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## redshift

Lets say we have an object that 5 billion years away from earth. There would be a certain redshift from the expansion of the universe. Lets say that we had some new technology that showed it was also moving at 1/3 the speed of light perpendicular to earth. How would we be able to calculate the redshift due to the motion perpendicular to earth and the redshift due to expansion and add those two redshifts together. Is there an easy equation? Thanks in advance.

2. Originally Posted by Copernicus
Lets say we have an object that 5 billion years away from earth. There would be a certain redshift from the expansion of the universe. Lets say that we had some new technology that showed it was also moving at 1/3 the speed of light perpendicular to earth. How would we be able to calculate the redshift due to the motion perpendicular to earth and the redshift due to expansion and add those two redshifts together. Is there an easy equation? Thanks in advance.
Is there an easy formula? To who? I know that there is going to be trig functions in that one, so I would say it's hard. A scientist would probably say it's "theoretically possible", probably owing to the fact that their instrument might or might not be collecting data for a cosmological redshift and a Doppler shift, the gravitation redshift and the angle of the target relative to the observer, all at the same time. Two or more different instruments might not yield good numbers to be used together.

I don't know what that formula would look like, but it sounds like a mess.

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## Flat Earth ?

Please explain what you mean by traveling perpendicular to the Earth ?

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Originally Posted by John Mendenhall
Please explain what you mean by traveling perpendicular to the Earth ?
One has a line from the earth to this distant galaxy. The galaxy is part of the expansion of the universe and is expanding away at whatever rate something 5 billion light years away is expanding. The expansion is directly away from earth. The galaxy is also moving at ninety degrees from that expansion, for whatever reason, at 1/3 the speed of light. How would one add the transverse motion to the expansion motion, and calculate a red shift.

For additional fun, what if the gravitational field was 10 times higher at earth, than at the galaxy?

5. Originally Posted by Copernicus
One has a line from the earth to this distant galaxy. The galaxy is part of the expansion of the universe and is expanding away at whatever rate something 5 billion light years away is expanding. The expansion is directly away from earth. The galaxy is also moving at ninety degrees from that expansion, for whatever reason, at 1/3 the speed of light. How would one add the transverse motion to the expansion motion, and calculate a red shift.

For additional fun, what if the gravitational field was 10 times higher at earth, than at the galaxy?
Calculate separately, multiply the results. So if cosmic expansion, relativistic doppler and gravitational red shift all individually doubled the wavelength between emitter and receiver, the combined red shift would amount to a factor of 2x2x2=8.

Grant Hutchison

6. Originally Posted by Copernicus
Lets say we have an object that 5 billion years away from earth. There would be a certain redshift from the expansion of the universe. Lets say that we had some new technology that showed it was also moving at 1/3 the speed of light perpendicular to earth. How would we be able to calculate the redshift due to the motion perpendicular to earth and the redshift due to expansion and add those two redshifts together. Is there an easy equation? Thanks in advance.
Also, as an additional comment, I may be misunderstanding, but I think we already have technology to show what you call the perpendicular motion, which normally I would think of as a the "proper motion." So in other words, how fast it moves over time in relation to the cosmic background or other stars.

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Originally Posted by Copernicus
Lets say we have an object that 5 billion years away from earth. There would be a certain redshift from the expansion of the universe. Lets say that we had some new technology that showed it was also moving at 1/3 the speed of light perpendicular to earth. How would we be able to calculate the redshift due to the motion perpendicular to earth and the redshift due to expansion and add those two redshifts together. Is there an easy equation? Thanks in advance.
As far as I'm aware the sideways motion has no effect on redshift, unless there is some weird GR effect that I'm not aware of when it's moving at close to c. Doppler shift is only a measure of the motion directly towards or away from you. Or are you asking about something else?

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Originally Posted by Dave241
As far as I'm aware the sideways motion has no effect on redshift, unless there is some weird GR effect that I'm not aware of when it's moving at close to c. Doppler shift is only a measure of the motion directly towards or away from you. Or are you asking about something else?
https://en.wikipedia.org/wiki/Relati...Doppler_effect

Weird SR effect, I am afraid!

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Thanks all. Grant, would the z values be multiplied.

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Originally Posted by Jens
Also, as an additional comment, I may be misunderstanding, but I think we already have technology to show what you call the perpendicular motion, which normally I would think of as a the "proper motion." So in other words, how fast it moves over time in relation to the cosmic background or other stars.
Hi Jens, From what I understand, it is currently impossible to measure transverse motion, even 50 million light years away.

11. Originally Posted by Copernicus
Thanks all. Grant, would the z values be multiplied.
No. As I said in my example, you multiply the factors of [received wavelength]/[emitted wavelength]. Which translates to multiplying together the (1+z) values for each redshift, doppler * cosmological * gravitational.

Grant Hutchison

12. Originally Posted by Dave241
As far as I'm aware the sideways motion has no effect on redshift, unless there is some weird GR effect that I'm not aware of when it's moving at close to c. Doppler shift is only a measure of the motion directly towards or away from you. Or are you asking about something else?
As Shaula said, weird special relativity. In the case of transverse red-shift, what you're seeing is the time dilation of the emitter.

Grant Hutchison

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Originally Posted by grant hutchison
No. As I said in my example, you multiply the factors of [received wavelength]/[emitted wavelength]. Which translates to multiplying together the (1+z) values for each redshift, doppler * cosmological * gravitational.

Grant Hutchison
I get it now. Thanks. What I don't get is can there really be a z value if the redshift come from two or more different mechanisms.

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Originally Posted by Shaula
Whoa!! Well how about that, I just learned something new today! Thanks Shaula and Grant, that's actually a really cool effect that I honestly didn't know about before.

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Originally Posted by Copernicus
I get it now. Thanks. What I don't get is can there really be a z value if the redshift come from two or more different mechanisms.
What would stop you combining the effects? It is no different than saying the speed of something rolling downhill is due to a combination of gravity and friction.

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Originally Posted by Shaula
What would stop you combining the effects? It is no different than saying the speed of something rolling downhill is due to a combination of gravity and friction.
Thanks, I got it.

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