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Sam99
2011-May-08, 11:13 AM
I understand the basic analogies of expansion & that the space between everything gets bigger. But why is it so small at close distances & huge at far distances?

Not sure why this is true:
The apparent recession velocity of a galaxy v is proportional to its distance d from the observer

caveman1917
2011-May-08, 11:27 AM
http://www.bautforum.com/attachment.php?attachmentid=14922&d=1304854244

You are at A.
The first line represents two galaxies B,C that are 1 and 2 units of distance away from you.
The second line represents the same two galaxies 1 unit of time later.
You see that B moved 1 unit of length further during that time, but C moved 2 units of length further.
So the recessional velocity of B is 1 s/t and C is 2 s/t.

You also might want to check the real wikipedia's entry, it gives a lot more information.

John Jaksich
2011-May-08, 01:07 PM
I believe the confusion resides in attempting to differentiate between Hubble's linear expansion and the contemporary theories of cosmic inflation ( due to dark energy ).

Hubble's 1936 velocity-distance relation is a linear law---->the "slope of the graphic relation is not zero but some real number value" . . . if you will

Hubble's original words----> may clinch it for you (here is a link to google's version):

But, as caveman1917 succinctly explained---it is a linear law ----> so if one were to work backwards ----from the analysis of the data given in Hubble's time---we have a straight linear graph that lend eventually lead to the hot BB theory.

There have also been other wrinkles but a good place to would be the wiki----> and here is Dr. John Gribbin's explanation on inflationary (dark energy ... as well as other nuances to the theories): http://www.lifesci.sussex.ac.uk/home/John_Gribbin/cosmo.htm

2011-May-08, 06:56 PM
So, if I understand correctly, because due to expansion are things are moving away from each other, galaxy B moves twice as fast relative to our viewpoint than galaxy a?

John Jaksich
2011-May-08, 07:32 PM
So, if I understand correctly, because due to expansion are things are moving away from each other, galaxy B moves twice as fast relative to our viewpoint than galaxy a?

I, don't have the exact number that you seek---but according to Hubble's relation--the Universe is expanding ---yes ---if its "twice as fast relative to some viewpoint" I could not say exactly---but I believe that the important point is that there is an expansion.

caveman1917
2011-May-08, 08:23 PM
So, if I understand correctly, because due to expansion are things are moving away from each other, galaxy B moves twice as fast relative to our viewpoint than galaxy a?

We are A.
C moves twice as fast as B, from our viewpoint (ie galaxy A).
It also works the other way around if you take our viewpoint to be C, but the picture is meant to put us at A.
C is twice as far away, it moves twice as fast.

2011-May-08, 08:44 PM
What I mean is, that while galaxy a is moving away from us, universe b is also expanding at an equal speed away from universe a, so seems to be moving at an increased speed to us.

I understand you're not actually adding the speeds together, I think.

Strange
2011-May-08, 09:14 PM
maybe it helps to think of it in terms of scaling; as the universe expands (linearly) distances get scaled by the same amount. So, if we say galaxy B is at distance 1 and C is at distance 2; then after a time when all distances have increased by 10%, galaxy B will be at 1.1 and C will be at 2.2 - i.e. C will have "moved" twice as far (0.2) as B (0.1). This is just a direct consequence of linear scaling/expansion.

Jeff Root
2011-May-08, 10:09 PM
Here's a GIF animation I made to show uniform expansion
throughout space. Every dot moves away from its nearest
neighbors at the same constant speed. The farther apart
two dots are, the faster they move apart.

http://www.freemars.org/jeff2/expand5c.htm

-- Jeff, in Minneapolis

WayneFrancis
2011-May-09, 12:27 AM
What I mean is, that while galaxy a is moving away from us, universe b is also expanding at an equal speed away from universe a, so seems to be moving at an increased speed to us.

I understand you're not actually adding the speeds together, I think.
ASCII art FTW

@ T0 A.....B.....C.....D
@ T1 A......B......C......D
@ T2 A.......B.......C.......D
@ T3 A........B........C........D

In this ASCII universe space is expanding at a rate of 1 unit for every 5 between T0 and T1

between T1 and T2 the rate slows to 1 unit for every 6
between T2 and T3 the rate slows to 1 unit for every 7

So in this ASCII universe the rate of expansion is slowing down but we can still measure things. currently in our universe it is speeding up but both that and a constant expansion rate is hard to illustrate using ASCII art.
at T0 each position is 5 units apart from its nearest neighbour.
at T1 each is 6 units apart.

So by the time T1 A has receded from B by 1 unit , from C by 2 units from D by 3 units. D looks like it is receding 3x as fas as B to the point of view of A

By the time T3 each point has receded from its nearest neighbour by 3 and the next one over by 6 units.

If we had a infinitely long ASCII art 1D universe you could see that the rate they receded from any point is based on how far they where from that point at the beginning. Double the initial distance and the rate double. Increase the initial distance by just 10% and its rate is increased by just 10%. Increase the initial distance by 150,000 times and the rate increases by 150,000 times.

Cougar
2011-May-09, 12:59 PM
...expansion... why is it so small at close distances & huge at far distances?

Just to clarify, expansion is the same everywhere. But when you consider billions of lightyears of space, it adds up.