The essence of the redshift

When we observe the remote galaxies’ spectrum we see that their spectral lines are moved to the red end. This phenomena is explained as metric expansion of space-the space is expanding in geometric sense and that’s why the distance between galaxies is increasing.
BUT!
How do we know that exactly the space is expanding? The redshift phenomena can be in principle explained by an ordinary motion, is not it so? Imagine that the galaxies are receding from our galaxy as spacecrafts and that’s why their spectral lines are shifted (Doppler effect)……..so, how do we know that redshift is caused by exactly metric expansion of space and not by galaxies’ motion?

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Because we see the same redshift in all directions. If it were from the proper motion of the galaxies, some of them would be moving towards us and we would see blueshift.

Unless we are at the exact center of the Universe, which has no center, and wouldn't be very likely if there were a center, space must be expanding because EVERYTHING is moving away from us (and everything else).

 

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AlexG

At some certain distance of course, right?

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Yes, the further the object the greater the redshift.

 

Why it is said we are at the center , we are not at the center of our solar system

 

if we envision ourselves inside a balloon that is expanding then no matter where we are inside it every other particle is moving away from us.
we can easily conclude that we are at the "center" of the balloon, even though we might not be.

 

if I'm correct, you are referring to the 'skin' of the balloon expanding, right? . . . .now, exactly what is causing the balloon skin to 'expand' . . . in your example it is likely some kinds of fluid (be it gas or liquid). As the balloon expands what is happening inside the balloon? . . . . more fluid is being added, right? In your example, the additional fluid is entering the balloon from the balloon opening (the Big Bang in the Standard Model), causing slight overpressure within the balloon, thus the 'skin' expands. The same 'effect' would be observed in an alternative model, in which the additional (new) 'fluid' is being continuously "created" within the bounds of the balloon skin.

Ah . . .that is the crux of the situation . . . . what is the "mechanism" causing the expansion!

wlminex

 

No, we are saying we are not at the center of the universe. There is no center.

 

i kind of figured this would come up, it's the reason i said expanding as opposed to someone blowing it up.

it's a mind experiment to illustrate how and why the red shift happens.
it also shows how everything can seem to be moving away from us even though we may not be at the center.

 

Is that realistic hypothesis ? There are asteroids moving toward us , We have a mood which we consider that some planet collided with the earth and produced our moon .

( I am not debating , I am just trying to understand )

 

The expansion of space is very small from a local point of view, local being even as large as the galaxy. It' is only when we measure very large distances, like to other galaxies and beyond that we see appreciable values that the balloon analogy can be useful in demonstrating. Even then it has limits, as all analogies do.

A bit more accurate analogy would be to use something like a compressed sponge or foam, and as you let it go to regain its shape, all the points within spread apart equally. So it doesn't matter if you're in the exact center or not, it looks the same.

 

I have the notion there was a beginning, the beginning was at the so called big bang , were and expansion started, so I assume the expansion was three dimensional, so why are we saying there is no center ?

 

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A bit more accurate analogy would be to use something like a compressed sponge or foam, and as you let it go to regain its shape, all the points within spread apart equally. So it doesn't matter if you're in the exact center or not, it looks the same.[/QUOTE]


Even your sponge ball gave a frame of reference , but you are dismissing a frame of reference ?

 

As of right now we have no reason to think we are the center. The sponge analogy breaks down because it has an outside. But the point remains that you could be anywhere inside the sponge and see the same expansion outwards. If you could not see the edge, then it would be more similar, with farther points moving faster away from you, no matter where you were.

 

Ok lets say I am on the road on a bicycle cars are passing me . I don't see the edge , there are cars coming against me. But to those passing me and to me I am the center , I measure how far and fast they are passing me

 

No. That does not work does it.

Consider stuff on our side of the balloon, that is expanding towards us.
Stuff on the other side is expanding away from us, thus we would see that difference.

Eg out speed is V expansion is E, we would see some stuff expanding at V+E and some expanding at V-E thus we would see at two tone universe, or rather a multi tone universe.

However we do not see that because we are at the centre of the universe.

Praise be to Allah.

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Nonsense.

 

It's not nonsense, I have just explained it to you.
Which bit don't you understand.

If you want to talk about nonsense just look at the big bang theory, more holes in it than a private health insurance policy.

A bang with a centre yet it has no centre, if you can swallow carp like that without crying nonsense then you are in no position to criticise the obvious truth.

Pot calling kettle black me thinks!!!

 

Everything is relative. To the cars you are moving and they are the center. But this is more analogous to the nearby objects and measuring their redshifts based on their velocities, not the general expansion of space itself, which for farther things has to be accounted for as well.

You misunderstand the example of the balloon. There wouldn't be anything expanding towards you at all. So we don't see what you think we should see.

 

The BB is not an explosion, expanding material into space. The expansion is occuring at every point in the universe. The balloon skin analogy is actually a good one. You just have to realize that it's a two dimensional analogy for a 3 dimensional expansion.

Since every point is expanding from every other point, there is no center. Everything is moving away from everything else.

However, on the 'local' scale, up to about 250 million light years, the effects of spatial expansion are overridden by the strong force, the electomagnetic force and gravity. Out beyond that we see redshift in every direction we look.