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 CountIbli
Joined: 6/1/2005
Msg: 88
how is the universe expanding?Page 6 of 6    (1, 2, 3, 4, 5, 6)
I haven't read all the replies but it looks like this thread got derailed very quickly. The question was what the universe is expanding into. To answer this we need to step back a little to get some mathematics involved (I'll try to make it as painless as possible). Warning: This response is almost as long as a typical post by Scorpionmover

Let's start simple with two points, A and B, on a line. The distance between them is just |B-A|, where | | is the absolute value. This definition doesn't work so well when we get into 3 dimensions, so we can modify it somewhat. Let's suppose we have two points A(x, y, z) and B(t, u, v). The distance between them is sqrt[(x-t)^2 + (y-u)^2 + (z-v)^2], where sqrt means square root. This reduces to the previous expression in one dimension, so we haven't fundamentally changed the definition of distance.

We can further generalize this definition of distance by introducing some coefficients.

d=sqrt[a(x-t)^2 + b(y-u)^2 + c(z-v)^2]

Why would we introduce these coefficients? For one thing, if we're using a coordinate system besides the Cartesian coordinate system (for example spherical coordinates where we have a distance, a latitude, and a longitude) we have to introduce these coordinates in order to get consistent results across different coordinate systems. Obviously the choice of coordinate system shouldn't change distance.

We can generalize even further by allowing cross terms into the definition of distance. For example there might be a term like f(x-v)^2 in that sqrt operation.

We can collect all these coefficients into a special matrix called the metric tensor. This metric tensor is crucial for the mathematics of General Relativity. The metric tensor basically tells us how to define distance. GR contains two rather nifty generalizations of this metric tensor. First of all, the coefficients can be functions of position so that the very definition of distance depends on where you are in the universe. This is basically what is meant by non-Euclidean geometry. Second of all, GR takes place in 4 dimensions, with the fourth dimension being time.

GR consists of a complicated mathematical relationship between this metric tensor and the amount of energy/matter present. From a physical standpoint this metric tensor represents gravity. But the metric tensor also tells us what distance is.

So finally we can answer the OP. Asking what the universe is expanding into is a meaningless question. I think the question probably comes from thinking about expansion like a balloon blowing up (a common analogy, but one that is horribly misleading). The universe is not expanding like a balloon, which needs to expand into something. Instead, the very definition of distance is changing over time.

On the question of extragalactic redshift, this has nothing to do with the Doppler effect. This is because the galaxies are not moving away from each other. Remember, the universe itself is not getting bigger and carrying galaxies with it (they are not like dots on the surface of a balloon). What's happening is that the very definition of distance between galaxies is changing. The extragalactic redshift is a gravitational effect not a Doppler effect. Caveat: there are small local motions that the galaxies undergo which cause Doppler shifting (which can be redshift or blueshift), but this is separate from the extragalctic redshift.

Hopefully this clears things up for those who managed to make it to the end.
 stargazer1000
Joined: 1/16/2008
Msg: 89
how is the universe expanding?
Posted: 1/18/2010 1:54:56 PM

On the question of extragalactic redshift, this has nothing to do with the Doppler effect. This is because the galaxies are not moving away from each other. Remember, the universe itself is not getting bigger and carrying galaxies with it (they are not like dots on the surface of a balloon). What's happening is that the very definition of distance between galaxies is changing. The extragalactic redshift is a gravitational effect not a Doppler effect. Caveat: there are small local motions that the galaxies undergo which cause Doppler shifting (which can be redshift or blueshift), but this is separate from the extragalctic redshift.


Except....we are talking expansion of spacetime between galaxies which is spreading the wavelength of light between two points spreading apart. The term Doppler shift is commonly used by cosmologists. Are you saying they're wrong in the name of the very tool they use?

As for what the universe is expanding "into," again, there are cosmologists who are asking that very question and trying to find answers. Then there are those who are looking into possible "collision" points between our universe and, perhaps neighbouring universes.

So what am I missing?
 CountIbli
Joined: 6/1/2005
Msg: 90
how is the universe expanding?
Posted: 1/19/2010 4:46:45 PM


Except....we are talking expansion of spacetime between galaxies which is spreading the wavelength of light between two points spreading apart. The term Doppler shift is commonly used by cosmologists. Are you saying they're wrong in the name of the very tool they use?


The short answer: Yes. Longer answer: http://scienceblogs.com/interactions/2007/03/redshift_basic_concepts.php



As for what the universe is expanding "into," again, there are cosmologists who are asking that very question and trying to find answers. Then there are those who are looking into possible "collision" points between our universe and, perhaps neighbouring universes.

So what am I missing?


Until we find one of these other universes all they're talking about is science fiction.
 stargazer1000
Joined: 1/16/2008
Msg: 91
how is the universe expanding?
Posted: 1/19/2010 5:17:48 PM

The short answer: Yes. Longer answer: http://scienceblogs.com/interactions/2007/03/redshift_basic_concepts.php


Well Count, after some additional research, I stand corrected. Thank you.

Actually, I am curious to know what your degree is in. Are you working in this field, related or just an area of interest?


Until we find one of these other universes all they're talking about is science fiction.


Certainly more in the realm of conjecture, however, one that many are taking it seriously which is why they are actively looking for some kind of theoretical model that can serve as a basis for experiment/observation to find 'em.
 CountIbli
Joined: 6/1/2005
Msg: 92
how is the universe expanding?
Posted: 1/20/2010 12:17:18 AM


Actually, I am curious to know what your degree is in. Are you working in this field, related or just an area of interest?


I have a BS in physics and a MS in mathematics, but I've never worked in those fields. The jobs just weren't there when I graduated and 15 years later my degrees aren't worth the paper they're written on.
 JustDukky
Joined: 7/8/2004
Msg: 93
how is the universe expanding?
Posted: 1/20/2010 4:25:29 AM
Did you go into math/physics for a job...or because you loved it? I suspect the latter, simply because it appears you've "kept up" and haven't forgotten everything. If you didn't enjoy it, you probably would have forgotten. Still, I noted a bit of disparagement in your post, so I thought I'd drop a couple of trite comments that appear to apply:


I've never worked in those fields.

If you do what you love, you'll never work a day in your life.

15 years later my degrees aren't worth the paper they're written on.

If they are important to you, who cares what anybody else thinks they are worth?

The basest of all treasures is money. It isn't worth the paper it's printed on, but everyone seems to value it. If nobody else values your degree, it's because money can't buy it. Its value can't be exchanged and must stay with you. The treasure is yours to keep.

Now have a beer & kill a few brain cells... It always works for me...
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