infinite universe

EL

The thought of a finite universe makes me sleep better at night too, it's easier for me to accept a universe with a finite amount of matter than an infinite amount...but, who says I'm supposed to sleep well at night .
Anyway, as you say, back to the main topic...

SpaceTiger

Well, no, I wouldn't say it's like that at all, nor do I think that every theory has to give us definite answers. Inflation is saying that we shouldn't be surprised that we're having trouble distinguishing between the finite and infinite universe. One of the main points of inflation was that it explained, without fine-tuning, how the universe could appear almost perfectly flat. In doing so, it also made other predictions that were testable, some of which have been verified.

If it happens that the universe conspires to make certain things difficult to observe, then we'll just have to live with that. If we rejected all such theories, there would be no quantum mechanics or chaos theory.

EL

Think of a 2D universe situated on the surface of a sphere. The area of this universe is finite, but anyway it has no boundaries where the 2D beings living there can "fall over a cliff".
Now you of course have to generalize this picture to a universe with 3 spatial dimensions, where the "area" is replaced by the "volume". Although our brains cannot really handle this generalization, this would give you a hint of how a universe could be finite and still unbounded.

ComputerGeek

actually, I think it is infinite yet bounded.

Danger

Now you're just teasing...

What I was getting at is that if it formed a finite time ago, and is expanding at finite speed, it can't be infinite. There's a maximum size that it could have reached by now. 'Infinite' implies that it had no beginning, not just no end.
Perhaps I'm using the wrong definition of 'infinite' here, but that's what it means to me.

Garth

How?

Garth

alfredblase

The universe is defined by what we as individuals percieve. As we all believe that we can only percieve up to a certain distance, then the universe is finite without question. And since there is no diference as to whether I look in this direction or in that direction, we can see equally far in all directions, so at any given moment in time the universe is spherical with me as the origin of that sphere.

Further I would add that speculating about what is outside our universe, as we believe that it can never be interacted with, is an exercise in philosophy at best. And philosphy can be very ...

mars2

Oh~~It is so involuted.

EL

Why can it not? If the universe was (spatially) infinite at Big Bang, which may have been the case, it should of course be infinite now also. (Or are you just talking about the today observable universe?)

EL

Sorry, I can't find "involuted" in my dictionary...what does it mean?

Danger

You lost me on that one. How could it have been infinite when it formed? If it started at zero volume/infinite density, and is now at medium volume/medium density, then it had to have passed through small volume/high density on the way.

EL

Why not? Why must it have been finite?
Why must it have startet at zero volume?
The Universe started as a singularity of infinite density, but not necesary of zero volume. Think of an infinite plane with small dots on it, representing the matter distribution today. Now proceed backwards in time, i.e. move the dots closer and closer together, until you reach an infinite density, i.e. the singularity. The plane still has an infinite area though! You get my point?
This of course requires an infinite amount of matter in the universe.

Hence the Universe could always have been spatially infintie, although the density has changed with time.

Edit: Just to make it clearer: When I say "move the dots closer and closer", I of course mean that the space between the points shrinks, not that some force acts on the dots.

Danger

I'm going to wait for an expert's take on this. Again, I might be misusing the term 'infinite'. To my mind, the only way that something can have infinite density is if it has zero volume.

hellfire

I fail to understand how the universe could have been spatially infinite at the beginning of time. For every cosmological model the volume contained within every boundary at the big bang is zero because for a zero scale factor a(t=0) = 0 the volume element is zero (or all distances are zero in the singularity). A different situation arises as soon as one considers any time slightly greater than zero, avoiding the singularity.

EL

I agree with that using the term "spatially infinite" at t=0 may not really have any meaning, since as you say the scale factor is zero. However, what I wanted to point out is that it need not have started as what we intuitively think of as a point either. I.e. the reasoning Danger is using, that it has to be of finite size, does not hold.
The point I wanted to make is that even though the Universe started as a singularity, it may be infinite at any t>0. Wheter one call it infinite or not at t=0 is more of a personal taste.
Anyway, we do not really know how to describe the universe as time approaches zero, since we don't know about what laws will hold.

SpaceTiger

I think Ned Wright addresses this in a fairly straightforward manner:

How can the Universe be infinite if it was all concentrated into a point at the Big Bang?

EL

Yes! And the question adressed is what confuses many people, since it is simply not the whole thruth. The Universe (i.e. the whole Universe) doesn't have to be concentrated into one point at the Big Bang.