Dark matter and doppler

[Le Repteux]

Hi everybody,

 

The calculations of the normal rotation period of the galaxies we observe depends on their observed mass, but it also depends on the doppler effect from their recession. The more they speed away from us, the more their period is stretched. When we calculate this period, thus when we calculate their rotational speed, we have to include doppler effect in the calculations, which increases that speed. Has anybody thought of not including the doppler effect in the calculations? If not, can somebody do these calculations for a couple of galaxies and tell me if their rotational speed is still too fast to account for their observable mass?

Edited October 1, 2014 by Le Repteux

[Nicholas Kang]

According to BBC Knowledge Asia Edition, recession of galaxies are not known as Doppler Effect, they are known as Cosmological redshift. Why?

Because Doppler Effect says nothing about motion of space-time continuum. It just tells you what happened to an object in motion THROUGH space-time. They speed away becasue the space-time is expanding. The galaxies hardly move at all. This is global space-time expansion, where gravitational effects become obvious. Locally, gravity effects are negligible. Therefore, the expansion of spacetime is not obvious, for example in Earth. Therefore, our distance remains constant if we are standing still. Recall that gravity is really a mathematical "trick" of space-time geometry.

[Enthalpy]

Hi,

 

The worry with galactic rotation is in the speed profile. The speed of globular clusters should decrease with their distance to the galaxy's center but it doesn't, telling that there is much mass in a galaxy which is not at its center and which we don't see.

 

The recession speed, acting equally on all globular clusters or a remote galaxy, doesn't change that. By the way, we also observe the abnormal rotation profile for our own galaxy.

[Nicholas Kang]

The more they speed away from us,

 

I thought this is the keyword.

[Le Repteux]

Hi Nicolas,

 

The redshift observed is calculated exactly the same way doppler effect is calculated, this is why I used the term doppler effect. Do you have an answer to my question about the effect of this redshift on rotational calculations?

[Strange]

Hi everybody,

 

Hellooooo!

 

The calculations of the normal rotation period of the galaxies we observe depends on their observed mass, but it also depends on the doppler effect from their recession.

 

Why would it depend on the Doppler effect or their recession?

 

As noted, it is the rotational profile (i.e. the way rotational velocity changes with distance from the center) that is important.

 

These rotational velocities are measured using Doppler shifts - which will be red shifted on one side (where the stars, gas, dust, etc. is moving away from us) and blue-shifted on the other side.

 

The more they speed away from us, the more their period is stretched.

 

OK. There will be time dilation effects for very distant galaxies. But this will not affect the relative velocities at different radii.

[Le Repteux]

The recession speed, acting equally on all globular clusters or a remote galaxy, doesn't change that. By the way, we also observe the abnormal rotation profile for our own galaxy.

Recession speed acts on the rotation period of galaxies, thus on their rotational speed, the same way it would act on the time periods of a clock: it stretches them.

These rotational velocities are measured using Doppler shifts - which will be red shifted on one side (where the stars, gas, dust, etc. is moving away from us) and blue-shifted on the other side.

If you remove the cosmological redshift to the calculations of the red shifted side, and add it to the calculations of the blue shifted side, you get a galaxy that rotates too fast. Thus if you don't, you might have a galaxy that rotates exactly at the right speed.

 

OK. There will be time dilation effects for very distant galaxies. But this will not affect the relative velocities at different radii.

We can consider different radii as different clocks, each of them being affected the same by cosmological redshift.

[elfmotat]

Recession speed acts on the rotation period of galaxies, thus on their rotational speed, the same way it would act on the time periods of a clock: it stretches them.

 

You should read Enthalpy's post again. It's not the average rotational speed that's important, it's the speed as a function of radius, v(r ), which is important.

The fact that v(r ) ≈ const. remains true regardless of whether or not you Doppler-shift the frequency of rotation.

Edited October 1, 2014 by elfmotat

[Strange]

If you remove the cosmological redshift to the calculations of the red shifted side, and add it to the calculations of the blue shifted side, you get a galaxy that rotates too fast. Thus if you don't, you might have a galaxy that rotates exactly at the right speed.

 

Many (most? all?) of the galaxies measured are local and so there is no cosmological redshift.

 

And it is not about rotational speed; it is the way the speed changes with the radius from the center.

 

A: predicted rotation curve

B: observed rotation curve

 

Slowing (or in the case of Andromeda, speeding) the rotation rate will not affect the curve.

 

We can consider different radii as different clocks, each of them being affected the same by cosmological redshift.

 

Exactly. They will be affected equally by the cosmological time dilation (if there is any). And so it will not make any difference.

[Le Repteux]

Many (most? all?) of the galaxies measured are local and so there is no cosmological redshift.

If you are sure of that, then my question is bogus.

 

Exactly. They will be affected equally by the cosmological time dilation (if there is any). And so it will not make any difference.

No difference between the different speeds, but difference in calculation of the abnormal mass.

 

[Strange]

If you are sure of that, then my question is bogus.

 

I am not sure. But it seems likely (if galaxies are far enough away to have significant time dilation then they may be too small to resolve the doppler shift from each side)

 

 

No difference between the different speeds, but difference in calculation of the abnormal mass.

 

I don't get it. You need to explain why adding a constant would change the distribution of speeds within a galaxy or, equivalently, the distribution of mass within the galaxy.

 

Note: as with the velocity, it is not just the absolute mass of the galaxy that is significant, but the distribution of that mass: concentrated at the center but distributed in a sphere, unlike the galactic disk.

Edited October 1, 2014 by Strange

[Le Repteux]

I am not sure. But it seems likely (if galaxies are far enough away to have significant time dilation then they may be too small to resolve the doppler shift from each side)

OK, if you are not sure, then my question might have a sense.

 

I don't get it. You need to explain why adding a constant would change the distribution of speeds within a galaxy or, equivalently, the distribution of mass within the galaxy.

You have to refer to the calculation of the frequency for a clock in motion. A galaxy is a clock, and cosmological redshift is considered as a motion as far as the calculations of their rotational periods are concerned. The way you measure the rotational speed is independent of the calculations: if you measure the speed at a certain radius, you get the period. In other words, when you observe a galaxy in motion, all its rotational speeds at different radii are changed by the same proportion.

 

Note: as with the velocity, it is not just the absolute mass of the galaxy that is significant, but the distribution of that mass: concentrated at the center but distributed in a sphere, unlike the galactic disk.

Yes, I know.

Edited October 1, 2014 by Le Repteux

[Strange]

You have to refer to the calculation of the frequency for a clock in motion. A galaxy is a clock, and cosmological redshift is considered as a motion as far as the calculations of their rotational periods are concerned. The way you measure the rotational speed is independent of the calculations: if you measure the speed at a certain radius, you get the period. In other words, when you observe a galaxy in motion, all its rotational speeds at different radii are changed by the same proportion.

 

I still don't understand what you are trying to say.

 

Lets look at an analogy. Imagine you have a circular racetrack with many lanes. There is a car going round in each lane. You have a theory that predicts (for whatever reason) that the cars in the inner lanes will be going faster than the ones on the out lanes. But when you measure their speeds, you find that all the cars are going at the same speed.

 

But then you find out that the racetrack is in a part of space where time runs at half the rate. So the cars are all going twice as fast (*) as you measured them to be. But they are all still at the same speed. I don't see how adding time dilation can make the relative speeds of the cars change.

 

(*) Or half the speed. I'm confused! But they are still all the same...

Edited October 1, 2014 by Strange

[Le Repteux]

If you are talking about the time dilation of relativity, this is not what I am talking about. I am only talking of the impact doppler effect has on the observed frequency of a moving clock. In your example, for him to observe this effect, the racetrack would have to be in direct radial motion with regard to the observer.

Edited October 1, 2014 by Le Repteux

[Strange]

If you are talking about the time dilation of relativity, this is not what I am talking about. I am only talking of the impact doppler effect has on the observed frequency of a moving clocks. In your example of the racetrack, to observe the effect I am talking about, it would have to be in direct radial motion with regard to the observer.

 

Firstly, if you are talking about cosmological red-shift then you are talking about a (general) relativistic effect, not Doppler. And there is an exactly corresponding time dilation (see supernova light curves, for example).

 

However, whether you are talking about local galaxies in relative motion or cosmological redshift, you still have';t explained how a constant offset can change teh relative velocities.

 

The velocities within the galaxy are measured by the varying red-shift cause by the rotation. There is then a CONSTANT added (subtracted?) to all of these for the cosmological red shift (or Doppler effect for relative motion).

 

So, if the velocities were decreasing with increasing radius, as predicted, and you add the same amount to all of them - then they will still be decreasing with increasing radius.

 

And, if the velocities are constant with increasing radius, as observed, and you add the same amount to all of them - then they will still be constant with increasing radius.

 

I don't understand how you think that the extra cosmological (or Doppler) red shift is going to change the graph from A to B (see post 9). All it can do is shift the whole curve up or down a bit.

 

What am I missing?

[Le Repteux]

Firstly, if you are talking about cosmological red-shift then you are talking about a (general) relativistic effect, not Doppler. And there is an exactly corresponding time dilation (see supernova light curves, for example).

OK, but if you want to calculate their rotational speed, you will have to plug this frequency gap into the equations, no?

 

The velocities within the galaxy are measured by the varying red-shift cause by the rotation. There is then a CONSTANT added (subtracted?) to all of these for the cosmological red shift (or Doppler effect for relative motion).

 

So, if the velocities were decreasing with increasing radius, as predicted, and you add the same amount to all of them - then they will still be decreasing with increasing radius.

 

And, if the velocities are constant with increasing radius, as observed, and you add the same amount to all of them - then they will still be constant with increasing radius.

 

I don't understand how you think that the extra cosmological (or Doppler) red shift is going to change the graph from A to B (see post 9). All it can do is shift the whole curve up or down a bit.

OK, I understand what you mean. Do you know by how much that curve would shift then? Would the predicted curve cross the observed curve, for instance? If we calculated a mean observed velocity, would it be closer to the mean predicted one?

Edited October 1, 2014 by Le Repteux

[Strange]

OK, I understand what you mean. Do you know by how much that curve would shift then? Would the predicted curve cross the observed curve, for instance? If we calculated a mean observed velocity, would it be closer to the mean predicted one?

 

I don't know. Or even have a gut feel for the relative magnitudes. I would have to do some research to find out. So I suggest you do it.

[Le Repteux]

I don't even know where to begin with!

 

The idea behind my question, but you may have noticed, was to do as if there was no expansion, and if the calculations I was talking about would have given the expected result, to try to find another way to explain the redshift. Your argument about the curves is very good, I'll think about it, and if I can link it to my idea, I'll be back.

[patton119]

So out of curiosity, dark matter surrounds our galaxy. If its imagined to be like the oort cloud, shouldn't there be a cloud of dark matter near the center of our galaxy... Just a question, no real facts behind it

[Strange]

Dark matter not only surronds the galaxy, it is spread through the galaxy as well. It is densest at the centre.