OT: Gravity, Dark Energy, Universal expansion

Suicide Junkie · #1 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.

MasterChiToes · #2 January 5th, 2008, 11:05 PM

In polar or spherical coordinates, r is often constrained to the positive values. Space and Time can be 'mixable' in General Relativity, since the transformations and behavior of distance and duration are essentially the same... and especially since the radial coordinate axis is "unreal" and choosing a measure of space or time (which have a 1:1 correlation) is unimportant. The past doesn't exist anymore, and the old space doesn't exist either. Whether one of the two is more conceptually real is unimportant to the math.

Yes, you can't travel through the middle, only the surface of the sphere is the universe, there is nothing else, unless you want the philosophical definitions that inside of the balloon is the past and outside the balloon is the future. However, you can also choose the philosophical definitions that inside the balloon are old universes and outside are coming universes.

Suicide Junkie · #3 January 5th, 2008, 11:42 PM

So, back to my initial issue: "twice as far away as the big bang" is absurd.

MasterChiToes · #4 January 6th, 2008, 04:26 AM

There is a 1:1 correlation between space and time, especially in this example. From an observer, the big bang is seen through space, at a distance. However, the objective and unobservable size of the universe is larger than that... and it can be over twice that observed distance.

Still it is semantics... so don't worry about it. If you understand my example, it would be better to ask... can the universe be continuous if it is un-traversably large, or is there some sort of mega-macroscopic quantization going on, which could imply some sort of emergent macro-forces might exist.

Suicide Junkie · #5 January 6th, 2008, 02:24 PM

Aside from your negative time problem, I guess the issue is in trying to relate your model to reality when it is just too different.

MasterChiToes · #6 January 6th, 2008, 05:07 PM

There is no negative time problem for an expanding 4 sphere surface just like a pocketwatch doesn't prove time travel is possible.

http://www.astro.ucla.edu/~wright/Balloon2.html
http://en.wikipedia.org/wiki/Metric_..._balloon_model

Suicide Junkie · #7 January 6th, 2008, 05:20 PM

You're the one who said "twice as far away as the big bang"

Anyways...

I think the original point is that indeed there are places you can't get to from here.

Assume space is expanding at 20% per year, just for example, and the fastest you can go is 1 light year per year.
In such a case, attempting to travel to Sirius (8.6 ly away) is a bad idea.

Completely ignoring the compound interest you'll suffer:
In the first year, you'll get 1 lightyear along on your journey, leaving 7.6, but in the meantime that 7.6 will have stretched to 9.1
Light trying to get here from Sirius will suffer the same fate, and we wouldn't be able to see it.