The recent space discovery that our universe is expanding at an accelerating rate, and will continue to do so forever, implies that the critical density of the Universe is 1.0, that is, it is neither closed like a sphere nor open like a saddle, but is , rather, flat, like a pancake. But I am having trouble understanding how the proven curvature of space time (gravity) can be 'flat' and 'curved' at the same time. Can someone explain??

Large-scale cosmology isn't my strong point, but I'll give it a shot since no one else has yet.

I think the 'flat' refers to the overall shape of the universe, but the curvature caused by black holes, stars, planets, people, etc., is a local curvature. It's like how we consider the earth to be a sphere even though it has local curvature in the forms of mountains, valleys, etc.

This is how I make sense of it at any rate.

Of course I find this after I post.

It turns out I was right:
http://arcturus.mit.edu/ask/universe.html#q4
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03-02-03, 08:58 PM
gerry
Methos5K..
Thanks for your response and the site! I agreed with your answer even before the 'fact', assuming that our universe is indeed 'flat'as the recent evidence appears to indicate.

03-26-03, 02:24 PM
Julieta Martinez
I still don't understand the concept of the universe being flat. Isn't the universe infinite in all directions?

03-26-03, 03:21 PM
methos
Julieta - I'll give it a shot, but I'm not sure I can explain this one clearly.

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Start by imagining a someone living in 2 dimensions. This would be somewhat like an ant on a surface that it can't dig into. Up & down have no meaning to it because it cannot move in those directions. Only forward & backward and left & right have meaning to it.

What happens when we place this surface in 3-dimensional space?

Now imagine that the surface the ant lives on is flat. In this case, the ant can travel in any direction forever and never end up in the same place. A triangle measured on this surface will always have 180 degrees.

Now imagine that the surface is a sphere, like the earth. You can walk in a straigh line along what seems to be a flat surface and eventually end up in the same place. some lines that are parallel at some parts of the surface intersect at other parts (think of the longitudinal lines on the earth that are parallel at the equator but intersect at the poles). If you measure a large triangle on this surface, it will actually have more than 180 degrees.

Finally, imagine the surface is shaped like a saddle. On this surface, lines that start out parallel eventually end up moving away from each other. Large triangles will actually have less than 180 degrees.

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Now, imagine living in 3 dimensions (like we do). Imagine these three dimensions exist in a fourth dimension (hard to imagine, I know, I certainly can't really imagine it).

There are 3 possible cases, similar to the cases above.

The first case is a "flat" universe. In this case, parallel lines never interstect, the universe goes on forever in every direction, and the universe will continue to expand forever (although how fast it is expanding will slow down to nearly zero eventually).

The second case is a "closed" universe. In this case, parallel lines will eventually intersect, if you travel far enough you will end up back where you started, and the universe will expand at first, but contract in the end (like a balloon you first blow up and then let the air out of).

The last case is an "open" universe. In this case, parallel lines will eventually move away from each other, the universe goes on forever, and the universe will continue to expand forever, without slowing to close to zerto as the flat universe would.

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What the evidence currently points to is the flat universe, which is pretty much what it sound like you picture the universe as (i.e. infinite in all directions)

03-26-03, 04:31 PM
Julieta Martinez
Okay this seems like an extremely hard subject to comprehend. I got lost in your second paragraph. But I will read it over and over, maybe it will sink in.

03-26-03, 07:42 PM
methos
Julieta - In my opinion, it's an extremely hard concept and not one I feel I have a complete grasp of. If you have any questions about my explanation, let me know and I'll try to explain or find a site that can explain.

the take-home messages of all of this (assuming the universe really is flat):
1) you could travel forever in any direction without circling back.
@) two object starting off on a parallel course will stay on parallel courses.
3) the universe will expand forever, although the expansion will slow down to nearly zero.

In other words (except maybe for that last part) the universe is exactly how it sounds as though you picture it. The closed and open possibilities are harder to understand, but they don't seem to be the case anyway.

03-28-03, 04:18 PM
Matiqua
I think I sorta get it, but its very complicated.

03-31-03, 02:47 PM
Julieta Martinez
WHen you say that you could travel in any direction forever, does that include up and down? I guess I am picturing that I am at the center point of a sphere, and I can travel in every single direction or to any point on the surface of that sphere.. I guess that's my question.

Am I making sense??

03-31-03, 04:04 PM
coldfuse
Ah - but the earth is not (likely) the center of the Universe! That distinction belongs to my teenage daughter. Wink

03-31-03, 05:14 PM
Julieta Martinez
I just gave the sphere as an example of what I was seeing in my head as going in all directions.

Thanks Fuse!

04-01-03, 09:27 PM
gerry
Julietta....
The Universe IS very difficult to comprehend, especially since it has more than our familiar 3-dimensions. The Universe has at least 4 dimensions, time being the fourth, which is inseparable from the other 3, and there likely are many more spatial dimensions that we can't see. To complicate the situation, the universe is also expanding, even if it is already infinite in extent. This defies common sense, since how can something already infinite expand to a size greater than infinity, although I think the mathermeticians will argue that twice infinity or even 1/2 infinity is still infinity.
Back to your question, yes, you can travel forever in any direction, up or down, left or right, or back or forth, always taking time with you, while probably unknowingly moving through other higher dimensions that are unseen.
It is sometimes helpful to assume our Universe is like the surface of an expanding balloon, as Methos described very clearly, and that we are 2-dimensional 'shadows' living on that surface. As such, we could only move along the surface of the balloon in 2 directions, back or forth or left or right, but also unknowingly moving up...in a 3rd dimension, as the balloon inflates, but never down unless the balloon deflates. As the balloon inflates, your 2-dimensional shadow-like friend will get farther and farther away from you, without actually physically moving relative to you. You will wonder, "where the hell is he going?", and he will wonder, "where the hell is she going", and in actuality, you are both curving inward into the 3rd dimension, which exists inside the balloon. The question, i believe, we all should be asking, is not what lies outside the Universe as we expand perhaps forever, but rather, what the hell lies inside the Universe?
Sorry, I'm sure I have confused you, as I have confused myself.

04-03-03, 12:09 PM
Julieta Martinez
gerry, I get it!!

Thanks for your great examples!!! I can surely understand the "where in the hell is he going?" better than pure scientific talk.

Big Grin Wink

I think I am going to buy a book about that so I can learn more. What an interesting subject.

This message has been edited. Last edited by: DorianGreyed,

Julietta...it is fascinating, isn't it? While there are many good books on the Universe, the 2 I recommend are Brian Greene's "The Elegant Universe", and Stephen Hawking's "The Universe in a Nutshell".

Thanks! I think I'll go to B & N tonight!!