Questions Regarding The Universe

[Nedersong]

If the universe is thirteen billion years old, and it can only expand at the speed of light, how big does that make the universe?

Could what's beyond the edge of the universe be another dimension in itself? If the universe stopped expanding, could that mean it hit a barrier of some sort? And if it is a barrier, could that mean that it would be the edge of another universe?

 

[TheLonelySquire]

What does God need with a Starship?

 

[StolenThunder]

Moving this outwith the GTD universe to Sci-Tech.

 

[Trekker4747]

If the universe is thirteen billion years old, and it can only expand at the speed of light, how big does that make the universe?

Uhhhh 13 billion light years?

But don't confuse the physical part of the universe (galaxies) with the medium it exsists in (empty space) which by all likelyhood is infinite.

It's not like if you zoomed past the most distant galaxy you'll crash into a brick wall.

 

[Shaw]

If the universe is thirteen billion years old, and it can only expand at the speed of light, how big does that make the universe?

Could what's beyond the edge of the universe be another dimension in itself? If the universe stopped expanding, could that mean it hit a barrier of some sort? And if it is a barrier, could that mean that it would be the edge of another universe?

One of the hardest things for people to realize is that the universe isn't expanding into some pre-existing volume... the only volume that there is is increasing.

And the idea that the universe is about 13 billion years old and has been expanding at the speed of light means that we are 13 billion light years away from ourselves. And part of the reasoning behind originally setting the speed limit of the expansion to the speed of light is that at a rate any faster than that we would be moving away from ourselves faster than the speed of light.

Consequently, when people think of the big bang, they don't take into account that they are themselves standing at the point of that event. Where you are sitting reading this right now is where the big bang took place... but so is where I'm sitting, and so is where the next galaxy over is sitting. Every place there is is where the big bang happened. And everywhere is the center of the expansion.


It should be noted that this does come with paradoxes... like is the universe really expanding, or is how we're measuring shrinking? The universe may be the same size as it was a year ago, but I may be a little shorter now than I was then. In that way, what we use as our universal measuring rod (the speed of light) may not be as fixed as we believe (but we don't have anything else to work with that is any better).

Getting passed the idea of boundaries and outside are pretty big hurdles, mainly because we really want there to be some sort of universal space grid and universal time to work with. But understanding that neither of those are fixed and that both can be effected both in large scales and small by mass/energy is the first steps towards that understanding.

And just think... I've hardly touched the surface of this subject! It gets even better as you come to understand these things.

 

[K'riq Sa]

..... YOU ARE HERE! ................

If my candybar is gonna get smaller I want a refund.

















coefficiently,


K'riq Sa
Minister of the Church of Quantum Harmonic Stringlets

 

[M'Sharak]

If the universe is thirteen billion years old, and it can only expand at the speed of light, how big does that make the universe?

Uhhhh 13 billion light years?

Your universe expands in a single direction only, does it?

 

[Ronald Held]

Our section the this universe is as large as the age is. If Inflation is correct, the size is infinite.
I expect to have more to say on this at my talk at the Con in Las Vegas.

 

[Gepard]

Getting passed the idea of boundaries and outside are pretty big hurdles, mainly because we really want there to be some sort of universal space grid and universal time to work with. But understanding that neither of those are fixed and that both can be effected both in large scales and small by mass/energy is the first steps towards that understanding.

And just think... I've hardly touched the surface of this subject! It gets even better as you come to understand these things.

Isn't there an idea out there that he universe is "closed" so that, if you flew in a straight line for ever and ever, you'd eventually return to your starting point? Sort of making the three dimensions of space folded around themselves into a four-dimensional hypersphere?

 

[Shaw]

Isn't there an idea out there that he universe is "closed" so that, if you flew in a straight line for ever and ever, you'd eventually return to your starting point? Sort of making the three dimensions of space folded around themselves into a four-dimensional hypersphere?

Yeah... most theories of the large scale topology of the universe have the spacial dimensions forming a closed manifold (3-surface) with no boundaries. And that if we could freeze frame the universe and could travel at unlimited speeds in this frozen instant of time that we would find that all large scale semi-geodesic paths eventually lead back to their starting point.

And the easiest topological model to apply to this type of 3-surface is it being a 3-sphere. But a 3-sphere would exhibit (on very large scales) a positive curvature, where as something like a 3-torus would have zero curvature (would be basically flat). So while the idea that the universe is a closed (unbounded) 3-surface is widely accepted... the overall topology is still in question, and the possibilities of positive, zero or negative large scale curvature can have some drastic effects on both the structure of how the universe got to this point and where it might be going into the future.

Early versions of the Hubble constant were basically a very simple (linear) study of the rates at which galaxies moved away from us based on how far away they were. More current calculations take into account things like mass/energy density which helps take into account aspects of gravitation at different stages.

The main problem with trying to figure out the overall topology of the universe is that the differences in the models (positive, zero or negative curvature) going backwards aren't so radically different from one another that one could conclusively hold those models up to what we have observed going back in time and say "yes, this is it". But these models project very different futures so while this ambiguity looking backwards may not be a "deal breaker" for most models, it obviously becomes important when trying to predict forward (not that we'd have any way to observe any of that either).

Because I love topology, I've always been quite interested in the different large scale topological models of the universe. And having a background in math rather than physics, I'm more than happy to play around all sorts of different possible topologies even if most are nowhere near that of the actual universe (it's one of the benefits of being in the math community over the physics community).



If you guys would like to play around in small versions of some of these topological spaces, I would suggest the application Curved Spaces. It can be quite fun to play with.

 

[Mr. B]

If the universe is thirteen billion years old, and it can only expand at the speed of light, how big does that make the universe?

Not that I know what I'm talking about, but I would have to believe the universe is expanding at a speed significantly slower than the speed of light.

 

[Gepard]

And the easiest topological model to apply to this type of 3-surface is it being a 3-sphere. But a 3-sphere would exhibit (on very large scales) a positive curvature, where as something like a 3-torus would have zero curvature (would be basically flat). So while the idea that the universe is a closed (unbounded) 3-surface is widely accepted... the overall topology is still in question, and the possibilities of positive, zero or negative large scale curvature can have some drastic effects on both the structure of how the universe got to this point and where it might be going into the future.

And just to be clear, this is pretty much the only option that would allow a finite universe, right? Otherwise it would just be infinite, like Trekker mentioned...

 

[Shaw]

And just to be clear, this is pretty much the only option that would allow a finite universe, right? Otherwise it would just be infinite, like Trekker mentioned...

Well, when most people talk about an infinite universe, they envision an infinite pre-existing volume and that the big bang was some explosion in that volume.

It is hard to move people beyond ideas based on physical experiences.

The big bang didn't happen somewhere... it happened everywhere. Every point in space was once all together in the same point. There could never be any of those camera angles of someone watching the big bang from outside, because there was no outside.

Honestly, I cringe every time I see some special on physics that shows an explosion to represent the big bang because I know that it is just reenforcing the idea that there was some pre-existing volume that the known universe must be expanding into.

So in a real since, this is the flat Earth problem all over again. People thought of the Earth as a big disk that was completely flat, and if you when too far you'd fall off the edge. We know now that the earth has no edge and that the surface is closed, finite and without a boundary (which is the nature of the surface of spheres).

But some concepts are really hard... and the universe expanding with a complex geometry but not being within some nice, regular, infinite reference frame is one of those things that just isn't going to be accessible to everyone because some ideas are really hard to let go of.


I don't know if anyone downloaded that program or not so I'll show a quick image from it...

This is a space of finite volume (about 3.43x10^13 km^3), and the only thing in it is the Earth (singular, only one Earth). The topology of space makes it seem like you can see lots of Earths running off into the distance, but it is like a house of mirrors in that there is really only one. When you move past the frames shown you come right back in from the other side.

In this space there is no outside, it doesn't exist within some other volume. All that there is is all that there is.

Now just take this up to a massive scale and have it increasing in size, and you've got a pretty close model of the universe. Only at some point the scale is so large that you don't see yourself anymore.

And even at this scale, the Earths you see in adjacent cells are actually being viewed as they would have looked about a second earlier as that is about how long light from them took to get to that camera. And the further away the images of the Earth are, the further back into the past we are seeing.


In mathematics we start off with standard Euclidean geometry, look at the geometries of surfaces in Euclidean space, and then remove Euclidean space altogether. But at each step of the way you gain some intuition before removing previous foundational concepts. I'm starting to wonder just how fair it is to just drop this type of thing on people and expect them to just see it.

 

[Meredith]

I like the small universe where I can give myself a proper backrub with my own hands.

 

[Ronald Held]

AFAIR, that fits the data less well than an infinite flat universe.

 

[Shaw]

AFAIR, that fits the data less well than an infinite flat universe.

In steady state theory, sure... in big bang theory the universe is finite. And the data supports big bang theory.

Einstein wanted the universe to be flat and infinite too... but when Hubble presented him with just the limited data on hand back then, it was enough to change his mind on the idea. Today there are few in the physics and astronomy community that would take your position, the debates all circle around refining the model further rather than the broad aspects of it.

But yeah... for people out side of this area, letting go of an infinite flat universe is one of the biggest hurdles. And more often than not the hurdle is based on the idea of the infinite flat universe being a metaphor for Euclidean space. It is understandable, Euclidean space is, after all, a very safe place when it comes to visualizing things... it is just that it doesn't work as well in looking at the universe as it does for, say, building a house.

I personally don't think people are exposed to enough non-Euclidean geometry and that makes them uncomfortable with these concepts when first confronted with them.

None of this is "my theories" or "my interpretation" of this stuff... I wouldn't share such stuff in a forum like this. This is just where the current theories are at based on the current data. But I know of no theory (just bad PBS representations) of the big bang as being some explosion in the middle of a pre-existing infinite flat universe.



But I encourage anyone who does believe this to run through the math. A big explosion in the middle of a pre-existing infinite flat universe would have characteristics that are easy to check. The values of trajectories would differ greatly from those of a closed volume that was increasing uniformly. It was the first types of calculations done on this subject back in the 1930s. But the obvious conclusion of trajectories from a big explosion like that is that we would be able to say it happened there, because everything would be generally moving away from that spot.

 

[Daedalus12]

I'm starting to wonder just how fair it is to just drop this type of thing on people and expect them to just see it.

Keep going. Your audience awaits. This stuff was always very interesting to me. I am not a physics major so I've never sat in any lectures about the topology of the universe (only general topology courses).

A question. Has there been any inroads made to to physically show the topological properties of the universe? AFAIK there has been many models proposed that is mathematically sound which is problematic in the same manner as the string theory. If not what on the theoretical horizon would offer some glimpse of hope that such endeavor is even possible?

 

[Shaw]

A question. Has there been any inroads made to to physically show the topological properties of the universe? AFAIK there has been many models proposed that is mathematically sound which is problematic in the same manner as the string theory. If not what on the theoretical horizon would offer some glimpse of hope that such endeavor is even possible?

The main issues are in figuring out the global curvature... and there doesn't seem to be (the last I heard) a straight forward way of checking the curvature.

The best analogy is the surface of either an apple or a basketball. On the large scale view of them, both are topologically a sphere... but if you were exceptionally small on the surface of either of them, their irregular shapes might keep you from seeing their large scale topology.

This is the same issue faced in cosmology today... we can see a segment of the universe going back in time almost to the big bang, but the irregular surface of space do to gravitational effects makes most methods for checking curvature more like guess work. Without knowing that curvature, we still have a very limited idea of where the universe might be going.

This makes this area one of the most exciting aspects of cosmology because it has so many unanswered questions and quite a few different models fit nicely with the data. And so far none of these has predicted things that the other's haven't or things that we can easily check to weed out the competing theories.

Of course unanswered questions are what scientists live for... but sometimes it is not as easy to get the general public excited about scientists saying we don't know. You and I are in fields that are similar enough to this stuff that we can see how exciting that is, but there are people who rest their hopes on science and a we don't know can be quite scary from that point of view.


But yeah, on a small scale we can check for curvature in just about the same way we would to figure out that the Earth has positive curvature... but without something similar that we could check across many clusters of galaxies in scale, we are left waiting for someone to figure out a better way of checking while astronomers continue to collect data.

 

[LaxScrutiny]

The universe is expanding, except there is no perspective outside the universe, so the universe is expanding relative to what?

It is expanding relative to the speed of light. The speed of light being constant, the distance between any point A and any point B seems to be increasing.

If the speed of light were slowing at a steady rate, the universe would appear to be expanding. If light can continue to slow, but never stop, then the universe would appear to be expanding infinitely.

 

[Gepard]

And just to be clear, this is pretty much the only option that would allow a finite universe, right? Otherwise it would just be infinite, like Trekker mentioned...

Well, when most people talk about an infinite universe, they envision an infinite pre-existing volume and that the big bang was some explosion in that volume.

It is hard to move people beyond ideas based on physical experiences.

The big bang didn't happen somewhere... it happened everywhere. Every point in space was once all together in the same point. There could never be any of those camera angles of someone watching the big bang from outside, because there was no outside.

Honestly, I cringe every time I see some special on physics that shows an explosion to represent the big bang because I know that it is just reenforcing the idea that there was some pre-existing volume that the known universe must be expanding into.

So in a real since, this is the flat Earth problem all over again. People thought of the Earth as a big disk that was completely flat, and if you when too far you'd fall off the edge. We know now that the earth has no edge and that the surface is closed, finite and without a boundary (which is the nature of the surface of spheres).

But some concepts are really hard... and the universe expanding with a complex geometry but not being within some nice, regular, infinite reference frame is one of those things that just isn't going to be accessible to everyone because some ideas are really hard to let go of.


I don't know if anyone downloaded that program or not so I'll show a quick image from it...

This is a space of finite volume (about 3.43x10^13 km^3), and the only thing in it is the Earth (singular, only one Earth). The topology of space makes it seem like you can see lots of Earths running off into the distance, but it is like a house of mirrors in that there is really only one. When you move past the frames shown you come right back in from the other side.

In this space there is no outside, it doesn't exist within some other volume. All that there is is all that there is.

Now just take this up to a massive scale and have it increasing in size, and you've got a pretty close model of the universe. Only at some point the scale is so large that you don't see yourself anymore.

And even at this scale, the Earths you see in adjacent cells are actually being viewed as they would have looked about a second earlier as that is about how long light from them took to get to that camera. And the further away the images of the Earth are, the further back into the past we are seeing.


In mathematics we start off with standard Euclidean geometry, look at the geometries of surfaces in Euclidean space, and then remove Euclidean space altogether. But at each step of the way you gain some intuition before removing previous foundational concepts. I'm starting to wonder just how fair it is to just drop this type of thing on people and expect them to just see it.

This pretty much all stuff I'm happy to say I figured out years ago; what Iwas wondering is if there are any other closed surfaces besides spheres/hyperspheres that have been considered for our universe, or if that's the only one that would work, of if there's no way to know, etc, etc....