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Thread: redshift

  1. #1
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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.
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    Quote Originally Posted by Copernicus View Post
    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.
    Solfe

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    Flat Earth ?

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

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    Quote Originally Posted by John Mendenhall View Post
    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?
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    Quote Originally Posted by Copernicus View Post
    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

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    Quote Originally Posted by Copernicus View Post
    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.
    As above, so below

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    Quote Originally Posted by Copernicus View Post
    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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    Quote Originally Posted by Dave241 View Post
    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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    Quote Originally Posted by Jens View Post
    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.
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    Quote Originally Posted by Copernicus View Post
    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

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    Quote Originally Posted by Dave241 View Post
    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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    Quote Originally Posted by grant hutchison View Post
    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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    Quote Originally Posted by Shaula View Post
    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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    Quote Originally Posted by Copernicus View Post
    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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    Quote Originally Posted by Shaula View Post
    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.
    The moment an instant lasted forever, we were destined for the leading edge of eternity.

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