OT: Gravity, Dark Energy, Universal expansion

Ragnarok-X · #1 January 5th, 2008, 04:37 PM

I think you are on crack because while i understand most of the words you are using, using them in conjuction drives me kinda insane

MasterChiToes · #2 January 5th, 2008, 05:42 PM

The universe expands like the surface of a 4D balloon, with the surface being three-space.

But the rub is... not only is the far side of the universe is 'farther away' than the big bang... it is so far away that light from one side will never meet light from the other... an effective 3.1415c receding velocity due to spatial expansion.

Renegade 13 · #3 January 5th, 2008, 06:10 PM

Quote:

Ragnarok-X said:
I think you are on crack because while i understand most of the words you are using, using them in conjuction drives me kinda insane

I tend to have that effect on people from time to time

Suicide Junkie · #4 January 5th, 2008, 07:01 PM

The big bang didn't occur in some particular part of spacetime, but rather it is when everywhere was the all the same place.

It makes no sense to say that something is twice as far away as the big bang.

MasterChiToes · #5 January 5th, 2008, 07:10 PM

Quote:

Suicide Junkie said:
It makes no sense to say that something is twice as far away as the big bang.

Yes it does... since in this context time = space (with a factor of c). The big bang was ~20 Billion years ago which makes it 20B*c far away [time] vs the farthest point in the universe which would be 20B*c*pi [space] (1/2 the circumference 2pi*r of the sphere described by r being the time since the BB [*c]).

Suicide Junkie · #6 January 5th, 2008, 07:46 PM

Come on, man.

Its not 20B light years away.
Its 20B years away.
Those are completely different units.

The size of the *visible* universe (roughly, time since photons were released * speed of light) is different from the size of the universe (which depends on its topology)!

Also, you are stating that there exists a farthest point in the universe from somewhere. This implies that you are assuming a closed universe. Why? Evidence indicates a flat or very slightly open universe last I checked.

The crux of the matter is that "speed of light perpendicular to 3-space" is nonsensical, since distance and speed are both undefined for that.

MasterChiToes · #7 January 5th, 2008, 09:41 PM

In my first post I said, "for the universe expanding at the speed of light"... which is an assumption. The rest follows from the balloon model of an expanding universe (the diagram above). In this case, the balloon model is an expanding 4-sphere [due to symmetry the number doesn't matter... it could be any n-sphere n>3], where the balloon's outer surface is 3-space.

Yes, the size of the visible universe is different from the size of the universe, which as I said, can make the universe untraversable... that is was one of my main points.

However, for an n-dimensional balloon model of the expanding universe, it does not by any means imply a closed universe, only a finite one... it can still be open if it expands forever. In any manifold/topology, any point can still have a farthest point... like two opposite sides of the balloon... it is a matter of spatial symmetry, and has nothing to do with the openness. Every point on a circle, or sphere, or n-sphere has a farthest point.

"speed of light perpendicular to 3-space" is not nonsensical, just non-physical... imagine a balloon who's radius is expanding at the speed of light. Mathematically, that perpendicular-ness allows speed and distance to be defined for more dimensions using the same symmetry that allows time being the 4th dimension (ie time= distance/c).

r^2 = x^2+y^2+z^2 where r=c*t (t being the age of the universe)
as t increases, any two "fixed" points (x1, y1, z1) and (x2, y2, z2) will be moving away from one another.

(Personally, I would like to draw the light cones for that diagram, but the curves are pretty hard to photoshop.)

Suicide Junkie · #8 January 5th, 2008, 10:16 PM

Presuming all of your topology and a fixed rate of expansion, why are you measuring "distances" through your time axis?

Go far enough back in time... and you start going forward again, but on the opposite side of the universe?

My problem is that you are mixing a space dimension with a time dimension.
You can't travel through the middle, since that is time travel. And you're measuring through to negative time which is definitely not right.