Tocky on 19/6/2009 at 05:15
Quote Posted by heywood
But you couldn't locate the center of expansion because it cannot be defined in the two dimensional coordinates you observe within.
But it isn't just two dimensional when the dimension of time is added. With time comes trajectory because even with all points moving apart isn't there still greater movement in a particular direction over others as with a fireworks burst? Perhaps not but I would be grateful for the explaination.
Does it have anything to do with immense gravity warping time and skewing early results or just that early is a ship that has sailed by any current measurment?
demagogue on 19/6/2009 at 13:05
haha ... yes.
Quote Posted by Tocky
But it isn't just two dimensional when the dimension of time is added. With time comes trajectory
Well, it's still 2 spatial dimensions and 1 time dimension.
But you're right that time and trajectory are important.
I used the word "points on a balloon", but I even corrected myself on this at one point. You're actually looking at vectors, the points moving in a particular direction.
Quote Posted by Tocky
even with all points moving apart isn't there still greater movement in a particular direction over others as with a fireworks burst?
No, but there is an illusion of it.
The illusion: From "your" perspective everything is moving in a particular direction; exactly *away* from you and you aren't moving. So it looks like you are at the "center".
The cold reality: The instant you move to another galaxy, everything is still moving *away* from you in exactly the same way and you aren't moving. It "looks" like the "center" has moved with you.
The balloon-skin-universe is still the best way to see it. But not explaining that again.
Here's a video: (
http://www.youtube.com/watch?v=I9Cjxd4Mjog)
Put your finger on one galaxy and see everything moving away. Then put your finger on another galaxy and see everything is also moving away from you in the same way.
Edit: Then this video shows how we know that other galaxies are actually moving like this, by the redshift of star light.
(
http://www.youtube.com/watch?v=e6G2Z6iD-9M)
And on redshift itself: (
http://www.youtube.com/watch?v=Man9ulEYSgk)
Quote Posted by Renzatic
Beyond wormholes, which from what I understand are basically like 4D holes punched through 3D space (like punching a hole through a sheet of paper in a way), there isn't any information out there beyond the theory. So really, do 4D objects actually exist and intersect with our universe, or is the only proof we have just scribblings on a blackboard?
I think you're confusing the "4D" of spacetime (3 spatial dimensions + 1 time dimension) vs. "4D" hypercube space (4 spatial dimensions).
Wormholes aren't 4D-spatial things in 3D space. They are incredibly curved parts of 3+1D spacetime that connect relatively flat parts of 3+1D spacetime. But it's all still run-of-the-mill 3+1D spacetime.
At least under general relativity (or what I know, anyway; only read a few books), there are not 4D hypercube-like objects in our universe (in 4+1D spacetime), only normal 3D cube-like objects (in 3+1D spacetime). ... Though they do use hypercube geometry for some equations, but (I recall reading somewhere) that's not about describing real objects. (One footnote might be the entire universe; in an analogous way to saying the 2D-balloon-skin-universe wraps around in 3D space. It's still all 2D, but you need the 3D to describe it fully.).
String theory is a different story, since it proposes tiny string objects that can have up to 10 or 11 spatial dimensions (plus 1 time dimension). But that really is still just scribblings on a blackboard, since they don't have any data to say it's talking about real objects yet.
Tocky on 20/6/2009 at 04:09
Quote Posted by demagogue
No, but there is an illusion of it.
The illusion: From "your" perspective everything is moving in a particular direction; exactly *away* from you and you aren't moving. So it looks like you are at the "center".
The cold reality: The instant you move to another galaxy, everything is still moving *away* from you in exactly the same way and you aren't moving. It "looks" like the "center" has moved with you.
Yes but if we can measure the rate at which each galaxy is moving away from each other and then apply that relative to each can't we then discern which direction each is moving more quickly away from to find the center?
I'm saying expanding yes, all over yes, but if the rate is still increasing greater relative to the invisable center the speed will telltale in a particular direction.
And I suppose you now burst my bubble with we can never find the center because the speed is exactly the same in all directions. Or quite possibly what we can discern of the universe is too small a segment. I would prefer to think we just don't have the means yet.
daniel on 20/6/2009 at 04:27
I can't see what the OP's beef is. A theory is the explanation of all evidence we have. Gravity is also a theoy and we see it everyday, there's no denying it's there. But we still don't know the origin of gravity, so it's a theory.
As for religious peoples, keep in mind that the Big Bang theory was proposed by a Catholic priest. Creationism is hardly an acceptable theory for how the universe was created - God gave us brains so we ought use them.
demagogue on 20/6/2009 at 15:22
Quote Posted by Tocky
And I suppose you now burst my bubble with we can never find the center because the speed is exactly the same in all directions.
Yep, I believe the theory is that whatever point in the universe you pick, the redshift (the relative speed of stuff moving away from you) will always be the same (as a function of the universe's age). But -- aside from the arbitrariness of measuring "speed" as a way to get an "answer" to this, since there is no "universal speed", it's always relative to your frame of reference -- anyway, saying that the speed of expansion is everywhere the same is beside the much much bigger point that the very idea of a "center" is incoherent in the geometry, a bit like asking what's the volume of a line. The question doesn't even make sense for this kind of geometry, much less make it hard to find the answer. There is no answer to find!
If the balloon analogy didn't really visualize it... Imagine a map of the world and point to its center. You can't. Or if you did (it's Greenwich, or the North Pole, or the GMT/Equator intersection) it'd be a totally arbitrary point; you could pick any point and make it the center and draw a bunch of lines through it, but the sphere itself doesn't care.
The universe has a (
http://en.wikipedia.org/wiki/File:End_of_universe.jpg) shape that's either curved, flat, or hyperbolic 3D space. But in all three cases there is no center in the same way there is no center to a world map. The only thing that throws people off is that maps are in 2D and space is in 3D. So "no center" is an easy and indisputable answer.
The real "hard question", the one where our limitations in measurements is a real problem, is what
shape is the universe (and with it its ultimate fate, a big fade-away & chill vs a big crunch). As far as I know we don't have that answered and may not ever be able to indisputably. That's where you should direct your preference for the few remaining mysteries of the universe that are possibly beyond human reach, and yet have fantastically important significance.
Bjossi on 20/6/2009 at 16:21
Quote Posted by dj_ivocha
So, considering the universe is more than a dozen billion light years across now
Much, much more than that; our view of the point of origin is only the partial radius of a circle, and don't forget that it is believed that the expansion is accelerating. Of course this is assuming that the shape of the universe is a sphere and it expands from a singularity where it all began.
heywood on 20/6/2009 at 16:51
Quote Posted by Tocky
Yes but if we can measure the rate at which each galaxy is moving away from each other and then apply that relative to each can't we then discern which direction each is moving more quickly away from to find the center?
I'm saying expanding yes, all over yes, but if the rate is still increasing greater relative to the invisable center the speed will telltale in a particular direction.
And I suppose you now burst my bubble with we can never find the center because the speed is exactly the same in all directions. Or quite possibly what we can discern of the universe is too small a segment. I would prefer to think we just don't have the means yet.
Yes, the speed is the same in all directions. We have the means to measure the red shift extremely accurately, and our measurements show that for objects of the same distance, the amount of red shift is the same in every direction. In other words, everything is expanding uniformly away from us.
And we can rule out our galaxy as the center of the expansion because if it were, you'd have trouble explaining how there are other galaxies as close as 42000 light years from us but still moving away. Also, we can rule it out because the red shift we observe is not consistent with motion of objects within space away from us. In other words, the red shift is not explainable by the Doppler effect (a common misconception). Hubble discovered that.
Given that the universe looks the same to us in every direction, one seemingly reasonable assumption is that the universe is uniform (isotropic and homogeneous) in the spatial dimensions, and only evolves in time. If you make that assumption and solve the general relativity equations, you get the Big Bang model of expansion which
is consistent with the red shift.
The Big Bang model doesn't predict an expansion of the universe
into 3D space, it predicts an expansion
of 3D space. So there is no such thing as the center of expansion; it simply doesn't exist. To understand this, I think you need to go back to the balloon analogy and think about it some more. In the balloon analogy, space is curved and has two spatial dimensions. These dimensions are expanding uniformly everywhere on the balloon, so no matter where you place an observer everything is moving uniformly away.
Also, the observer on the balloon can't directly see the curvature of his own 2D space and thus the shape of his universe, just as we can't directly see the curvature of our 3D space and the shape of our universe. We can't even estimate the degree of curvature very well because our assumptions about the density of the universe depend on dark matter which we can't directly observe.
Quote Posted by daniel
I can't see what the OP's beef is. A theory is the explanation of all evidence we have. Gravity is also a theoy and we see it everyday, there's no denying it's there. But we still don't know the origin of gravity, so it's a theory.
As for religious peoples, keep in mind that the Big Bang theory was proposed by a Catholic priest. Creationism is hardly an acceptable theory for how the universe was created - God gave us brains so we ought use them.
I don't see what the OP's beef is either. Not only is the Big Bang a theory, it is based on some assumptions that have to be taken on faith without evidence.
Obviously, I agree that Biblical creationism is not an acceptable theory because it doesn't predict a lot of observational data and its assumptions are neither verifiable nor tautological. Essentially, it's derived by "making shit up".
However, I would argue that the standard model of cosmology is not a good theory either. It certainly started well. First with just one assumption, that the laws of physics are the same in every frame of reference (principle of relativity). From there we got a theory of gravity that worked on cosmological scales. Then we assumed the universe was homogeneous and isotropic (cosmological principle) and got the basic Big Bang theory. But beginning with the discovery of the CMB, we keep finding conflicting observational data that requires ad hoc modification of the theory based on assumptions that are not verifiable and not at all self evident, like inflation and dark energy.
And just to be clear, I'm not saying we show throw out the basic theory. I just think that cosmologists need to recognize when they cross the line into "making shit up", acknowledge the limitations/contradictions in their model, and stop presenting it as truth to the lay audience.
Tocky on 21/6/2009 at 06:05
gah
You folks are incredibly patient. Space being curved isn't a difficult concept to visualize and explains perfectly the laser pointer hitting you in the back of the head when you point it in front of you. Any amount of curve comes back around eventually. But the balloon visualization has to be too simplistic because it makes the center in the air of the balloon but in the nothing when applied to space. Nothing can't be visualized. Nothing has no center. Nothing has no physics. Nothing to apply to nothing. Nothing is unknowable except nothing. We have it as concept but never actuality.
I had a horrid thought. What if religion and science were both right. God exists and doesn't. God is unknowable and is the center of the universe. God is nothing.
Shoot me now before I have another thought.
Too late. This ties in with if for everything there is an opposite then the sum total of all positives and negatives is... shoot me. Quickly.
Mortal Monkey on 21/6/2009 at 16:44
Quote Posted by heywood
Yes, the speed is the same in all directions.
[...]
Also, we can rule it out because the red shift we observe is not consistent with motion of objects within space away from us. In other words, the red shift is not explainable by the Doppler effect (a common misconception).
Wait, what? Aren't these statements completely contradictory to one another? If red shift is not explained by the Doppler effect, then what is it and how does it tell us the speed of an object?
