Discussion in 'Astronomy, Exobiology, & Cosmology' started by Adam, Jan 14, 2003.
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are they on the other side of the universe and if so whats in the centre of the universe?
i mean if we can see the other side cant we see the middle?
There is no "other side" and there is no "middle" of the universe. When an astronomer says "the most distant object in the universe" she means most distant <i>from Earth</i> that we have detected so far.
Doesn't the entire Big Bang/Big Crunch thing involve some central area, if not point, that everything is expanding away from?
If you have stars so:
Or in whatever distribution, they can't all be expanding away from each other unless from a central area. If no central area, then some must be moving toward each other.
Here's a common analogy:
Think of a balloon. Draw dots on the surface. Those dots represent galaxies. The universe is the <b>surface</b> of the balloon. This part is important - ONLY the surface is part of this model universe; the centre of the balloon in 3 dimensional space is not part of the balloon universe, since galaxies (dots) only exist on the surface.
Now, blow up the balloon. From the point of view of every galaxy on the surface, all the other galaxies move further away. None of them get closer. Where is the central galaxy? There isn't one. The balloon's surface has no centre.
Our universe and the big bang picture is like this, only with an extra dimension. Think of all our galaxies in 3D space as existing on a 4-dimensional hypersurface. As space expands, all the galaxies move further apart. But there is no centre to our universe, and no galaxies approach us (except a few which are locally gravitationally bound to the Milky Way).
Unfortunately our universe is three dimensional space. No doubt this is covered in thigns I have not studied yet. but it seems to me that if you picture a 3D grid (cube) full of objects, and say the cube is expanding and all objects maintain their same co-ordinates (assuming that the unit measurements on the axes grow with the cube), they must all be expanding away from some point.
This is only 2D of course. But, over time, there is expansion. Notice all points move away from the central point. The only way to say they don't all move away from some point is to remove all objects within the area being discussed, like so:
My problem with the model you describe is that the universe is not a hollow space. We are related to other objects by three spacial dimensionss. There is depth. I find it hard to see how the balloon surface idea can accurately model this universe.
Isn't where the Big Bang has once been now the center of the universe? I mean, all matter in the universe has once been in that tiny *shrug* thing (I don't know what you call it) which then exploded, creating the Big Bang. Then all is heading away from that 'thing', making it the center!?
Adam, in your last diagram the objects are still moving away from one point, you just didn't draw it.
I was trying to do an analogy of the balloon thing.
now im REALLY confused
i have herd that theory before but there still has to be another dimenton involved
so there is an edge and if there is an edge there has to be a middle
i mean if i flew in a straight line the OPOSITE way from the way the universe is expanding what would i find?
Re: James R
Nope. Perhaps your visualization skills are not strong enough, but each of the points in your cube will only see all its neighbors moving apart, at a speed proportional to their distances. They will not detect expansion from some point. A similar analogy uses a loaf of rising raisin bread. As the bread rises, "space expands," and each of the raisins moves further from the others. There is no center of expansion, though; if the raisin bread were infinite in size, the picture would still look the same for any piece you cared to look at.
The addition of a dimension does not change the qualities of a space.
There is no edge. There is no middle.
It's not possible. Space itself expanding. No matter which direction you move, it'll look the same. There is no "direction" associated with the expansion.
Perhaps my distribution of obects should not have been so uniform. I do see that the distances increase in a uniforum distribution of objects. However, we know that things do not uniformly expand away from each other on the cosmological scale, such as those pictures of colliding galaxies.
You're forgetting gravity.
That's exactly what I mean, actually. Objects are not uniformly distributed, and there is gravity influencing everything.
Wouldn't this be a more acurate diagram?
The same could be drawn for a three dimensional diagram.
The problem with your picture now is that you still have edges. Take your diagram which looks like this (except in 3D):
and join the edges. For example, if the stars on the right side look to their right, they see the stars on the left side. If somebody travels from one of the top stars upwards they end up at the bottom. (Think of the computer game Asteroids, if you know it.)
Now do the expansion. All stars move away from each other, but the view is exactly the same from any star. There is no centre of the expansion.
Please Register or Log in to view the hidden image! Hmm, in that case I don't understand the problem.
That objects are not uniformly distributed implies that the universe had varying matter/energy densities in it's early history (which as I understand it is confirmed by COBE). This doesn't mean that expansion is not uniform. It means matter/energy is not distributed uniformly.
Your example of colliding galaxies is of two gravitationally bound objects. ie. the force between these two galaxies is great enough that instead of being moved apart by the expansion they are being brought together by gravity.
The expansion is negligible over short distances. It's about 1.9cm-2.1cm a light year per second.
Imagine you have a length of rope. It is 1000metres long. Imagine that after one second for every meter of the rope it's length increases by 1cm. Over the entire rope we get a total increase of 1000x1cm=10m. Now the rope is 1010metres long.
Again, imagine that another second passes and the rope increases in length by 1cm for every 1m of rope. This means the total length is now 1010m + (1010 x 1cm) = 1020.1m. Do this long enough and the ends of the rope will be moving apart at greater than light speed, however, the expansion rate is still only 1cm/sec/1m.
ie. any two points along the rope that are separated by 1m will be separated by 1.001m a second later.
You might want to now point at the center of the rope and say "there is the center of the rope", but what if I join the ends together? Where is the center of the rope then?
This is analogous to how space expands. Except that it expands in every spatial dimension (maybe it expands temporally too but I'm not qualified to make that judgement) and not just along the 1 dimension as with the rope.
WRT, to gravity. This would equate to two points on the rope moving towards each other regardless that the rope is increasing in length all the time. All it would take is a gravitational attraction that is greater than the rope increase.
In real terms the expansion of space is something like 1.9-2.1cm/sec/light year. The escape velocity on earth is around 11km a sec.
hope that helps
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Re: James R
While on one side of the baloon the planets and galaxies are encerasing the distance between them (right half of the baloon) the same happens on the left side. They will collide. (the right expansion with the left one)
§lîñk€¥™, (or Paul),
If you join those ends, that would make a circle. The center would be the center of the circle. Everything else would be moving away from that center.
How can you say, "per light-year", when you (along with a group of others) state that there is no center or edge to the universe, making it impossible to have a measure of light-years!
Oh and there's one more thing I would like to know. Does space expand at a sonstant (or constant accelarating rate) from ANY objects around it?
If so, that would make no sense at all. Since the Big Bang, things must have moved away from THAT BIG BANG. There's no reason at all for these things to be moving away from each other. Therefore, the universe wouldn't be able to expand at a rate of 1.9-2.1cm/sec/light year, simply because cm/sec/ligh-year cannot be applied to this problem.
Re: §lîñk€¥™, (or Paul),
His analogy was that of a 1D space, a line -- a rope. If you join the ends of the rope together, you still have a 1D space, a line -- just one which is closed and has no boundary. If you were a 1D creature who lived on the line, and were only capable of seeing forwards or backwards along the line, you would not be able to make the statement that "the center of expansion is the center of the circle formed by the rope," since you aren't capable of anything more than 1D perception. In the space of the 1D rope, there is no center of expansion; you have to invoke some higher dimension in which to view a center. In a very literal sense, for the 1D creature, the "center of expansion" does not exist in his universe.
In real 3D space, you'd have to hypothesize a fourth spatial dimension, and observer the 3D space's curvature within it. However, our universe seems to only supply us with three dimensions, and no center of expansion exists in our universe.
If you join the ends of the rope together, do the concepts of foot and meter suddenly disappear?
It seems experimentally that space is accelerating in its expansion. I am rather unsettled by this fact, but it seems to be the reality dealt us.
It seems you have not spent long enough thinking about the low-dimensional analogies presented to you here, since they explain the way expanding space looks quite well. A uniform expansion of space means any point in that space sees all the other points moving away from it -- and furthermore, that their speed of recession is proportional to their distance.
This is something of a "non-conclusion."
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