As the young boy was depressed because the universe is expanding, I have to add that there is a question I’ve been pondering recently.
Doesn’t the fact that it is expanding indicate that it is expanding into something? Unless you define the universe as those objects that occupy space, then I guess you could say the objects are moving within that space, but by saying the universe is expanding, doesn’t that imply that space as a whole is expanding? And therefore it is expanding into something?
Or am I just spinning my wheels here? And for that matter, where is here, anyway?
(emphasis added)
Yes, but space is definitely not expanding into other space.
I tried last New Year’s Eve to explain this to a fellow member of the local Bertrand Russell Society at a party. He freely admts to being mathematically challenged. He insisted that if our s-t continuum was expanding then it must be “pushing higher dimensions aside” and that isn’t right either.
Neither of us had been drining much, but it was still a mistake, because he is a lawyer and loves to argue. When I tried to explain “Flatland” he wanted to argue against that, because he wanted to talk about higher dimensions, not lower. :rolleyes: (This is an example of listening with your mouth instead of your ears.)
I have since thought of a “perverse” ilustration which may help get the reality across.
There are many, many threads on this board on this topic and some start with the same question. Try searching on “expanding universe”.
In a nutshell, the problem is that it is very difficult, if not impossible, for the human mind to conceive what is going on in any intuitive way. The whole concept is dealt with mathematically.
The usual analogy to explain the expanding universe is the surface of an expanding balloon. However, the purpose of this analogy is to illustrate how everything can be moving away from everything else without there being a “center of expansion”; that is not exactly the same as your question. But people think about the balloon and how it expands into three-dimensional space, so think our universe is doing the same thing. That’s where the analogy breaks down.
Its expanding, i.e. the limits to the universe are going further away (from everywhere).
**It is expanding into nothing (literally), which is why this is called a universe and there is nothing else.[/**QUOTE](emphasis +)This may serve only to confuse, though, since “nothing” is usually viewed as empty space.
It’s incorrect to say that the limits of the Universe are getting further away, since the Universe does not have limits. The simplest way one can describe the expansion of the Universe is to say that things in the Universe are getting further apart (at least on sufficiently large scales). And the further apart two objects are, the faster they’re getting even further apart.
(emphasis +)This may serve only to confuse, though, since “nothing” is usually viewed as empty space.
Okay, before I give the “perverse” illustration, here is an old one, used by Hoyle in his popularizing essays.
Paint dots on a balloon to represent galaxies. When you blow up the balloon the galaxies move away from each other in proportion to their distance. (The key advantage of this very limited illustration is that each galaxy can be seen as the center of the expansion.) The two dimensions of the surface represent our (minimally) 3-space. The radius of the balloon, said Hoyle, represents time. I think that it may also represent a dimension (or three) “higher” than or own (minimum) of 3 spatial dimensions.
The one major drawback is that the “galaxies” (painted dots) themselves would be expanding as well, and they don’t. (I recall trying to rework it in my head with glued-on beads, although that would be hard to arrange physically, since they would tend to get unglued very quickly.)
I should say that “galaxies” really refers to units that are rarely single galaxies, and usually clusters of galaxies.
I think though, that the main problem is that the univetrse/total galaxies are finite here. There are ways to have an infinite numer of galaxies in an illustration.
Some time back, the Steady State Theorywas prominent. Galaxies continually condensed out of “Background Material” AKA intergalactic medium (gas and dust). This would run out at some point, but according to SS matter is continually being “created” everwhere, and so it all balances out. Even the total matter in “the Universe” supposedly balances out, since galaxies that move too far out for us to see (per “Visible Universe”) are no longer counted.
Evidence accumulated against SS, so it is no longer considered fashionable. Some diehards have insisted, though, that we are in a Big Bang it is only a bubble in a SS Universe.
Basically expansion means that the distance between any two objects (on a level that is suitably large enough that universe appears homogeneous and isotropic and disregarding the movement of the objects) increases as a function of time.
On a more technical level it means that in a frame constructed out of co-moving observers in local frames where the universe is homogenous and isotropic, the cooridnate distance between two spatial points is increasing as a function of cooridinate time.
There aren’t any edges (in standard big bang cosmology) or other non-removable anisotrophies or inhomogeneities as the assumption is that there exists a frame where the universe is homogeneous and isotropic ( this assumption is called the Copernican principle).
Here is the way I think of it. Space-time is itself a thing. Even completely void and empty, space still has physical properties, including some curvature from gravity and a bubbling background noise of virtual particles. Anything that is not space-time cannot be said to exist - without those physical properties I mentioned we can’t even measure “nothing” there. There’s less than nothing - it doesn’t even have the framework to be nothing.
For people who get confused about the balloon example because the balloon is expanding into air, here’s a metaphor that doesn’t have that breakdown: A chess board. Take a standard chess board of 8x8 squares. The distances in the rules of chess are all measured in the unit of a square, but we could say that a square is a billion light years if that helps the analogy, since we measure the rules of the universe in our units of distance. Now, take that chessboard with all its pieces on it and start drawing new lines, cutting each square into half vertically and horizontally. Now the board is 16x16 squares. The distances have all doubled inside the board as far as the pieces are concerned and each piece perceives that other pieces are moving away from it. None of this requires that the board itself be expanding into something else.
As with all analogies and metaphors for something so alien as space-time, this is going to break down in various ways, but hopefully it will help in showing that you can have “expanding” without having “expanding into.”
Yes, but not as much as you’d think. Most (about 70%) of the “stuff” in the Universe appears to be dark energy, which in turn appears to be a property of spacetime itself, so as the Universe expands, you end up with more dark energy filling it. The density of all the rest of the stuff (including dark matter) is decreasing, but it isn’t all that significant compared to the constant-density dark energy.
I know this is going nowhere, but I will try anyway. Let’s start out with a 1-dimensional universe, i.e. a string. Imagine that the string is actually a finite loop, rather than an infinite string (although this is not crucial). Now imagine that string. You imagine it as embedded into a space of a larger dimension, right? To a mathematician, though, a string can be defined independently of any ambient space. As such, it can grow (or shrink) in the sense that points on it can get further apart or closer together. You probably cannot imagine this free of an ambient space.
Incidentally, a string cannot be knotted in itself–the concept is meaningless and cannot be knotted when embedded in a 2-dimensional space or in any space of dimensions 4 or higher. Only in 3 dimensional space and knottedness is a property of how it is embedded in 3 dimensional space, not of the string itself. Similarly an ordinary sphere could be knotted in 4 dimensional space, but not in any higher or lowere dimension.
Now getting back to our ordinary 3 dimensions of space, the universe could be a 3-sphere (the 3 dimensional analog of a sphere) or 3-torus or something more complicated. If you could think of it not as being part of a 4 dimensional space (let’s leave time out of this), but just as having its own intrinsic geometry, then it can expand without expanding into anything.
That’s actually very close to what I was going to give as a “perverse” illustration.
Suppose that the distance between “galaxies” actually remained constant. While that is going on, each “galaxy” and everything in it shrinks in identical proportion.
To any observer the only thing that really seems to be changing is the mutual distance between “galaxies.” Their own “galaxy” (and individual galaxy), planetary system, planet, surface features, population centers, and so on, would be assumed to be remaining with contant sizes. Especially since any measuring instrument would be shrinking along.
So, yes, the density of matter would be dropping, and by a factor of the cube of linear change. For example, if all lengths and distances within a “galaxy” shrink by a factor of 10 within x years, the matter density would decrease by a factor of one thousand during those years.
