There is no “edge”. It’d be like getting in a boat here on earth and sailing about looking for the edge of the earth. You’ll never find one. Supposedly you could fly all the way around the Universe and come back to where you started but I think you’d have to travel faster than light to ever manage that trick and FTL travel is a whole other can of worms with its own issues.
Sorry…to further add to what I just wrote…
“Stuff” in space is pretty uniformly distributed. Certainly there is local lumpiness and features but in the bigger picture there is pretty much stuff all over the place in generally equal proportions.
To go back to the balloon analogy (I know real cosmologists think it is flawed and it is but it is so handy):
Imagine taking a marker and putting dots on an uninflated balloon all over it. Now blow up the balloon and you still have dots all over it but each dot is moving away from every other dot and there is no one place you look and say “no dots…must be the edge”. As it happens when astronomers look at the Universe they see something along the same lines. Everything is, generally, moving away from everything else just like the dots on the balloon do when it is inflated.
Another way to look at it:
Before the universe was the size of a pearl, we go back further in time in which the universe was just a point. (Conjecture, of course, current equations fail us at that ‘point’ of ‘time.’)
And so, if you ran time backwards, you would see that everything would fly towards each other and collapse in a single point which was once at the big bang.
Thus, at one point, everything was at the center. All space-time can trace its history back to that single point. From the point of view of every current point of space-time in the current universe, it was once at the center of that original point and has witnessed all other space-time expand from it. Every current point in space-time can claim that it is the center of the universe from its own point of view.
Peace.
Is it accepted wisdom that the universe is curved round on itself, or is it just one of many models? If it is the best model we have, is it conceivable that one day we may have powerful enough telescopes to be able to see and recognise the same galaxy cluster *looking in two different directions * thus proving it beyond doubt?
So, when the universe was pea or penguin sized, was all electrons and protons and neutrons and whatever lumped together? I.e. was the universe so small because there was no vacuum between those particles.
I guess there were no particles either, but what was that pea made of?
Actually it seems the current best guess is the Universe is flat like a sheet of paper. This is based on recent measurments (link below).
You have a few choices that are determined by the overall density of the Universe.
Sphere – Universe is “closed” and positively curved, density is high enough to cause eventual contraction to a Big Crunch as the Universe implodes (in a manner of speaking).
Saddle – Universe is “open” and negatively curved, density is less than that needed to cause eventual contraction and expands forever. Universe dies in an eventual “heat death” (everything cools to near absolute zero).
Flat – Universe is “open” with no curvature. Density is exactly equal to the critical density to cause one of the above two options. Universe expands forever and again suffers an evetual heat death.
Link to pictures of shapes described above
Note that those shapes are a bit misleading as they are 3D and this is all in 4D (or more) but they get the idea across.
Link to quick story on WMAP measurments that indicate Universe is flat
There are other ways you could get wrap-around, even with a flat Universe. However, using data from the Cosmic Microwave Background (which is at a greater distance than any galaxy we might observe, and in fact the greatest distance we can observe using any sort of light), we’ve looked for this sort of matching patterns exhaustively enough to be certain that if there were any such matches, we would have found them by now. It’s still possible that the Universe does wrap around, but if so, it’s on a larger scale than we can observe.
Does this mean that it is still possible that the universe does not wrap around? In turn does this mean there might be an “edge”? I am a bit confused, trying to crowbar my perception of 3D space into a flat 4D balloon!
Never fear…picturing 4D shapes for us mere mortals is beyond most of us. I remember Chronos telling me several years ago he can do it and he explained it (time shifting a shape) but I admit I never really got what he was doing and still suspect it is a trick and not really visualizing a 4D shape. Then again he is smarter than I am and studies this stuff much more so it is quite possible my brain wattage is simply too low to grasp it all.
The way I handle it is not visualizing 4D space but imagining I am a 2D creature in Flatland and how I would see things that way compared to my 3D reality.
In short picture you are a 2D creature living on the surface of a sphere. As such you have NO concept up “up” or “down” directions. There is only right, left, backwards and forwards. To you your world is perfectly flat. If someone told you to find the “center” of your universe you simply could not do it as there is no center to the surface of your Universe. Now from my 3D perspective I can say there is no center to the surface of the sphere but there is a center to the sphere itself because I can see the third dimension. To our poor 2D version of ourselves this is simply incomprehensible except in the most abstract, thought experiment manner.
So just extend that analogy to yourself. Except now you are a 3D being with no concept of what is “left of right”. We are stuck on the surface of a 4D hypersphere or hyperplane or hypersaddle or whatever and simply cannot comprehend except in an abstract (or mathematical) sense any other directions but up, down, right, left, backwards and forwards. But using our thought experiment of our 2D selves we can at least see what is going on in theory.
As for an “edge” to the universe I simply cannot see how that would work…even in a flat universe. The universe is “everything” so what can be “outside” or off the edge of everything? Run into a wall? Where does the wall go? Peer over the edge? What could possible be there?
I imagine there muct be some cosmic something keeping us from those occurrences. Perhaps a flat universe is really a 4D Moebius Strip such that we can move all over it and never find an edge even if it is ultimately finite and not a sphere (it’s 4D aspect means we cannot find an edge in any direction we can go). I admit I just made that up and have never heard it proposed like that…just speculating but it gives me a way to think of something less mind bending than peering off the edge of the Universe.
I suppose the thing I really struggle with regard to the universe the size of a pearl theory (just after the big bang) is not that all the matter was contained within such a small volume, but that all of space was contained in that volume as well. To me it is conceptually easier to imagine an infinite volume with nothing in it, that we are gradually expanding to fill. I recognise, however, that this view is not one that stands up to scientific scrutiny. As such, an “edge” to the expanding universe is also anathema to cosmologists. Anyway, thanks for trying to help my understanding. I am now going to take a strictly 3D route to my bed, before my head hurts any more!
It is currently not only possible, but presumed true that the Universe does not wrap around. This is not really a statement based on observation (we can never rule out the possibility of wrap-around on very large scales), but merely on Occam’s Razor: Since we can’t see any wrap-around, we’ll presume it’s not there. If this is the case, then the Universe is infinite in extent.
Two-dimensional analogies might help here: A closed universe (one with spherical geometry) is like the surface of a sphere. It’s finite, and has curvature measureable by someone living on that surface (more on this in a moment). A spatially flat universe can be like an infinite plane, which has no edges by virtue of its size, or it can be like the map in many video games, which wraps around at the top/bottom edge and the left/right edge, and hence really has no edges at all. In either case, there would be no curvature measureable by someone living on the surface. And you could also have an open universe, with hyperbolic geometry, which can be locally considered as being like the surface of a saddle. Such a universe could be infinite, or it could be finite due to wrap-arounds similar to those on a video game map.
In all of these cases, any point in the universe looks like any other. So no point can be identified as being the unique “center”, and there are no edge points. You can keep on going in any direction you want, and you may or may not eventually come back to where you started, but you’ll never come to a region which is empty forevermore.
Now, more on curvature. We’re used to thinking of a curved surface in terms of a higher dimensional space that it’s embedded in, but that’s not actually necessary. You can measure the curvature of a surface without ever actually leaving that surface. One simple way to do this is by measuring triangles. You’ve probably learned that the sum of the angles of a triangle is 180 degrees. This is not necessarily true, however; if it is, we say that the space those triangles are in is flat. Note that this has nothing to do with the number of dimensions of your space: A 3-d space, for instance, is considered “flat” if all of its triangles behave this way. But let’s consider some other “space”. For instance, take three points on the surface of the Earth. Let’s use the North Pole, the point at 0 latitude, 0 longitude, and the point at 0 latitude, 90 degrees W longitude. Now, draw a big triangle connecting those points, but restrict the lines of your triangle to the surface. You’ll find that each angle of this triangle is 90 degrees, for a total of 270 degrees in the triangle! In fact, for any triangle on the surface of a sphere, the sum of the angles is greater than 180 degrees, and how much greater depends on the size of the triangle. So if a group of creatures living on the surface of a sphere measured large triangles and found that the angles added up to more than 180 degrees, they could determine that they were living on a sphere, and even how big that sphere was. Note that they never needed to leave the sphere for this: Not only can they not tell you whether the center is “up” or “down”, they don’t even need to have a concept of “up” or “down” at all. Similarly, if you measure triangles and find that the angles add up to less than 180 degrees, you know you’re living in a hyperbolically curved universe (one like a saddle), without ever needing to refer to higher dimensions.
We have made similar (but more complicated) measurements in our own Universe, and we find that, to the best that we can determine, our Universe is, in fact, flat. That is to say, the largest triangles we can measure still seem to be 180 degrees. Note that we can’t be completely certain that it’s flat: It’s quite possible that it’s curved, but that we just can’t measure triangles which are large enough. A small piece of a curved surface will seem to be flat. But even if we found that our Universe was curved, that would not necessarily mean that there’s some other dimension that’s it’s curved in.
Chronos:
How do you go about measuring the topography of the Universe on the earth?
I followed you use of triangles no problem but unless we have sent probes to deep space to setup a massive traingle with laser beams I don;t see how the results could be reliable.
The problem (as I see it) is the earth itself is a gravity well. The sun is a gravity well and indeed the whole solar system warps the space about it. Space is essentially curved locally and that to me would seem to skew the answers of your triangle measurements.
Not saying it can’t be done. Indeed if you say it has been done I believe you and have no doubt scientists accounted for my objection and probably a few dozen other ones I have not thought of. Just curious.
but, but… it has not always been that way has it? Was I correct in thinking that the established view is that not only all of the universe’s mass and energy were contained in a pearl sized volume, but that all of space was contained in it as well? I confess I struggle with this idea, but if it correct, then at least at that moment the universe was not infinite, no? Even if it has been expanding at the speed of light ever since, it still cannot be infinite in extent can it? I feel I am not adding much in the fight against ignorance here, but I appreciate the help in developing my undertanding.
Here is one possible answer that avoids the Big Bang singularity and (I think) allows for infinite space. Brane Theory