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Thread: Redshift Components

  1. #1 Redshift Components 
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    This is my first post on this forum so I would like to thank the webhost and moderator(s) for making it available.

    As I understand it, there are 3 components that make up the measured redshift observed from a distant galaxy: doppler due to the relative speed between the observed galaxy and the observer; gravitational due to an increase or decrease in gravitational effect on the photon; and cosmological due to the expansion of space.

    Does anyone know if there is a way to determine the contribution of each component?

    Has the gravitational shift (red or blue) been determined for a distant quasar?

    (Please do not turn this into a debate over the validity of the big bang theory)


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  3. #2 Re: Redshift Components 
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    Quote Originally Posted by 1946cosmo
    Has the gravitational shift (red or blue) been determined for a distant quasar?
    Very interesting questions. I have found something on measurements of the gravitational redshift:
    http://www.einstein-online.info/en/s...rfs/index.html
    Quote Originally Posted by 1946cosmo
    (Please do not turn this into a debate over the validity of the big bang theory)
    I support you in this request.


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    Thank you Dishmaster! The link that you provided led me to some very interesting and useful information.

    Using the data for the white dwarf Sirius B (mass = .978 of our sun and radius = .00864 of our sun) and the equation of general relativity to predict gravitational redshift z = G*M/r*c**2 indicates that if our sun were replaced with Sirius B then the redshift measured by a distant observer would increase by a factor of 113.

    As I recall, quasars are typically considered to be very high density - possibly comparable to the density of a white dwarf. I am eager to learn about their gravitational redshift.
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    Quasars are believed (for a good reason) to be active galactic nuclei driven by a supermassive black hole. They can have a range of different phenomenologies depending on the viewing angle (cf. unified scheme). They all have in common an accretion disc (not part of the Black Hole) emitting X-rays due to friction as well as high-velocity and well confined bipolar jets.
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    A number of years back, it was thought a quasar was moving away from us at a speed faster than light. A dumb mistake which was quickly realised in that they had not taken into account the gravitational component of the redshift.
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    Type Ia supernovae are the "standard candles" used to determine the distance from earth to distant galaxies. Since a Type Ia supernova is the result of a white dwarf exploding after reaching a mass of 1.4 times that of our Sun, it's light would exhibit a significant gravitational redshift. Presumably, the gravitational redshift is being subtracted when computing a Type Ia supernova's receding velocity.

    Also, I assume that the majority of Type Ia supernovae being observed are in a binary system and accumulate their critical mass from their binary partner. As such, the white dwarf will exhibit a doppler shift associated with it's rotation about its companion star. At the distances involved and the period of rotation it may not be possible to determine the magnitude and sign of the rotational doppler shift.

    Has it been determined that the spectrum of a Type Ia supernova is comparable to that of a typical white dwarf? Is it possible that the violence of the explosion alters the spectrum from normal? If space can expand as proposed by many cosmologists, isn't it possible that happens in the vicinity of the supernova and contributes to the observed redshift?

    Has anyone determined if the redshift associated with a distant body changes when passing in the vicinity of a massive body? Gravitational lensing alters the direction of light. Observers on earth will see light after it is receding from a gravitational mass that has altered its course. Perhaps the gravitational blueshift as the light approaches the body is less than the gravitational redshift when the light recedes from the body.

    Hubble's law assumes that the farther a distant galaxy the faster it is receding from the Milky Way. Many cosmologists and astronomers estimate that galaxies more than seven billion light years distant may be receding at a significant percentage of the speed of light. Is time dilation incorporated into the recession velocity computation?
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    As far as I know, there is no need for accounting for relativity, because the recession is not relativistic, i.e. the recession velocity is due to the expansion of space, not movement through space. That is why it is possible to have recession velocities exceeding the speed of light.

    Presumably, the gravitational redshift is being subtracted when computing a Type Ia supernova's receding velocity.
    As far as I know the gravitation shift is very small in comparison to that of expansion. Not sure if it is subtracted though. it probably would be if it was significant.

    As such, the white dwarf will exhibit a doppler shift associated with it's rotation about its companion star. At the distances involved and the period of rotation it may not be possible to determine the magnitude and sign of the rotational doppler shift.
    The shifts will vary between both red and blue as it revolves around the companion star, giving an average. But again, the effect will be very small in comparison to that of expansion at such large distances afaik.

    Perhaps the gravitational blueshift as the light approaches the body is less than the gravitational redshift when the light recedes from the body
    Why would it be?
    Last edited by KALSTER; January 13th, 2012 at 10:14 AM.
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    Quote Originally Posted by 1946cosmo View Post
    Type Ia supernovae are the "standard candles" used to determine the distance from earth to distant galaxies. Since a Type Ia supernova is the result of a white dwarf exploding after reaching a mass of 1.4 times that of our Sun, it's light would exhibit a significant gravitational redshift. Presumably, the gravitational redshift is being subtracted when computing a Type Ia supernova's receding velocity.
    It has not been definitively established that SNe Ia are white dwarf explosions. You are welcome to calculate the gravitational redshift of such stars, though. SNe Ia are not standard candles. They do, however, show amazing regularity that allow them to effectively be used as something like standard candles, regardless of their astrophysical cause.
    Also, I assume that the majority of Type Ia supernovae being observed are in a binary system and accumulate their critical mass from their binary partner. As such, the white dwarf will exhibit a doppler shift associated with it's rotation about its companion star. At the distances involved and the period of rotation it may not be possible to determine the magnitude and sign of the rotational doppler shift.
    The light measured from SNe Ia comes well after any given star is destroyed. It is fairly certain that the light comes from a shell of material. This material would not be rotating around a star.
    Has it been determined that the spectrum of a Type Ia supernova is comparable to that of a typical white dwarf? Is it possible that the violence of the explosion alters the spectrum from normal?
    Yes, it's fairly certain that the light we see from SNe Ia is not that from the surface of a star, white dwarf or otherwise. It is the light that may be from the remnants of an explosion, but the exact properties of white dwarf explosions are hard to work out.
    If space can expand as proposed by many cosmologists, isn't it possible that happens in the vicinity of the supernova and contributes to the observed redshift?
    The "expansion of space" is merely the action of gravity. Work out the gravitational effects and you work that out.
    Hubble's law assumes that the farther a distant galaxy the faster it is receding from the Milky Way. Many cosmologists and astronomers estimate that galaxies more than seven billion light years distant may be receding at a significant percentage of the speed of light. Is time dilation incorporated into the recession velocity computation?
    Indeed. A great example of this (and of the accuracy of SNe Ia use) is found in Goldhaber et al 2001. Pre-print: [astro-ph/0104382] Timescale Stretch Parameterization of Type Ia Supernova B-band Light Curves
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  10. #9  
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    Looks like I was talking bollocks.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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  11. #10  
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    Quote Originally Posted by PhysBang View Post
    The light measured from SNe Ia comes well after any given star is destroyed. It is fairly certain that the light comes from a shell of material. This material would not be rotating around a star.
    When you say "well after", what kind of time scale are you thinking for when it starts?

    Some light would come from the initial explosion, but I suppose the surface area is very small, and dense so there is a limited to the number photons that have a path to free space.

    Then you have the explosion traveling out at 1/6 of the speed of light, but the density of the mattieral is dropping as it expands etc.
    Would thermal collisions drop away after only after hours, so it is infact rapidly cooling?

    So are you thinking most of the light show starts when the solar mass of matterial traveling at 0.166 c hits the ISM?
    So maybe 3 or 4 hours?

    Or would magnet fields and radiation heat up the surounding ISM before that?
    Last edited by PetTastic; January 13th, 2012 at 12:23 PM. Reason: "for when it starts" added
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  12. #11 SNe Ia duration 
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    It is my understanding that the typical SNe Ia increases in brightness for 18 days and then dimishes for a month. That's why Saul Perlmutter used a three week interval when searching for them.
    Last edited by 1946cosmo; January 14th, 2012 at 07:39 AM.
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  13. #12  
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    Yes. but is there any chance of see light from the event its self rather than the impact with the ISM and decay of colbolt, iron etc.
    Would a space telescope perminantly looking for a supernova be able to see anything of the original explosion from a reasonable distance, or will we onlly ever see the aftermarth starting a few hours later?
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  14. #13 space expansion due to dark matter 
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    Quote Originally Posted by PhysBang View Post
    Quote Originally Posted by 1946cosmo View Post
    If space can expand as proposed by many cosmologists, isn't it possible that happens in the vicinity of the supernova and contributes to the observed redshift?
    The "expansion of space" is merely the action of gravity. Work out the gravitational effects and you work that out.
    I thought that the cosmological expansion of space has been attributed to dark energy instead of gravity.
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  15. #14  
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    Quote Originally Posted by 1946cosmo View Post
    I thought that the cosmological expansion of space has been attributed to dark energy instead of gravity.
    Dark energy is invoked to explain the (apparent) acceleration of expansion.
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    Quote Originally Posted by 1946cosmo View Post
    Has anyone determined if the redshift associated with a distant body changes when passing in the vicinity of a massive body? Gravitational lensing alters the direction of light. Observers on earth will see light after it is receding from a gravitational mass that has altered its course. Perhaps the gravitational blueshift as the light approaches the body is less than the gravitational redshift when the light recedes from the body.
    According to General Relativity, any light will be redshifted on the way out of a gravitational field by the same amount as it is blueshifted on the way in, so there is no change. Gravitational redshift is purely due to the difference in the gravitational potential of the source object, when compared to the observer - the difference between the climb out of the gravitational well of the distant galaxy and in our case the fall into the gravitational well of the Milky-Way. No object in between will have any net effect on that redshift component.

    Cosmological redshift can be thought to work in the same way, but on a much grander scale - the difference in the gravitational density of the universe between the time the light was emitted and the time it is detected, due to the expansion of the universe.
    Last edited by SpeedFreek; January 13th, 2012 at 06:51 PM.
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  17. #16  
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    Quote Originally Posted by PetTastic View Post
    When you say "well after", what kind of time scale are you thinking for when it starts?
    In what we see, the light peaks some ten to thirty days after it begins to be detectable. In the white dwarf models, the star explodes and the outer shell that is blown away is slowly warmed from within by the energy within. I'm not sure of the timeline of this, but I imagine that it takes at least a day. In these cases, the explosion has happened and the star is gone, it's the collection of gas around where the star was that is heating up and throwing off photons.
    Some light would come from the initial explosion, but I suppose the surface area is very small, and dense so there is a limited to the number photons that have a path to free space.
    I'm not sure that anyone has seen the initial explosion that lead to a SN Ia.
    Then you have the explosion traveling out at 1/6 of the speed of light, but the density of the mattieral is dropping as it expands etc. Would thermal collisions drop away after only after hours, so it is infact rapidly cooling?
    I'm not sure what you mean. SNe Ia can last for over a month.
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    Quote Originally Posted by 1946cosmo View Post
    I thought that the cosmological expansion of space has been attributed to dark energy instead of gravity.
    Cosmological expansion is an effect of gravity. So-called dark energy is a type of energy that creates certain gravitational interactions that accelerate cosmological expansion. One can have expansion without any dark energy.
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  19. #18 acceleration of expansion 
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by 1946cosmo View Post
    I thought that the cosmological expansion of space has been attributed to dark energy instead of gravity.
    Dark energy is invoked to explain the (apparent) acceleration of expansion.
    My mistake. Thank you for the correction.
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  20. #19 expansion without dark energy? 
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    Quote Originally Posted by PhysBang View Post
    Quote Originally Posted by 1946cosmo View Post
    I thought that the cosmological expansion of space has been attributed to dark energy instead of gravity.
    Cosmological expansion is an effect of gravity. So-called dark energy is a type of energy that creates certain gravitational interactions that accelerate cosmological expansion. One can have expansion without any dark energy.
    As I recall Saul Perlmutter's doppler redshift data indicated that until about 7 billion years ago the expansion rate of space was decreasing. After then the expansion rate of space began increasing. It was as if the gravitational force was slowly being overcome by some other force. It seems logical that both forces (gravity and dark energy) may have always been present but dark energy is effective over a greater distance than gravity. I suppose one could also argue that dark energy has been increasing with time. I don't understand how "one can have expansion without any dark energy". The expansion of space represents work since mass (galaxies) are being separated over increasing distances against the force of gravity which requires energy.
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  21. #20  
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    Quote Originally Posted by 1946cosmo View Post
    I don't understand how "one can have expansion without any dark energy". The expansion of space represents work since mass (galaxies) are being separated over increasing distances against the force of gravity which requires energy.
    Before the discovery that the expansion of the universe was accelerating, it was assumed that the rate of expansion had been decelerating since the Big Bang. The "work" you refer to was the initial impetus for the expansion - the inflationary epoch. Once the universe was in a state of expansion, the only work being done was by gravity - slowing down that rate. The question was whether the energy density of the universe large enough to slow that expansion to a halt or not.

    Then we found that the deceleration had turned into an acceleration after around 7 billion years, so it seems as if there is another player on the field apart from gravity - dark energy. But dark energy was not required in order for the universe to be expanding - originally the expansion was simply an "initial condition" of the Big-Bang and more recently it has been tentatively explained by inflationary theory.
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  22. #21 no gravitational redshift correction 
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    Quote Originally Posted by PhysBang View Post
    Quote Originally Posted by 1946cosmo View Post
    Hubble's law assumes that the farther a distant galaxy the faster it is receding from the Milky Way. Many cosmologists and astronomers estimate that galaxies more than seven billion light years distant may be receding at a significant percentage of the speed of light. Is time dilation incorporated into the recession velocity computation?
    Indeed. A great example of this (and of the accuracy of SNe Ia use) is found in Goldhaber et al 2001. Pre-print: [astro-ph/0104382] Timescale Stretch Parameterization of Type Ia Supernova B-band Light Curves
    Thank you for the link to a very informative and pertinent paper! During my first quick reading of the document I found no mention of incorporating gravitational redshift correction in the analysis. However, since SNE Ia apparently are fairly uniform, it can probably be assumed that the gravitational redshift would be fairly consistent. If so, it would be associated with a fixed offset of the spectral plot and not a change in the width of the plot. I will investigate further.
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