. We haven't seen the end to the galaxies yet. We just see more and more the farther we can see out.


Well who said it was unending in every direction? And even if that is so there are several ways to make that happen. The surface of a basketball is unending in every direction but has a finite area. The current 'inflation' theory is about the curved geometry of space-time being inflated like a 4 dimensional balloon... that its always been unending in every direction but it just happens to be bigger now than it was 10 minutes ago.

The difficult part is comming to terms with not being able to leave the 'surface' of the universe.

(hard core science guys would use other terms)

[Dorian Gray]

Perhaps there is a finite but absolutely vast area that the universe can expand into, but to compensate, the universe shrinks by half every 20 minutes, while remaining in proportion. The odd thing is, this could not be disproven, or even detected. Or proven.

[Badger]

Originally posted by Eos of the Eons



Thanks for trying to explain it everyone. It still just does not make sense.

Nothing can't be something, or light would bounce off of it.

Sighs. I just wish we really knew more.

Thus, I don't believe the universe is expanding, but all the matter in the universe is in motion. The universe has always been here, just the matter is in a constant flux. The universe can't expand because it is already unending in every direction. We haven't seen the end to the galaxies yet. We just see more and more the farther we can see out.

No, I don't think the world is flat :D

Yes, I'm weird.

No, I don't profess to be right, it's just the only thing that makes sense to this layperson :)

Eos, I'll give a shot at this.

It's hard for the average person to get their head around because we are wired to exist in a four dimensional environment (3 dimentions moving through time). It's hard to think of more dimensions.

If you imagine the universe we're in as a soap bubble that's increasing in size, the outside surface of that bubble is kind of the outer limit of where our theories of physics are valid (the physics that rule this particular universe having been created at the moment of the big bang). What's outside the soap bubble? Well, we can't tell because the physics that govern us are inapplicable outside the wall of the soap bubble, so we can't detect what's out there. So, to us there's "nothing" out there......abscence of anything, even space for there to be no atoms present in. It's hard to wrap ones head around that.

This is my best understanding of it, anyway, and I'm no specialist in this area, so I await correction!

[Larspeart]

So, the universe is expanding at 1ly a year into 'nothing', and we say it is nothing because we HAVE to say that, because we can't come up with a better answer???

Come on!

I think I am inclined to go with the other guy who said 'nothing is something, so it can't expand into nothing'. Seems more reasonable. That, or it ISN'T expanding, and it is a closed system. Dark matter may play into this, allowing just enough mass to keep the Universe constant (or, egad, shrinking). We don't have a clue how much dark matter and dark energy is out there (yet), so it is tough to say.

Maybe the Universe isn't expanding, but what is around US is, and so we THINK it is expanding. Possible, right?

Good topic. I love to watch the 'scientists' on this board try to tackle philosophy.

:)

[wollery]

Originally posted by Larspeart
So, the universe is expanding at 1ly a year into 'nothing', and we say it is nothing because we HAVE to say that, because we can't come up with a better answer???Wrong, the Universe isn't expanding at 1ly a year into nothing. The Observable horizon of the Universe is expanding at 1ly per year. The problem you have with the statement that it is expanding into nothing is that you think of the nothing as being the same as the vacuum of space. It isn't, our Universe defines space and time, not the other way round. It would be more correct to say that the Universe is expanding, but it is not expanding into anything, it's just expanding.

Come on!

I think I am inclined to go with the other guy who said 'nothing is something, so it can't expand into nothing'. Seems more reasonable. That, or it ISN'T expanding, and it is a closed system. Dark matter may play into this, allowing just enough mass to keep the Universe constant (or, egad, shrinking). We don't have a clue how much dark matter and dark energy is out there (yet), so it is tough to say.Answered above.

Maybe the Universe isn't expanding, but what is around US is, and so we THINK it is expanding. Possible, right?No, not possible, the Universe is definitely expanding, if it wasn't then it would be contracting and we'd all be in for a "Big Crunch". This was one of the major predictions of relativity, although Einstein hated the idea so much he added an extra force which allowed the Universe to be static.

Good topic. I love to watch the 'scientists' on this board try to tackle philosophy.

:) No need to tackle philosophy on these points, this is pure science. Must add though that I love to watch the 'philosophers' trying to tackle science. :D

[Soapy Sam]

Throughout history our perception of the size of the universe has been subject to a sort of punctuated equilibrium. Every so often, it jumps by another order of magnitude. Or several.
My dark suspicion is that we are nowhere near the end of this process.
One day, I fear we will find that "THE Universe" as referred to in the thread will turn out to be one tiny bubble in a froth of bubbles expanding , shrinking and bursting like the suds in a bubbling hot tub and that the entire multiverse of bubbles is but a transient wrinkle on the surface of a hyper bubble which is a surface stain on the Toenail of the one true BATHER.

Failing that, when asked the size of the universe, I find "81/2 by 11" makes as much sense as any theory of cosmology.

(Of course that should be "8 1/2 x 11 x 1 x .000001 x...etc, but I omit the other 9 dimensions for simplicity.)

[Larspeart]

Wollery, I think you may be just a little too quick to make an assumption.

Last I had heard, they now believe that the Universe is 14-15billion years old. 30 years ago, they thought it was 8-10bil. 100 years ago, they were throwing around 3-5bil. 2,000 years ago, everyone said something in the neighborhood of 5-10,000 years.

As time goes by, 'science' changes. Science is so busy trying to find truth that it often overlooks the 'truths' it disproves.

I wouldn't be surpized if they find that the Universe is 'actually' 20 billion years old, or 50 billion, before I die.

[wollery]

Originally posted by Larspeart
Wollery, I think you may be just a little too quick to make an assumption.

Last I had heard, they now believe that the Universe is 14-15billion years old. 30 years ago, they thought it was 8-10bil. 100 years ago, they were throwing around 3-5bil. 2,000 years ago, everyone said something in the neighborhood of 5-10,000 years.

As time goes by, 'science' changes. Science is so busy trying to find truth that it often overlooks the 'truths' it disproves.

I wouldn't be surpized if they find that the Universe is 'actually' 20 billion years old, or 50 billion, before I die. You're right, science does change, you don't have to tell me that, it's something I deal with on a day to day basis. Science advances and closes in on the real values and rules.

2,000 years ago the estimates were based on biblical stories, 100 years ago they were based on geological measurements of the age of the Earth, 30 years ago they were based only on innacurate measurements of the recession velocities of relatively low redshift galaxies.

The assumption on which the current value of the age of the Universe (13.8 billion years) is based is that relativity gives us an accurate description of the way the Universe works. That's it, no other assumptions necessary. All of the evidence we have suggests very strongly that this is the case. The data used to calculate this value are extremely accurate measurements of the fluctuations of the Cosmic Microwave Background at several different angular scales.

Of course all of our scientific theories could be wrong and as a good (I hope) scientist I'm more than willing to state that if someone comes up with a better description of how the Universe works then I'll be happy to go along with it. That's the point of science, to come up with the best possible explanation and then test it to destruction, at which point you need to find another, even better explanation. We don't forget the truths we've disproved, we improve on them. Most scientists are well aware of how their own particular field has evolved and of the theories that have been disproved over time.

[T'ai Chi]

Originally posted by Cecil

So the answer to your question is

4/3 * pi * r^3 = ~265 cubic gigaparsecs, or 9.2*10^30 cubic ly.

I guess you are assuming it is shaped like a sphere.

[TillEulenspiegel]

Wollery is correct but in one respect too genteel .

He states that science is modifiable in reaction to contrary proofs of accepted theory . This malleability of scientific theories does not make science a weak model of observation and quantifaction , but rather more robust in investigating the claims of people who hold non-demonstraitable beliefs. Faith is not grounded in any protocol that calls for accountability and replicability , science does.

"Extrodinary claims require extraordinary evidence".-Sagan. Show me the differential between the actual condition of the universe and show me where any alternative ( how ever popular ) has supplanted the outcome or methodology of the scientific model in any form. The attempt of science is to catalog the working of our universe without regard to religious or other dogma........And the thing that is not surprising is that it functions regardless of the intentions of the experimenter. ( For the most part, with a tip o' the hat to the QM guys. )

[Eos of the Eons]

Originally posted by rockoon


Well who said it was unending in every direction? And even if that is so there are several ways to make that happen. The surface of a basketball is unending in every direction but has a finite area. The current 'inflation' theory is about the curved geometry of space-time being inflated like a 4 dimensional balloon... that its always been unending in every direction but it just happens to be bigger now than it was 10 minutes ago.

The difficult part is comming to terms with not being able to leave the 'surface' of the universe.

(hard core science guys would use other terms)

I've heard the ball/balloon thing before. Balloons have a diameter that ends, and they are in the universe. Balloons are in something.

Nice analogy though, just doesn't work quite right.

[Sindai]

Originally posted by Eos of the Eons
Nice analogy though, just doesn't work quite right.

Virtually any analogy breaks down if you stretch it beyond the concept it was intended to illustrate.

[Cecil]

Originally posted by T'ai Chi


I guess you are assuming it is shaped like a sphere. I'm assuming we can see about 13 Gly in every direction, which has the consequence that the observable universe is spherical.

[Dorian Gray]

We might assume that, but do we know for sure? I mean, most galaxies tend to be basically a flat circle, many times bigger in x and y than in z - like a plate, or something. Does anyone suppose that the universe is like this also?

[T'ai Chi]

Originally posted by Cecil
I'm assuming we can see about 13 Gly in every direction, which has the consequence that the observable universe is spherical.

While sensible, I don't see how that necessarily follows at all. It just seems to say that the distance we can see is limited the equally in every direction.

[peptoabysmal]

Originally posted by Cecil
I'm assuming we can see about 13 Gly in every direction, which has the consequence that the observable universe is spherical.

Doesn't that also have the condition that we are in the exact center of the universe? How likely is that?

[TeaBag420]

Originally posted by wollery
You're right, science does change, you don't have to tell me that, it's something I deal with on a day to day basis. Science advances and closes in on the real values and rules.

Okay, argument by appeal to authority. Fair enough. I know jack NOTHING about this subject, so I'm going to offer my thoughts.


2,000 years ago the estimates were based on biblical stories,

Only, to be brutally honest, among Jews. Buddhist cosmology posits a universe quite a bit older than modern science does, and did it several hundred years B.C.E. Notice I said "posits".

100 years ago they were based on geological measurements of the age of the Earth, 30 years ago they were based only on innacurate measurements of the recession velocities of relatively low redshift galaxies.

The assumption on which the current value of the age of the Universe (13.8 billion years) is based is that relativity gives us an accurate description of the way the Universe works. That's it, no other assumptions necessary. All of the evidence we have suggests very strongly that this is the case. The data used to calculate this value are extremely accurate measurements of the fluctuations of the Cosmic Microwave Background at several different angular scales.

Of course all of our scientific theories could be wrong and as a good (I hope) scientist I'm more than willing to state that if someone comes up with a better description of how the Universe works then I'll be happy to go along with it. That's the point of science, to come up with the best possible explanation and then test it to destruction, at which point you need to find another, even better explanation. We don't forget the truths we've disproved, we improve on them. Most scientists are well aware of how their own particular field has evolved and of the theories that have been disproved over time.

The idea that the universe has an age (oh by the way, someone please define "universe" for purposes of this discussion.... and enforce the definition :D ) is absurd. To have an age, it must have had a beginning. To have a beginning, it must have had a cause. No science needed (although I did batch to the Sarah Silverman character on Star Trek when she played the astronomer in the two-parter with Ed Begley Jr.)

[T'ai Chi]

Originally posted by TeaBag420

The idea that the universe has an age (oh by the way, someone please define "universe" for purposes of this discussion.... and enforce the definition :D ) is absurd. To have an age, it must have had a beginning. To have a beginning, it must have had a cause.

That is presicely why I personally feel that the estimates of the universes' age that are updated every so often will keep getting not so suprisingly older and older and older...

[wollery]

Originally posted by TeaBag420
Okay, argument by appeal to authority. Fair enough. I know jack NOTHING about this subject, so I'm going to offer my thoughts.

2,000 years ago the estimates were based on biblical stories

Only, to be brutally honest, among Jews. Buddhist cosmology posits a universe quite a bit older than modern science does, and did it several hundred years B.C.E. Notice I said "posits". True, but I was simply responding to the post.

The idea that the universe has an age (oh by the way, someone please define "universe" for purposes of this discussion.... and enforce the definition :D ) is absurd. To have an age, it must have had a beginning. To have a beginning, it must have had a cause. No science needed (although I did batch to the Sarah Silverman character on Star Trek when she played the astronomer in the two-parter with Ed Begley Jr.) I would define the Universe, for the purposes of this discussion, as the four dimensional space-time that we inhabit. I'm not sure that I have the authority to enforce it, but if anyone would care to argue the point......

Yes, it did have a beginning, it's called the Big Bang, and that did have a cause. We can never really know what that cause was, but the most viable current theory in science is that in the nothing before the BB (before is possibly not the best word, since time is a function of the Universe) there was a quantum fluctuation which produced negative energy and matter / antimatter which expanded rapidly rather than annihilating.

(Don't remember that episode!:( )

[wollery]

Originally posted by T'ai Chi
That is presicely why I personally feel that the estimates of the universes' age that are updated every so often will keep getting not so suprisingly older and older and older... Actually over the last five years or so they've been getting younger. Current estimate is 13.8 billion years, five years ago it was 15 billion years. As I said earlier, science homes in on the true values and we aren't afraid to change our answers as better theories and data arise.

[c0rbin]

We are in the center of our field of perception. Therefore we might as well be in the center of all that there is because we don't have any other observation platforms (at any signifigant distance) to change the shape.

This is not hubris, this is the way we perceive our _surroundings_ which _surround_ us in a sphere.

[diddidit]

Originally posted by Eos of the Eons
What is the universe expanding in/to?

Marlon Brando.

did

[diddidit]

Originally posted by Larspeart
Wollery, I think you may be just a little too quick to make an assumption.

Last I had heard, they now believe that the Universe is 14-15billion years old. 30 years ago, they thought it was 8-10bil. 100 years ago, they were throwing around 3-5bil. 2,000 years ago, everyone said something in the neighborhood of 5-10,000 years.

As time goes by, 'science' changes. Science is so busy trying to find truth that it often overlooks the 'truths' it disproves.

I wouldn't be surpized if they find that the Universe is 'actually' 20 billion years old, or 50 billion, before I die.

13.7 billion +/- 200 million years, established by the Wilkinson Microwave Anisotropy Probe, and reported here. (http://map.gsfc.nasa.gov/m_mm/mr_age.html)

did

[diddidit]

Originally posted by Eos of the Eons


I've heard the ball/balloon thing before. Balloons have a diameter that ends, and they are in the universe. Balloons are in something.

Nice analogy though, just doesn't work quite right.

I don't think you got the analogy quite right. Our universe, in the balloon analogy, is the surface of the balloon, not the volume within the balloon. As the volume of the balloon increases, the surface area of the balloon increases, but there's no center or edge to the expansion on that two-dimensional surface. In other words, our universe is expanding in four spatial dimensions (in addition to the "time" dimension); in order to "see" the center of the expansion (or the edge of the universe), you'd have to be able to sense that fourth spatial dimension.

Wackiness!

did

[LFTKBS]

Originally posted by peptoabysmal
Doesn't that also have the condition that we are in the exact center of the universe? How likely is that?

If you go with the assumption that there was indeed a Horrendous Space Kablooie at the beginning of time, then all points are indeed at the center of the universe. That's just a consequence of the topology of a sphere.

Real mathematicians, please correct the above as necessary.

[Dancing David]

Originally posted by T'ai Chi


That is presicely why I personally feel that the estimates of the universes' age that are updated every so often will keep getting not so suprisingly older and older and older...

Well, it would appear that a limit is appoaching, the highest estimate is seventeen billion and in the last five years mosts are pegging out below 15 billion.

Which is why when they found a star at 14 billion, people were real excited because at the time the outer limit was beween 13 and 14 billion, in recent times the outer limit has been decreasing.

And again the age of our portion of space time has no connection to other potentail 'spacetimes'. there is this recursive factor where like the worm Ouruboros you could have a space time bubble creating space time bubbles that eventualy get back to creating the original space time bubble.

[Dancing David]

Originally posted by Larspeart
Wollery, I think you may be just a little too quick to make an assumption.

Last I had heard, they now believe that the Universe is 14-15billion years old. 30 years ago, they thought it was 8-10bil. 100 years ago, they were throwing around 3-5bil. 2,000 years ago, everyone said something in the neighborhood of 5-10,000 years.

As time goes by, 'science' changes. Science is so busy trying to find truth that it often overlooks the 'truths' it disproves.

I wouldn't be surpized if they find that the Universe is 'actually' 20 billion years old, or 50 billion, before I die.

This is a very good point, science does go through revision all the time.

However, even though they are now finding galaxies at the edge of the observable space, they are very 'young' galaxies, and there appears to be a difference in quality to thier nature.

So there does to be a convergence on the 13 to 15 billion year date for the universe.

It could be revised soon, but the most recent revision have been down from the 15 to 17 billion and into the 13 to 15 billion range.

Truth in science is the search for the best approximation to the observed behavior of the universe. the main reason for the changes in the age of the universe was the calibration of distance, distance to distant objects is done by calibration of the brightness of certain stars (Cephid variables) there seems to be a convergence on that yardstick as well, in that there are many diffrent ways of gauging distance now and there appears to be convergence between them.

Could happen that the age of the universe gets suddenly revised again, but I think that nucleo sysnthesis since the big bing and after the big bing is also converging on the 14 billion year age. In that the amount of matter that we see of different types alos points to the age of the universe as well.

Maybe we will start to see the elephants soon as well!

[Dancing David]

Originally posted by peptoabysmal


Doesn't that also have the condition that we are in the exact center of the universe? How likely is that?

Good point with kind of a weird answer, in expanding space time all points of the universe will appear to be at the center of the expansion.

It kind of weirded out a lot of scientists at the time, they had just removed man from the center of the universe.

[Bottle or the Gun]

I've always had a problem with how the size and age of the universe is calculated. Measurements taken in the 'known universe' are relative, aren't they? Are we in the main 'mass' of the universe or just a 'bubble' adjacent to other 'bubbles' that are in different phases of expanding, collapsing, heating, cooling, etc, that are part of the larger universe?

Is it accurate to say that our knowledge of the age and size of the universe is limited to what we can detect 'locally', or is there evidence that could only come from one event? Don't say 'background radiation from the Big Bang', because again, couldn't that be a local wave?

[wollery]

Originally posted by Bottle or the Gun I've always had a problem with how the size and age of the universe is calculated. Measurements taken in the 'known universe' are relative, aren't they? Are we in the main 'mass' of the universe or just a 'bubble' adjacent to other 'bubbles' that are in different phases of expanding, collapsing, heating, cooling, etc, that are part of the larger universe?Some measurements are relative, while some are direct. It doesn't really make any difference, you just have to be aware of any uncertainties and take account of them. There's only the one Universe that we are aware of and can measure, no bubbles, no differential expansion or cooling rates.

Is it accurate to say that our knowledge of the age and size of the universe is limited to what we can detect 'locally', or is there evidence that could only come from one event? Don't say 'background radiation from the Big Bang', because again, couldn't that be a local wave? No, the background radiation could not be a local wave. Furthermore it was predicted a long time before it was discovered. In fact the discovery itself was an accident, Penzias and Wilson (who discovered it) were really annoyed by this 'noise' they were getting wherever they pointed their microwave antenna, and at one point thought that it was due to bird droppings in the antenna!

And also as Dancing David pointed out there are measures from other methods, Cepheid variables, Type 1A Supernovae, nucleosynthesis to name a few, and they all agree to a fairly small margin of error.

[Dancing David]

Under inflationary theory, there can be budding universes that are 'adjacent' to our own, but they are closed to us as we are closed to them.

In one way it is a very good question, it would appear that the universe is much, much larger than the portion we can observe, and it is a big assumption that isotropy extends beyond the observable universe. Given the nature of big bang and inflationary theory, if there was some weird anomoly in the early iniverse it should have visible effects across a huge area of the universe.

[rockoon]

Originally posted by Eos of the Eons


I've heard the ball/balloon thing before. Balloons have a diameter that ends, and they are in the universe. Balloons are in something.

Nice analogy though, just doesn't work quite right.

You've missed the point and applied the 3D balloon to the 3D universe - the balloon is indeed 3D but its SURFACE is 2D - if 2D life was to exist on this surface where they couldnt 'see' the 3rd D, they would find themselves living.. after much study and information collection, living on a surface of constant curvature which curves in a dimension they cannot percieve directly.

Now extrapolae that idea to 3D life. They may find that they are living on the surface of a 4-dimensional surface (or volume if thats a better term for you) of constant curvature which curves into a dimension they cannot percieve directly.

A 1D object of constant curvature is a circle.
A 2D object of constant curvature is a sphere.
A 3D object of constant curvature is a hypersphere.