Shape of the universe

[JMFNYC]

If suddenly the universe stopped expanding, and froze in time, and if the universe was shrunk so that each galaxy was the size of, lets say, a pea...what would the universe LOOK like? Would it look like a sphere with all the galaxies on the outside of it?

 

[DrRocket]

If suddenly the universe stopped expanding, and froze in time, and if the universe was shrunk so that each galaxy was the size of, lets say, a pea...what would the universe LOOK like? Would it look like a sphere with all the galaxies on the outside of it?

IF..... IF..... IF..... IF......

IF frogs had wings, they wouldn't bump their ass.

 

[SpeedFreek]

If suddenly the universe stopped expanding, and froze in time, and if the universe was shrunk so that each galaxy was the size of, lets say, a pea...what would the universe LOOK like? Would it look like a sphere with all the galaxies on the outside of it?

No, there are galaxies throughout the universe, so whatever shape it is, it is filled with galaxies. There may not be an "outside" from which to view the universe - it might go on forever.

 

[JMFNYC]

Ok so ....what would that shape be? Anyone?

 

[DrRocket]

If suddenly the universe stopped expanding, and froze in time, and if the universe was shrunk so that each galaxy was the size of, lets say, a pea...what would the universe LOOK like? Would it look like a sphere with all the galaxies on the outside of it?

No, there are galaxies throughout the universe, so whatever shape it is, it is filled with galaxies. There may not be an "outside" from which to view the universe - it might go on forever.

Speedfreak, I know that you know this, but for the benefit of other people reading this it is important to point out that the question of whether the universe goes on forever (is open) has absolutely nothing whatever to do with the question of whethre there is an "outiside" from which to view it (the question of whether our space-time is embedded in some larger manifold).

Also, there is no evidence that the uiverse is embedded in anything, and since by definition anything that can affect us is part of this universe, it doesn't make any difference.

 

[JMFNYC]

Speed and Dr. I appreciate your responses but you are not reading my (simple) question correctly.

 

[Saiph]

Topologically the shape is described in three general fashions, as a 2d shape as if we 'squashed' everything down into a single sheet: Spherical (a closed universe), an flat plane (slows but never quite stops), or a saddle shape (hyperbaloid, for accelerating universes).

Now, the trouble is picturing this in 3D...as each of those pictures described above are just a 2d shape that we can imagine, bent into a 3rd dimension. Trying to picture those as a 3d construct projected into a 4th dimension is....very odd. Especially as Dr. Rocket points out, there is no indication that there is a 4th dimension to use, so if you try you could well be horribly wrong.

 

[SpeedFreek]

Speedfreak, I know that you know this, but for the benefit of other people reading this it is important to point out that the question of whether the universe goes on forever (is open) has absolutely nothing whatever to do with the question of whethre there is an "outiside" from which to view it (the question of whether our space-time is embedded in some larger manifold).

Also, there is no evidence that the uiverse is embedded in anything, and since by definition anything that can affect us is part of this universe, it doesn't make any difference.

Yes, a very good point. Sometimes simplifying an answer far too much (like I did) can lead to misconceptions down the line.

Aside from the question as to whether our our space-time is embedded in a larger manifold, there is always the simpler question as to whether there is a place where there are no more galaxies - an "edge" of the universe, where the galaxies stop and there is just blackness beyond (a finite amount of stuff expanding into an infinite space). This is the way a lot of people are thinking about the universe when they ask these questions. The implications of this way of thinking would be that if you could shrink the universe down small enough so you could see the empty space around it, you could visualise the shape of the universe. The question is posed using the same way of thinking as when we take our viewpoint here and zoom out of the Milky-Way galaxy so we can consider what it looks like, but taking the principle and applying it to the universe.

It was that way of thinking that my answer was trying to address. If the universe is flat or open, is it infinite or finite?

Speed and Dr. I appreciate your responses but you are not reading my (simple) question correctly.

We are reading your question correctly, but just as my earlier answer above was too simplistic to make much sense, so was your (simple) question!

You asked:

If suddenly the universe stopped expanding, and froze in time, and if the universe was shrunk so that each galaxy was the size of, lets say, a pea...what would the universe LOOK like? Would it look like a sphere with all the galaxies on the outside of it?

What would it look like? It would look like it does now, but everything would be smaller and we have to ask the question "smaller, relative to what?". Your idea of shrinking the universe so that each galaxy was really small seems to imply that you are looking for some sort of "overview" of the situation - you want see more of the universe than we can see from here, you want to shrink the whole thing down so you can "stand back" and take the whole thing in. You want to see what "shape" the whole thing is by taking your point of view "outside" of it and looking at the universe sitting an empty space.

We cannot see a physical edge to the universe from here and thus we don't know if it has an edge. Very simply put, the theory of General Relativity (which seems to describe what we can see very well) describes the universe using geometry, so you might think of it as being able to describe the geometry of the whole universe, but we reside within that very geometry which makes this difficult, if not impossible. The fact that we have no idea how much larger the whole thing is than the parts of it we can see only compounds the problem.

But surely, if we can work out what the geometry of the universe is doing around here, we can apply that knowledge to the whole thing? Unfortunately it is not that simple.

Imagine you are a brick! You know you are a brick and you know what bricks are for, but your only sense is that you can feel the bricks directly next to you. You feel bricks around you on all sides. What is the shape of the structure you are part of? In fact, how do you, as a brick, know that your edges are straight? Your edges might be curved along one axis, so any wall you are part of might be curved. The structure your bricks make up might be a wall that curves around to meet itself, or it might be a flat wall. Does the wall go on forever, or does it stop somewhere? Is it a wall three bricks thick, or a building, or a pile of bricks that goes on forever in all directions? So far all we know is the brick, and we aren't even sure if the edges are straight, let alone how many bricks there are.

 

[JMFNYC]

Thank you Speed and Saiph for trying to educate me. Now let me further describe my question and please tell me if I am incorrect.

Lets assume that it is mathematically POSSIBLE for the universe to cease expanding and for every galaxy, and let's for our purposes limit our proposed model of the universe to ONLY the galaxies (forget dark matter etc.) NOW: if we wanted a MODEL of our Milky Way and, say our two closest galactic neighbors we would have a 2 dimensional (for the most part) triangle with three dots. Now lets add a fourth galaxy and a fifth etc. so now we have a 3 dimensional model (presumably). In other words, each galaxy RIGHT NOW is somewhere, and what I do not understand (because I am dumb) is why we cannot continue adding galaxies to our model until we have all 200 billion accounted for.

 

[JMFNYC]

Here is a present model of our Milky Way and its two closest neighbors (or any 3 galaxies anywhere in the universe not adjusted for scale):



.............. o


o



... o

Any problems so far? Incidently, if our galaxy was the size of this: "o" how far away would from it the nearest galaxy be?

 

[MeteorWayne]

many thousands of miles away.

 

[SpeedFreek]

Lets assume that it is mathematically POSSIBLE for the universe to cease expanding and for every galaxy, and let's for our purposes limit our proposed model of the universe to ONLY the galaxies (forget dark matter etc.) NOW: if we wanted a MODEL of our Milky Way and, say our two closest galactic neighbors we would have a 2 dimensional (for the most part) triangle with three dots. Now lets add a fourth galaxy and a fifth etc. so now we have a 3 dimensional model (presumably). In other words, each galaxy RIGHT NOW is somewhere, and what I do not understand (because I am dumb) is why we cannot continue adding galaxies to our model until we have all 200 billion accounted for.

Whenever you see figures quoted for the possible number of galaxies or the size of the universe, astronomers are talking about the observable universe, the parts of the universe from which light can have reached here by now. We think the universe has a finite age and therefore there is a limit on the distance that light can have travelled during the history of the universe. We will only be able to see galaxies whose light has been able to reach us in 13.7 billion years (the age of the universe), so our observable universe has a radius of 13.7 billion light years, in terms of "light travel time". The whole universe could be many magnitudes larger than that, or possibly infinite, so there will be parts of the universe we can never see.

As we seem to be able to see the same distance in all directions, we cannot see any "edge" to the universe where there are no more galaxies (except for the edge of time at the beginning!). Our universe is 13.7 billion years old, and due to the expansion we think that the most distant parts of the universe that we have seen will now be something around 46 billion light-years away from us. Our observable universe puts us in the centre of a sphere with a radius of 46 billion light years, but there could be galaxies a trillion light-years away that we can never know of. Our little sphere seems to be part of a larger universe and we have no way to find out if it has a "shape".

The only reason our little sphere is a sphere is because the speed of light is constant, so light that is coming in from all directions will, if limited to a certain time it can have been travelling for, have originated from emission points that form a sphere around us and this would be the same whether the universe is shaped like a sphere, a pancake, a donut or a mobius strip (a loop of ribbon with a single twist in one place which leads to a surface with no end!) as long as we cannot see an edge.

 

[SpeedFreek]

Any problems so far? Incidently, if our galaxy was the size of this: "o" how far away would from it the nearest galaxy be?

If the Milky-Way is 100,000 light years across and our nearest neighbour, Andromeda, is 2.5 million light years away, then:

o. . . . . . . . . . . . . . . . . . . . . . . . .o

At that scale (around 1 million light years per inch (depending on your screen size!)) our observable universe, with a radius of 46 billion light years, would have a radius of just over 1.1 miles.

 

[JMFNYC]

Speed we are coming excitingly close to answering my question. So if we only concern ourselves with the OBSERVABLE universe, namely those galaxies we can see and have discovered within the 14 billion light years, and we built a model of ONLY those galaxies, what would our 2.2 mile diameter model LOOK like?

 

[SpeedFreek]

Speed we are coming excitingly close to answering my question. So if we only concern ourselves with the OBSERVABLE universe, namely those galaxies we can see and have discovered within the 14 billion light years, and we built a model of ONLY those galaxies, what would our 2.2 mile diameter model LOOK like?

It would look like the picture in the link below:

http://www.atlasoftheuniverse.com/universe.html

 

[JMFNYC]

Thanks Speed. Is that a sphere with all the galaxies on the outside of it like dots on a balloon? Or is it a sphere with galaxies on the outside AND inside?

 

[SpeedFreek]

It is a sphere that contains all the galaxies in our observable universe - they fill the sphere and the sphere represents the limit on the regions of space that we can know about.

The balloon with dots on it is a different thing - it is a model to illustrate how the universe expands and in that model the galaxies are on the surface of the balloon. The two dimensional surface of the balloon represents the three dimensional universe and as the balloon expands, all the dots on the surface become further apart.

 

[JMFNYC]

Ok so we have a sphere filled with galaxies on the inside and on the outer edge. Got it thanks.

 

[SpeedFreek]

Ok so we have a sphere filled with galaxies on the inside and on the outer edge. Got it thanks.

We simply have a sphere that we are at the centre of, filled with galaxies. There is no "outer" edge except in an conceptual sense, there is just as distance past which any galaxies are outside of our observable part of the universe. To a galaxy that is right on the edge of our observable universe right now, our Milky-Way galaxy is right at the edge of their observable universe.

 

[DrRocket]

Thank you Speed and Saiph for trying to educate me. Now let me further describe my question and please tell me if I am incorrect.

Lets assume that it is mathematically POSSIBLE for the universe to cease expanding and for every galaxy, and let's for our purposes limit our proposed model of the universe to ONLY the galaxies (forget dark matter etc.) NOW: if we wanted a MODEL of our Milky Way and, say our two closest galactic neighbors we would have a 2 dimensional (for the most part) triangle with three dots. Now lets add a fourth galaxy and a fifth etc. so now we have a 3 dimensional model (presumably). In other words, each galaxy RIGHT NOW is somewhere, and what I do not understand (because I am dumb) is why we cannot continue adding galaxies to our model until we have all 200 billion accounted for.

The reason that you can't do what you are describing is because there is an implicit assumption in the process that you describe, and that assumption appears to be false based on current understanding of general relativity and cosmology.

The assumption is that one can construct the universe by first starting with an empty 3-dimensional Euclidean space and then simply locating within that space the various galaxies. If space really were Euclidean 3-space thn the process that you describe could, as an idealizatoin, be carried out and the result would be a complete model.

But, the data indicates that space is much more complicated than that. In fact, space doesn't exist at all. Neither does time. Instead, according to Einstein's theory of general relativity, what we have is a complicated structure that combines both space and time, a 4-dimensional manifold called space-time. And even if you are able to visualize a 4-dimensinal space (it is not as hard as you think) that mental model is not complete. That is because the dimensions, both space and time, are all mixed together and you cannot separate them, except locally. This is result of what mathematicians call curvature. Space-time is curved, and it is the curvature that accounts for what we call gravity.

So the problem in not that one could not, as a mental exercise, carry out the process that you describe. The problem is that the result would not be a model of the real universe in which we live.

 

[DrRocket]

Thanks Speed. Is that a sphere with all the galaxies on the outside of it like dots on a balloon? Or is it a sphere with galaxies on the outside AND inside?

There is a crucial point here. Sometimes you will see references to the possibility that "space" is spherical. That is indeed one of the possibilities alllowed by general relativity.

But a sphere is NOT a ball. A sphere is the surface of a ball. When considered as a manifold, which is how these things are treated in general relativity, a sphere has neither and inside nor an outside. There is no edge.

This idea also works in higher dimensins, so that the sphere that is discussed in cosmology is the surface of a 4-dimensional ball. You may have some trouble visualizing this, but the balloon analogy is OK so long as you recognize that for the purposes of a model of the universe the 'inside' and "outside" simply do not exist.

 

[JMFNYC]

Dr.: Could we not build a 3 dimensional model of all the galaxies in, lets say. the Virgo cluster? At 11:37 EDT, all the galaxies in this cluster were in a certain PLACE - is this not correct? I do not understand why this is not a fact. And if it is a fact, then I do not understand why we cannot add every other galaxy in our observable universe to it. In otyher words, The Milky Way and Andromeda are somewhere, right now, in relation to eachother. Isn't every other galaxy in the universe somewhere, right now, in relation to us?

 

[SpeedFreek]

Dr.: Could we not build a 3 dimensional model of all the galaxies in, lets say. the Virgo cluster? At 11:37 EDT, all the galaxies in this cluster were in a certain PLACE - is this not correct? I do not understand why this is not a fact. And if it is a fact, then I do not understand why we cannot add every other galaxy in our observable universe to it. In otyher words, The Milky Way and Andromeda are somewhere, right now, in relation to eachother. Isn't every other galaxy in the universe somewhere, right now, in relation to us?

For starters, have a look at the Relativity of Simultaneity. There is no absolute sense in which you can say that two things happened at the same time, if those events are separated by space. I should note that this is not due to the difference in the time it takes light to travel to different observers, it is a fundamental effect that remains even after you calculate out light-travel time.

 

[JMFNYC]

Ok so we can never have an ABSOLUTELY ACCURATE model or map of anything ever? When I look at a map of the US it is already flawed becuase the continents are shifting. Bet we still have a fairly accurate map of this place at a certain time. What is so special about the rest of the universe that we cannot have the same (inaccurate) map?

 

[SpeedFreek]

Consider one of the most distant galaxies we have seen. The light from that galaxy has been travelling for nearly 13 billion years and a lot could have happened to the galaxy since - how might we predict where it is today? We can make a good approximation of the distance it might have receded in that time, but we cannot predict how it might have interacted with the other galaxies around it - they may have merged. The original galaxy might not even exist any more.