Big Bang?

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Big Bang?

Postby Nick » Wed Apr 26, 2006 1:49 am

How do we know that the Bing Bang Theory is right? Just because the Universe is expanding doesn't mean that they will contract into a 0-dimensional point. If we turned back time, and all of the planets were moving opposite the direction they are now, doesn't it make more sense for them to move past each other? There must be a reason this is such a widely accepted theory, but I don't understand how.
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Postby houserichichi » Wed Apr 26, 2006 2:30 am

Pretend the universe is a tennis ball but instead of the inside being empty and full of air it's actually solid the whole way through like a bowling ball...but now pretend that instead of it being "solid" like we normally use the word, instead it's "solid" as in "full" but it's actually made of a stretchy rubber...

So now our universe is a stretchy, rubber, solid, tennis ball.

Now, assume that the universe is expanding as time increases. That means over time the tennis ball will grow to the size of a bowling ball. Heck, if we let time keep on going then it will grow to the size of a basketball.

If we reverse time, however, then logically the universe will contract. So if we started as a tennis ball then we shrink to, say, a ping-pong ball. Keep shrinking til a grain of sand, then smaller and smaller and smaller.

Remember, however, that the POINTS inside the ball are not shrinking. What's happening when we shrink the ball is that everything inside it is getting pressed closer and closer together whereas when we expand the "universe" the points are getting further and further apart from eachother. Can you see it in your head why this is true?

Anyway, back to shrinking tennis balls. It's a result from a branch of math called topology that if you shrink a ball down forever it will become a point. That means that all the points INSIDE the tennis ball universe are now infinitely close together. Since we're dealing with infinity (small) they're all, for sake of argument, at the same place at the same time (but not really...just pretend). So now we have a 0D universe with infinitely close points. That's how it works.
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Postby Nick » Wed Apr 26, 2006 10:29 am

But you have not answered my question. How do we know for sure that all of the points in the universe contract to a single point? How are we so certain that they do not simply pass each other?
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Postby houserichichi » Wed Apr 26, 2006 2:08 pm

I'm not sure what you mean by "pass each other"...

Image

Pretend, at one point, our universe was as big as the ball labeled with the BLUE number one. For now take it for granted that we're expanding so we know that eventually we'll get as big as the ball labeled BLUE number two and then BLUE number three. Do we agree on that?

Now if we reverse time from BLUE number three (but because we're reversing time I'll call it RED number one now) we go back to RED number two and then RED number three...so this is where we just were.

If we continue to reverse time then the ball universe will shrink more to RED number four, then RED number five, then RED number six, etc etc etc. It will get smaller and smaller.

From topology, if a ball of whatever size without any holes or empty spaces in it shrinks down small enough then it becomes the same thing as a point.

Since our ball-universe is shrinking down smaller and smaller it would eventually become indestinguishable from a point. Thus there would be no room inside this very tiny universe for things to pass eachother.

If you were to live through the whole ordeal yourself, from RED number 1 through RED number 6, the atoms, particles, and everything else in your body would be infinitely close. The planets would be infinitely close. The universe would be infinitely small. Sort of the opposite of the cold-universe scenario if the universe expands forever...eventually particles will be so far apart that nothing can exist in its current form because the distances will be too great for any kind of particle interactions on the large scale.

I'm not sure if the picture helped or if I just repeated myself but I broke this one up into paragraphs so, because I'm not quite sure what you mean by "pass eachother" maybe you can point me to which paragraph I assume that they're not...sometimes the internet is a pain for explaining things so bear with my attempts.
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Postby Nick » Wed Apr 26, 2006 9:02 pm

No, I understand you completely :D

See, I thought that the Universe was as big as the number of planets in it, and that the planets were expanding on their own... but what you're saying is that the universe is a sphere and that all of the planets in it are just sort of there...

I have no idea how to use paint, so bear with me here. You have two planets, and let's *assume* they existed there forever. They are forever moving away from each other. However, suddenly they start contracting. What I'm wondering is why the two planets would collide with each other when contracting.. when it's much more probable that they would move past each other.

Am I wrong to assume that the Universe is as big as the planets, instead of the planets are as far apart as the Universe?
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Postby houserichichi » Mon May 01, 2006 3:23 pm

Again, I just don't know if I'm getting what you're saying...the joys of internet, I suppose.

Picture a cube. Picture that cube being filled with points. You can specify the location of anything within the cube by setting up a set of axes and defining where on that axis a point lies.

Now, picture a sphere. Picture that sphere being filled with points. You can specify the location of anything within the sphere by setting up a set of axes and defining where on that axis a point lies. (same paragraph, different shape).

Now, pretend you were to make that sphere get bigger by blowing air into it or something. You may not create new points. When you blow air into the sphere the little dots (points) get farther apart, yes? When you let that extra air out the points get a little bit closer again. That seems intuitive I think.

Now pretend we have a set of axes centred in the middle of the sphere. That is, the coordinates of any point inside that sphere is (x,y,z), for lack of better variables. Now, assume the distance between point A = (x1, y1, z1) and B = (x2, y2, z2) is D1 (assuming we use the Euclidean metric) when the bubble is at size S1 at time T1. Lots of variables, sorry.

Now pretend we blow air into the sphere. The axes do not get bigger with the bubble, they stay the same size. So now on the axis (because it didn't grow with the bubble), point A is now at point A' = (x1', y1', z1') and B is now at point B' = (x2', y2', z2'). The distance between them is now D2 when the bubble is at size S2 at time T2.

Take it for granted (unless you'd like to read a textbook on topology) that D2 is bigger than D1, that is the distance between A' and B' (the two points after we blow air into the sphere) is greater than the distance between A and B (before air). Clearly, S2 is bigger than S1 (the sphere is bigger after we blow air into it) and T2 is bigger than T1 (time increased as we blew air in).

Reverse the process. Start with A' and B', S2 and T2 and D2. Let air out of the bubble. A' goes to A, B' goes to B, S2 goes to S1, T2 goes to T1, and D2 goes to D1. Everything got smaller, distances included. Continue the process even more but throw in a little calculus so that we continue until the distance D between the two points is zero. It's a limit. Notice how as we got bigger time increased and as we got smaller time decreased? So take the limit as time decreases to zero. What happens to the distance between the points? It also decreases to zero.

It decreases to zero means that they all tend toward the same place, that is, a singularity.

NOW, there's a caveat - you can't set up an axis in the real universe because of its global geometry, but at least you can use this as a rough visual aid (hopefully).

Let me know if I can try and explain it any better!!
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Postby Nick » Mon May 01, 2006 10:21 pm

Allow me to explain what I'm saying in further detail (I learned to use paint to do this, lol!)

Firstly, here is the universe at a singularity:
Image

OK, then it begins to expand, and two planets are created:
Image

The blue outline is the border of the universe. I know it's a little off, but pretend that it's a perfect elipse. The size of the universe is dependent to the distance of the two planets.

Oh no! Suddenly, the planets begin to get closer to each other:
Image

However, the big red one (which has more mass) swings the small black one around in its orbit.

The result is this:
Image

Though they were contracting, one planet moved to the side of the other, and they switched. The Universe never again became a singularity. :)
Last edited by Nick on Mon May 01, 2006 11:50 pm, edited 1 time in total.
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Postby houserichichi » Mon May 01, 2006 10:39 pm

I think I see the flaw in your logic...

by your diagrams, when the universe contracts not only will the red and black planets come closer together BUT also

1. the blue border of the universe shrinks (that's what is actually contracting) so there is now less space for the two planets to move in. Eventually that space will get so small that the two planets will be touching...

2. not only is the border of the universe shrinking but also the distance between ANY two points within the universe. That literally translates to the planets getting "smaller". Remember, if the radius of a planet was initially R1 then as it contracts the radius gets smaller (to an observer outside the universe, technically), so R2<R1.

Here is what I'm trying to say in picture format:

Image

I'm not sure if that helps any...but do you see how as the blue circle gets smaller eventually the black and red balls will "touch" even though they get smaller over time too...eventually the blue circle will be infinitely small so that means that there is zero distance between the red circle and the black one.

Hope that graphic helps explain. And FYI, the Paint was fantastic, only thing is on this forum you need to save the files as JPG, not BMP for them to work.
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Postby Nick » Mon May 01, 2006 11:47 pm

Ohhh! I understand now. The size of the Universe is not dependent on the distance of the planets, but rather the distance of the planets is dependent on the size of the Universe.

I'm gonna fix those images right now..
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Postby Keiji » Wed May 03, 2006 4:03 pm

houserichichi wrote:only thing is on this forum you need to save the files as JPG, not BMP for them to work.


<.<

Golden rule: NEVER SAVE JPEGS IN PAINT. ;)

Use PNGs. If you don't have Windows XP, go download a better image editor. :P
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Postby moonlord » Wed May 03, 2006 4:56 pm

Perhaps a golden rule is DO NOT USE MICROSOFT PRODUCTS AT ALL...
"God does not play dice." -- Albert Einstein, early 1900's.
"Not only does God play dice, but... he sometimes throws them where we cannot see them." -- Stephen Hawking, late 1900's.
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Postby papernuke » Tue Jul 11, 2006 5:28 pm

first of all, we dont even know if the universe will contract (big crunch) and the proof for the big bang is that they are going away from eachothers (yea you said that) and because of the BK rad that was from the big bang other theories arent that good for example everything was there just then suddenly
"Civilization is a race between education and catastrophe."
-H.G. Wells
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Postby houserichichi » Wed Jul 12, 2006 2:52 am

Rob wrote:Golden rule: NEVER SAVE JPEGS IN PAINT. ;)

Use PNGs.


I'll ping you in the jpeg!
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