# Please tell me if im imagining this correctly.

1. Jul 4, 2008

### zwest135

Just so you know...i just got out of high school, so my understand of physics is limited to say the least, but I have read that Einstein's theories of relativity predicted the expanding universe, here is what I imagine and tell me if this makes logical sense.

Ok we're 3d objects in a universe with 4 spatial dimensions. So I imagine us as dots on the surface of a sphere with an even grid on it representing the spatial fabric. Then I put all the mass (dots) at the top. This mass stretches the spatial fabric up towards it, so now what you have is a sphere with a bunch of lines at the top and close to non at the bottom. This represents space just before the big bang. Now what happens, because space is so stretched, the universe (by some force i guess) goes to correct this by the sphere blowing up like a balloon, as it does this all the dots spread out rapidly and the energy and heat dissapate and the lines (spatial fabric) even out and become uniform, which has not happened fully yet.

However I don't think this is accurate because if the lines evened out...there would be no more gravitational pull right? and I'm not even sure that if the balloon were to expand the lines would become even...i don't think they would...they would all stay the same distance from each other in proportion to each other, right? I don't know why but I thought that the warping of space and time by mass caused the universe to expand in an effort to correct the warps in the spatial fabric, but I don't think that is correct...

I'd just like to hear your opinions on this and explain, in a non mathematical way, if thats possible, how Einstein's theories predict the expanding universe.

Thanks, Zach

2. Jul 4, 2008

### marcus

What's correct? Let's say you want to imagine the mainstream picture. You haven't got it right then.
In the mainstream picture the dots are all more or less uniformly scattered all over the sphere(the 2D sphere analog of our 3D)

and in the early universe the sphere has a small radius, so the dots are close together.

You don't start with the dots clustered at the top, or clustered anywhere. The big bang does not start with a clustering of matter, leaving the rest of space relatively empty. It starts with space having a small volume in which a lot of matter is concentrated----more exactly with a state of very high density.

as the volume increases the density thins out.

Yes! that is part of the mainstream movie. You've got part of your imagining in line with the usual cosmology picture show. Just not the clustering at the top at the beginning.
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just out of highschool, you probably have not ever solved a differential equation yet.
that is a rule that lets you calculate the next small change in a system from looking at its present condition.
most physics systems are run by (often very simple) differential equations----
the present size, pressure, density, speed or whatever, tell you what the next little change will be
In cosmology there are the two Friedmann equations which govern how the metric evolves. it is a lot simpler than it may sound.
You ask how can one get from Einstein Gen Rel equation to the business of distances increasing.
The Friedmanns are a simplified no-frills version of the main einstein GR equation, stripped down to essentials for lazy people. the trick is to assume a more or less uniform universe, then things boil down to a few numbers like the density, the scale-factor (size indicator)...
you can solve the Friedmann equations and get solutions where distances are either increasing or decreasing. but the decreasing case doesn't fit reality (things are not crashing in on us). So the solutions that fit are ones with increasing distance and we go with that..

it is pretty basic. you just have to feel comfortable with a differential equation, not too different from what Freshman Engineer majors have to solve for homework. or maybe they have a computer package to do it for them now.