A physics question

Stanley_Marsh · 2007-03-19 09:19:56

This question isn't in the textbook
I want to know , Whether the position of galaxies , stars in the universe are stable , not moving around .
The method to determine whether the universe is expanding is through the redshift or blueshift
If the positions of galaxies in the universe arent relatively stable, then how can they determine the expand through redshift or blueshift?

Stanley_Marsh · 2007-03-19 09:21:03

If the positions are stable , then what keep them in place

mathsyperson · 2007-03-19 09:58:06

If the universe wasn't expanding, and galaxies were just moving around a bit, then we would observe red-shift in some galaxies and blue-shift in others. Some galaxies would have red-shift for a bit, then blue-shift, and switch between the two periodically.

However, because we always observe red-shift from all of the galaxies*, all of the time, that means that everything is getting further away from us and so the universe must be expanding.

(*This isn't strictly true. The galaxy closest to us is showing blue-shift, which means that it will eventually crash into us. But all other galaxies, where our gravity isn't enough to affect their movement substantially, are moving away.)

Stanley_Marsh · 2007-03-19 10:02:25

So this is a theory , it holds for what we know . Have they found out the Structure of the universe? I mean the position of galaxies are random or intentionally placed.

lightning · 2007-03-19 10:05:56

Stanley_Marsh wrote:

This question isn't in the textbook
I want to know , Whether the position of galaxies , stars in the universe are stable , not moving around .
The method to determine whether the universe is expanding is through the redshift or blueshift
If the positions of galaxies in the universe arent relatively stable, then how can they determine the expand through redshift or blueshift?

i know all about space i'm gonna be a asronermer maybe stable huh? let me see.... ah-ha! well we move arond the sun but in 2billion years(or something like that) the sun will expand called the red giant the red gaint goes to mars. after 100000 years (or something like that again) the red giant will start to fade till it becomes a white dwarf or a black dwarf

black:stays but eventully die
white:turns into a black hole

and well if you need more help i'm right here lmao

Ricky · 2007-03-19 12:54:41

Red shift and blue shift give relative movement between our planet and another star. You, on the other hand, talk about movement as if there is some type of movement that isn't relative. There isn't. All movement is relative.

There is no such thing as "cosmic rest". Given any system with n objects moving all in different directions, you can take any of these n objects and call it "at rest". They are all different systems, but the numbers work out exactly the same in each.

Stanley_Marsh · 2007-03-19 12:59:21

I know , Ricky. I mean even if all galaxies and stars are moving away from the earth ,that cant determine the universe is expanding , because their unstable movement.

Ricky · 2007-03-19 13:03:47

I mean even if all galaxies and stars are moving away from the earth ,that cant determine the universe is expanding , because their unstable movement.

What do you mean unstable movement? Every star is moving away from us a speed proportional to their distance. The further away they are, the faster away they move. What other conclusion can you draw other than the universe is expanding if everything is moving away from us?

Stanley_Marsh · 2007-03-19 13:09:19

oh ,right . lol , I am so stupid.

Ricky · 2007-03-19 14:00:14

Stupid? Why?

Now here is where things get interesting. Lets say we have three points in space, A, B, C. Each lie on the same line, we are standing at A, and B is inbetween A and C.

(Us) A ---------- B --------- C

What we find is that B moves away from us slower than C does. As it turns out, proportionally slower. We find that this means space itself is actually increasing. It's not just that things are moving away from each other. It's that the space in between is increasing in size. Here is what I mean.

A ----- B
A ---------------C

Now what happens if I replace every '-' with '--', i.e. the space is increasing?

A ---------- B
A ------------------------------C

B moved 5 dashes away while C moved 15 dashes away. As you can see, the speed at which C moves away from A because it's distance is longer.

Another effect of space increasing in size is that everything is moving away from everything else. Look back at the distance between B and C. In the first frame (before I doubled the dashes), B was 8 dashes from C (about). In the 2nd frame, B is now 16 dashes away from C. So even though B is moving away from A, it is also moving away from C as well.

Pretty freaky stuff. This means that from anywhere in the universe, when we look at a star, we will see red shift.

Stanley_Marsh · 2007-03-19 15:49:58

OMG, this happens when they have acceleration? Pretty Cool actually .
In my opinion maybe universe is like a room without air, and all the stars and galaxies are like a pack of air. The Big bang made them be released , the stars and galaxies are moving aways , like the air trying to fill up the whole room.

Ricky · 2007-03-19 17:37:50

OMG, this happens when they have acceleration? Pretty Cool actually .

Well, not when they have acceleration. This causes everything to accelerate away from everything else.

In my opinion maybe universe is like a room without air, and all the stars and galaxies are like a pack of air. The Big bang made them be released , the stars and galaxies are moving aways , like the air trying to fill up the whole room.

Evidence says that it is actually space that is expanding, not the stars and galaxies that are moving, though they do that as well.

mathsyperson · 2007-03-19 21:55:33

The air in balloon model is slightly flawed. As well as for Ricky's reason, the particles of air in a balloon will not necessarily expand. They just move around randomly, and their expansion is a statistical likelihood.

George,Y · 2007-03-20 02:29:14

Logically the expanding universe or the whole of existence expanding is a paradox.

I met it in a philosophy forum.

If the whole of the existence or the universe is expanding, where can it expand to?
Vanity. The universe must fill more vanity in order to expand, transfering vanity into part of universe.
But then the universe ain't the whole of the existence since vainity is larger than it.
vanity shall, instead, does not expand.

The Ancient Greek philosopher Parmenides of Elea commented:
Non-existence(vanity) does not exist by definition.

Last edited by George,Y (2007-03-20 02:30:20)

luca-deltodesco · 2007-03-20 03:29:23

perhaps its not that the universe is expanding, but that everything within it is becoming smaller.

Ricky · 2007-03-20 04:59:20

If the whole of the existence or the universe is expanding, where can it expand to?

The universe defines space. Outside the universe there is no space. So the question "where" can it expand to does not make sense. You might as well ask what is the sum of 1 and an apple.

Stanley_Marsh · 2007-03-20 05:46:07

We'll , perhaps , never see the day of revealation . Though I am dying to see what's outside the universe and why it's expanding.

Ricky · 2007-03-20 07:22:27

Though I am dying to see what's outside the universe and why it's expanding.

The question, "What's outside the universe" also makes no sense. Outside and inside are spacial descriptions. Space is what defines the universe.

Though why it's expanding is one of the top questions in astrophysics right now. It ranks right up there with, "What the heck is 'space' anyways?".

Stanley_Marsh · 2007-03-20 09:01:41

If M theory is right , then the universe is wrapped by a membrane , there's "outside" of the universe.

Ricky · 2007-03-20 09:06:09

From what I understand, a membrane is simply a more fundamental piece of "stuff" than strings are. But I am unsure of this.

egeniius · 2007-03-20 09:18:53

So I wonder (and perhaps niavely assume that the universe is symmetrical) as everything is moving away from everything - what is happening at the center of the universe?

mathsyperson · 2007-03-20 13:08:34

Well, depending on what the universe actually is, it may not have a centre. One theory is that the universe somehow bends around 4th dimension so that if you go in a straight line forever then you'll eventually end up where you started.

If that's the case, then the universe wouldn't have a centre. At least, not one in our dimensions.

George,Y · 2007-03-20 14:19:24

luca-deltodesco wrote:

perhaps its not that the universe is expanding, but that everything within it is becoming smaller.

Good Point!

George,Y · 2007-03-20 14:26:48

Sure no space outside of the universe as Parmenide has put it. The space is only at maximum the one of the whole universe. Thus the universe's space cannot be enlarged.

However, suppose now the space of the universe is larger than that of the previous, how can the difference of the space be added. There was no more space outside of the previous universe. How do you produce the difference of the space? And where can you put it?

Simple question, Ricky.

Ricky · 2007-03-20 15:17:44

However, suppose now the space of the universe is larger than that of the previous, how can the difference of the space be added. There was no more space outside of the previous universe. How do you produce the difference of the space? And where can you put it?

That's not a bad question. As of now it can't be answered because we have yet to even discover what space actually is.

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