How to differentiate Doppler and Cosmological redshift

[Jacques]

What I mean is Doppler redshift is caused by the motion of the source away from the observer, and the Cosmological redshift if caused by space expanding.

I read that distant galaxies doesn't move, it is space that expand.

Why ? Why do we say that the galaxies doesn't move. Is there something special in the redshift ?

Thanks

[Martin]

What I mean is Doppler redshift is caused by the motion of the source away from the observer, and the Cosmological redshift if caused by space expanding.

I read that distant galaxies doesn't move, it is space that expand.

Why ? Why do we say that the galaxies doesn't move. Is there something special in the redshift ?

Thanks

 

You can't distinguish by just looking at the light---and measuring the wavelength shift.

 

figuring out what part of the shift is cosmo and how much is individual motion doppler involves inference and looking at the context---it is bound to involve some uncertainty (though not as much as you might expect, compared with other kinds of uncertainty in astronomical measurement)

 

For example if you see a CLUSTER of galaxies, you can assume they are gravitationally bound and all at approximately the same distance.

then any individual differences in redshift would be Doppler effect of individual motion WITHIN THE CLUSTER.

that still doesnt address the question does the cluster itself have some overall center-of-mass motion---there is going inevitably to be some vagueness, which we can try to combat by making sharp definitions.

 

To get a good grasp you need to understand the phrase cosmologists love to use: "at rest with respect to CMB"

or "at rest with respect to the Hubble flow"

 

when they say that the cosmo redshift is caused by the increase in distance between two STATIONARY objects, they mean stationary wrt CMB

 

the formula is

1+z = distance(now)/distance(then)

 

1+z is the ratio of the wavelengths

and it is equal to the amount the universe expanded while the light was traveling

that is

 

the ratio the wavelength is increased is equal to the ratio by which the

universe expanded, which is the ratio of the distance to the object now compared with the distance to the object when it emitted the light.

 

the ratio of wavelenghts does not depend in any simple way on the recession SPEED at the time of emission or at the time the light is received, it depends on the whole HISTORY of expansion during the flight-time, which can be faster or slower at various epochs during the light's journey.

 

to get the formula to work out perfectly you have to allow for our earth and solarsystem motion with respect to CMB.

this has been measured rather accurately by COBE and WMAP and is around 376 km/second (if I remember) in the direction of the constellation Leo.

there is a doppler hotspot in the CMB sky in that direction. we have to correct the data to get rid of that.

 

so we have to adjust our observations so that WE are not moving wrt CMB

 

and there is always going to be some uncertainty about how much the other guy is moving wrt CMB

 

but fortunately these local motions are SMALL and average out and don't affect the overall picture much.

 

in the big picture everybody is more or less at rest wrt CMB or, equivalently, the Hubble flow and their minor scurrying around averages out to insignificant.

 

================

Hubble flow: a technical term for the expansion of the universe. Hubble noticed that the galaxies in the Leo direction were not receding as fast as expected (according to his law) and the galaxies in the opposite direction were receding TOO fast. So he deduced that this was caused by the SUN AND EARTH moving in the Leo direction and in effect "catching up" a little with the recession in that direction. So Hubble back around 1930 already had the notion of a universal rest frame.

Then, in the 1990s it was made much more precise by measuring the Leo hotspot in the CMB.

It is the same notion of being at rest as Hubble had back many decades ago, but it is measured more precisely using the CMB temperature variation.

[grifter]

Okay i did have an explanation but i saw Martin's Superior one above and I figured that mine was pretty much covering old ground, nice one Martin

 

however, if you want any further info feel free to give me a shout and ill be happy to stick something up here.

 

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back by popular demand: my original post...

 

I think this is what you are referring to... if not i apologies.

 

Cosmological redshift is explained by a different mechanism to standard doppler namely Hubble's law which is the apparent correlation of redshifts & distances (often required by cosmology models derived from GR that have a metric expansion of space...(another story)) this means; photons which are propagating through the expanding space are "stretched", thus creating the cosmological redshift, which differs from the classic Doppler effect described above, which can be explained by the velocity increase or "Lorentz transformation" between the source and observer that is not due to the classical transfer of momentum and energy, instead the photon's wavelength and redshift increases as the space through which they are traveling "expands"

 

I hope this in some way helps, if you don't get it then post below and I'll try and explain it a bit clearer

 

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[foodchain]

I think this is what you are referring to... if not i apologies.

 

Cosmological redshift is explained by a different mechanism namely Hubble's law which is the apparent correlation of redshifts & distances (often required by cosmology models derived from GR that have a metric expansion of space...(another story)) this means; photons which are propagating through the expanding space are "stretched", thus creating the cosmological redshift, which differs from the classic Doppler effect described above, which can be explained by the velocity increase or "Lorentz transformation" between the source and observer that is not due to the classical transfer of momentum and energy, instead the photon's wavelength and redshift increases as the space through which they are traveling "expands"

 

I hope this in some way helps, if you don't get it then post below and I'll try and explain it a bit clearer

 

Then I would guess that from two equal points in distance the redshit should be different? Say if you have to galaxies of equal distance from the earth, that the redshift exhibited should vary, or be exactly the same? More or less would two exact copies of photons from exact situations in a set and equal distance from a point express any variation in regards to redshift?

[grifter]

In a word...No, space is expanding equally in all directions, so no matter if you were looking "right or left" the distances would have expanded the same ammonunt, going back to what has to be my favourite analagy: a baloon, with lots of dots on, consider looking form one of the dots to two other dots, in different directions, the balloon is blown up, the distance between the two of the dots will increase by the same factor, get it?

[Jacques]

the ratio of wavelenghts does not depend in any simple way on the recession SPEED at the time of emission or at the time the light is received, it depends on the whole HISTORY of expansion during the flight-time, which can be faster or slower at various epochs during the light's journey.

Is there any evidence that it is true or is it deduced from the Big Bang theory ?

If we suppose that the redshift depend on the recession speed, does it create any contradiction with logic or observation ?

[Martin]

 

---------------------edit---------------------

 

back by popular demand: my original post...

 

I think this is what you are referring to... if not i apologies.

 

Cosmological redshift is explained by a different mechanism to standard doppler namely Hubble's law which is the apparent correlation of redshifts & distances (often required by cosmology models derived from GR that have a metric expansion of space...(another story)) this means; photons which are propagating through the expanding space are "stretched", thus creating the cosmological redshift, which differs from the classic Doppler effect described above, which can be explained by the velocity increase or "Lorentz transformation" between the source and observer that is not due to the classical transfer of momentum and energy, instead the photon's wavelength and redshift increases as the space through which they are traveling "expands"

 

I hope this in some way helps, if you don't get it then post below and I'll try and explain it a bit clearer

 

---------------------edit---------------------

 

Excellent explanation---short and to the point.

I think it's good to have two that may differ in style and emphasis but basically overlap like plywood.

If one had to be erased I would withdraw mine because not concise enough.

Bravo to you, as usual!

[Jacques]

(often required by cosmology models derived from GR that have a metric expansion of space...(

OK

That answer my questionning

Thanks