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Copernicus
2018-Dec-31, 01:37 PM
Can we tell if a galaxy is moving radially or transverse to our position.

grant hutchison
2018-Dec-31, 02:04 PM
Radially, from redshift / blueshift.
Transverse motion requires the measurement of a shift in position over time, and has only been applied to members of the Local Group, I think.
But there are tricks that can be used (https://arxiv.org/abs/1005.4430) to estimate the mean transverse motion of a cluster of galaxies.

Grant Hutchison

Copernicus
2018-Dec-31, 08:18 PM
So at what distance can we no longer tell if the motion is radial or transverse.

Ken G
2018-Dec-31, 08:29 PM
So at what distance can we no longer tell if the motion is radial or transverse.The radial motion can be determined at any distance, because it's just Doppler shift (though of course at cosmological distances the situation gets complicated). The transverse motion is independently determined, and that requires faster speeds at larger distances, because it's really a rate of angle not a rate of distance. The faster the speed, the larger the distance you can detect it.

Copernicus
2018-Dec-31, 09:01 PM
The radial motion can be determined at any distance, because it's just Doppler shift (though of course at cosmological distances the situation gets complicated). The transverse motion is independently determined, and that requires faster speeds at larger distances, because it's really a rate of angle not a rate of distance. The faster the speed, the larger the distance you can detect it.

How do we know it is radial?

grant hutchison
2018-Dec-31, 09:10 PM
How do we know it is radial?Maybe you could just tell us what you have in mind? Are you invoking the so-called "transverse Doppler", observed in objects with relativistic velocity across the line of sight?

Grant Hutchison

Ken G
2018-Dec-31, 09:35 PM
If it's transverse Doppler, note that is order (v/c)2, while the normal Doppler shift is order v/c, which for most non-cosmological applications is vastly larger. So outside of cosmology, frequency shifts are always radial velocity effects.

swampyankee
2018-Dec-31, 11:43 PM
So at what distance can we no longer tell if the motion is radial or transverse.

Radial motion can be detected to the edge of the observable universe; transverse motion requires requires tracking over a long period of time and precise measurements for the period. I think Hipparcos has milli-arcsecond precision over a one year period, that is, it could detect a transverse motion of one AU per year at 1,000 parsecs.

Copernicus
2019-Jan-01, 02:12 AM
Are we then assuming that the observed red shift at one billion light years is almost all radial?

cjameshuff
2019-Jan-01, 02:48 AM
Are we then assuming that the observed red shift at one billion light years is almost all radial?

...it's not an assumption. It's a fundamental result of how Doppler works.

Copernicus
2019-Jan-01, 03:51 AM
...it's not an assumption. It's a fundamental result of how Doppler works.

Transverse also has the doppler, but it is a different equation.

Ken G
2019-Jan-01, 05:27 AM
Are we then assuming that the observed red shift at one billion light years is almost all radial?Yes, this is called "the cosmological principle."

Copernicus
2019-Jan-01, 06:08 AM
Yes, this is called "the cosmological principle."

I was not aware of this. But I suspected. Thought experiment. If by some magic we are at near the center of a universe, the universe is static, as in not expanding, is there evidence that contradicts that almost all of the red shift past one billion light years, is from transverse motion?

Ken G
2019-Jan-01, 08:18 AM
I was not aware of this. But I suspected. Thought experiment. If by some magic we are at near the center of a universe, the universe is static, as in not expanding, is there evidence that contradicts that almost all of the red shift past one billion light years, is from transverse motion?Probably not, the issue would be, could you fit that into an entire story of the history of the universe that satisfied a well-tested set of unifying physical laws? There's a kind of "primrose path" in physics, where you can take a fixed set of observations, and cobble together some specific situation that might fit everything in that set of observations, and not be directly refuted by any other observation, but what you are cobbling together is not based on any unifying principles that simplifies your understanding of anything else. It's just artificially constructed to push through some particular idea (like all cosmological redshifts are transverse Doppler shifts), while not explaining anything but the redshifts you built it to explain. Then you'd need some completely different reason for the H/He ratio, and yet another one for the existence of a CMB.

If you go far enoug down that road, where every observation has its own individual explanation and there is no unification of anything and no predictions of what to look for next, you ultimately arrive at what might be called a "just is" model of the universe-- every observation just is, period. Nothing you see has any reason for it, nothing can be predicted, it all just is-- that's my model, and no observation refutes it. If you use your physics and make a prediction, and it works, and you say, "see, I knew there was a reason for why things happen," I'll just say there was no reason your prediction worked, it just is that your prediction worked this time, and it doesn't mean it will work next time. Statements like that fit all existing data, but they are lousy science, because they cannot be tested, and don't empower you to have expectations. So beware the idea that "fitting the observations" is all the science is about, it's just not true. (Ergo Occam's razor and all the other auxiliary principles of science that make it more than just "what fits".)

Shaula
2019-Jan-01, 09:48 AM
I was not aware of this. But I suspected. Thought experiment. If by some magic we are at near the center of a universe, the universe is static, as in not expanding, is there evidence that contradicts that almost all of the red shift past one billion light years, is from transverse motion?
The required speeds seem unlikely. Take a redshift of 0.1 (just over a billion light years). That requires a transverse speed of 0.5c.

I haven't done the maths but I'd question whether the required speeds are compatible with observations of Icarus - the star (z 1.5, v/c of 0.91) was lensed by a galaxy cluster(z 0.5, v/c of 0.74) and there was even a microlensing event within this.

Move up to GN-z11 and you are looking at 0.997c.

Copernicus
2019-Jan-01, 02:03 PM
The required speeds seem unlikely. Take a redshift of 0.1 (just over a billion light years). That requires a transverse speed of 0.5c.

I haven't done the maths but I'd question whether the required speeds are compatible with observations of Icarus - the star (z 1.5, v/c of 0.91) was lensed by a galaxy cluster(z 0.5, v/c of 0.74) and there was even a microlensing event within this.

Move up to GN-z11 and you are looking at 0.997c.

Yeah, that would not make sense.

Ken G
2019-Jan-01, 02:37 PM
I hadn't worked the numbers, but they come out interesting. To see a lateral speed, you need to be able to detect an angle change over a time, which is a speed divided by a distance times the elapsed time. The speed would need to obey (v/c)2 = 2z for small z, so the angle is ct/d *(2z)1/2 if you can wait a time t. Our current model has z = Hd/c, so if we use the same d with each z (say from supernova data), we get that the angle over time t is Ht*(2/z)1/2. So the angle you'd see (in radians) is a few (4-5) times the fractional age of the universe that you can wait. That fractional time per year is order 10-10, so if we measure angle in that weird astronomy unit of .000005 radians (called an arc second), we get about .0001 arc seconds per year at z=0.1. That's pretty small!

The upshot it, as usual with proper motions in astronomy, distance is not your friend. If you interpret z as a transverse Doppler shift, the effect at very great distance saturates at about .00002 arc seconds per year. You'd be better off looking at the closest cosmological galaxies out to the Virgo cluster, like Hubble himself did, where the effect would be about 20 times larger, or some .002 arc seconds per year. That would be hard to do for absolute calibration, but if you were patient enough...

Copernicus
2019-Jan-01, 03:14 PM
Yeah, that would not make sense.

I get a red shift of about z=1.15 at v=.5c using transverse motion, not .1

Shaula
2019-Jan-01, 03:19 PM
...we get about .0001 arc seconds per year at z=0.1. That's pretty small!
Hipparchus could hit an accuracy of 0.1 milliarcseconds and Gaia could to 10x better. So those numbers are just within the range of modern precision measurements, although this is probably going to be complicated by the fact that at closer redshifts the targets would not be point sources.

Shaula
2019-Jan-01, 03:24 PM
I get a red shift of about z=1.15 at v=.5c using transverse motion, not .1
Have you left on the +1?

1 + z = \frac{1}{\sqrt{1 - \frac{{v}^{2}}{{c}^{2}}}}

https://en.wikipedia.org/wiki/Redshift#Redshift_formulae

The more accurate value for the speed would be 0.4c, sorry, rounded up.

Copernicus
2019-Jan-01, 03:26 PM
Have you left on the +1?

1 + z = \frac{1}{\sqrt{1 - \frac{{v}^{2}}{{c}^{2}}}}

https://en.wikipedia.org/wiki/Redshift#Redshift_formulae

You are right, I get 0.15 now. I didn't scroll down wolframalpha far enough.

Ken G
2019-Jan-01, 03:30 PM
Hipparchus could hit an accuracy of 0.1 milliarcseconds and Gaia could to 10x better. So those numbers are just within the range of modern precision measurements, although this is probably going to be complicated by the fact that at closer redshifts the targets would not be point sources.Yes I don't really know how hard it would be, note that not only does Gaia deal with point sources, but I wouldn't be surprised if it settles for relative proper motion. It somehow has to get absolute proper motion, and I don't know how that is done, but I wouldn't be amazed if it uses as background objects the very sources we are talking about. So Gaia may need the distant galaxies to not be moving transversely in order to work. If so, the very fact that it does seem to work would likely rule out the possibility of transverse redshifts.

antoniseb
2019-Jan-02, 05:03 PM
You might be able to do better with a millimeter array, or perhaps something like the Event Horizon Telescope, comparing the positions of nearby SMBHs compared to background quasars over the course of a decade or two. Granted, the SMBH might be moving with respect to the center of mass of its host galaxy, but at least it would be a start toward answering the OPs question. Obviously this would take some years to accomplish.

Ken G
2019-Jan-02, 10:45 PM
The problem is, no one is going to undertake it, because they likely couldn't even get the results published if their conclusion was merely that "background cosmological sources are not moving at transverse relativistic speeds." It's because we always have to decide what we are going to test, and what we are going to take as given because it seems absurd otherwise. It's not that we're never wrong when we do this (the distance to the stars, in ancient times, comes to mind), it's that we generally don't think it's worth spending time on tests that would require so many of the things we hold to be true to be wrong that it just doesn't seem worth spending time on. But it's an interesting question to put out there nonetheless.

Copernicus
2019-Jan-03, 10:31 AM
Thanks for all your help. There is a lot of stuff in physics that is too difficult for me to understand. I guess I will just have to believe that the cosmological principle is correct. The conviction of things not seen.

Ken G
2019-Jan-03, 02:20 PM
And remember there is a gulf between something being "correct" and something being so far from the truth that distant galaxies could be flitting around past each other at relativistic speeds. That latter model would require that much more than just the cosmological principle be wrong ,because we'd need a way to accelerate them to such speeds! I admit that saying "expanding space did it" is not quite the same thing as an explanation in the radial picture we now have, but we do observe gravitational redshifts that fit Einstein's model, and it still seems easier to accomplish radial redshifts than transverse relativistic speeds. Nevertheless, it doesn't mean we must hold that the cosmological principle is the complete story either-- it seems likely to me that there is no such thing as an "absolutely correct" physical principle, there are only the principles that we know how and when they break down, and the ones that we don't yet.

ngc3314
2019-Jan-03, 02:28 PM
Possibly the most relevant data right now are those employed in producing the INternatioanl Celestial Reference Frame (https://en.wikipedia.org/wiki/International_Celestial_Reference_Frame), an all-sky network of active galactic nuclei whose relative coordinates have been repeatedly measured by radio interferometry. The current noise level on these coordinates is given as 40 microarcseconds. This is the kind of measurement used, for example, to search for motion of an AGN if what we see is mostly coming from a jet rather than the core itself.

(As an aside, Vera Rubin was said to have once mused that it was interesting that cosmic expansion manifested itself only on the axis we could measure...)

Ken G
2019-Jan-03, 02:46 PM
Ah, that's an interesting number, 40 microarcseconds. The above calculations (assuming I did them right) say that in the sweet spot of the nearby Hubble law, it would take about 20 years of monitoring for transverse explanations for the redshifts to manifest themselves clearly, extending up to 2000 years for the most distant sources. So it is certainly a measurable issue on a human lifetime, as unlikely though it may be (notwithstanding Vera's insightful provocation!).

By the way, for those who don't know that history, Fritz Zwicky discovered dark matter but couldn't get anyone to believe him, and Vera Rubin found a way to show clearly its existence if Newton wasn't missing any long-distance gravity corrections. Neither won the Nobel prize for it, and can't now. Some people are just too far ahead of their time (or too caustic, or too female, the debates rage).

George
2019-Jan-03, 03:45 PM
The problem is, no one is going to undertake it, because they likely couldn't even get the results published if their conclusion was merely that "background cosmological sources are not moving at transverse relativistic speeds." It's because we always have to decide what we are going to test, and what we are going to take as given because it seems absurd otherwise. It's not that we're never wrong when we do this (the distance to the stars, in ancient times, comes to mind), it's that we generally don't think it's worth spending time on tests that would require so many of the things we hold to be true to be wrong that it just doesn't seem worth spending time on. Ah, "too much squeeze for the juice" eh? I love that saying. :)

Ken G
2019-Jan-03, 04:00 PM
Yes, I'd say your expression hits the nail on the head! (A more common but less vivid expression of it own.)

George
2019-Jan-03, 04:40 PM
By the way, for those who don't know that history, Fritz Zwicky discovered dark matter but couldn't get anyone to believe him, and Vera Rubin found a way to show clearly its existence if Newton wasn't missing any long-distance gravity corrections. Neither won the Nobel prize for it, and can't now. Some people are just too far ahead of their time (or too caustic, or too female, the debates rage). Is there a thread on this somewhere? It is a another fun history story. She, with W. Kent Ford, did their work in the 60s and published in 1970, and Fritz was still alive. Given two separate lines of evidence for DM -- Fritz was with fast motions within galactic clusters -- one would think it would merit strong consideration for a Nobel, perhaps that happened?


But I suppose "discovery" wasn't a fair word for it given it was only for one galaxy. Many more galaxy rotational curves came along throughout the 70s establishing the likelihood for DM, but Fritz had passed by then.

Also, V.R., in 1980, predicted DM would be discovered within 10 years. [I assume direct evidence was implied.] 10 years later, Martin Rees predicted DM would be discovered within 10 more years. 11 years thereafter, Martin Rees predicted it would be within another 10 years, repeating this same claim after 5 years to an audience that VR attended and she stood up upon hearing it an said, "I know of earlier predictions." :) [from Sc. Am. (https://blogs.scientificamerican.com/guest-blog/vera-rubin-didnt-discover-dark-matter/) .]

One other incentive to dive into her work (another thread) is the fact that her rotational curves were very similar to a Schmidt mass model. So what's with that?

Roger E. Moore
2019-Jan-03, 04:56 PM
Was not familiar with this bit on the history of dark matter.

http://cdsads.u-strasbg.fr/abs/2016AAS...22711406T

Lost in the Dark: A proto-history of dark matter

Trimble, Virginia L.; History 1
American Astronomical Society, AAS Meeting #227, id.114.06
Publication Date: 01/2016

The Greeks were probably not the first to think of everything, but they were quite often the first to write about it. Thus the first dark matter candidate was the counter-earth of Philolaus (c. 460 BCE), with its illuminated face forever turned away from us. The eclipsing binary interpretation of Algol brought forward the idea (Pigott & Goodricke 1780s) of stars not yet lit up, while the incorporation of thermodynamics into the astronomical tool kit suggested dark, dead stars. Jeans reported a number for these about three times the number of illuminated stars in 1922, the same year that Kapteyn set a comparable limit to what he called dark matter. The phrase appears as an index item in Russell et al.'s 1927 Astronomy</u> and cannot, therefore, have been invented any later. The first extragalactic investigation seems to have been that by Knut Lundmark, writing in German in the Meddelande of the Lund Observatory in 1930. One of the columns of his Tabelle 4 is headed: (Leuchtende + dunkle Materia)/(Leuchtende Materie) and lists values from six up to 100 for six galaxies, e.g. Messier 51 (10), Andromedanebel (20), and NGC 4594 (30). Binary galaxies came from Holmberg (1937), Virgo from Sinclair Smith (1936), and flat rotation curves from Babcock (1939, Andromeda) and Oort (1940, NGC 3115), the latter writing cautiously that the distribution of mass seemed to be very different from that of the light. Then there was a war, but by the time of a 1961 symposium in Santa Barbara focused on the large velocity dispersions in clusters of galaxies, the votes for dark matter slightly outnumbered those for unbound clusters and other alternatives. The idea of a constant of gravity increasing with distance came a smidge later from Arigo Finzi in 1963. The tipping point was arguably 1974 with a pair of short papers summarizing M/L ratios vs. distance scale (which could, of course, have been plotted before WWII). I mention only the slightly earlier and much less often cited one by Einasto, Kaasik, and Saar (published in Nature</u>, in case you are thinking of more Meddelande). I feel enormous respect and affection for Vera Rubin and Fritz Zwicky, but the published papers as are they are.

Ken G
2019-Jan-03, 05:01 PM
Interesting, it was clearly quite a community effort! And she doesn't even mention ignored Russian papers-- there's almost always a few of those in any astronomical context....

George
2019-Jan-03, 05:27 PM
Was not familiar with this bit on the history of dark matter.

http://cdsads.u-strasbg.fr/abs/2016AAS...22711406T

Lost in the Dark: A proto-history of dark matter

Trimble, Virginia L.; History 1
American Astronomical Society, AAS Meeting #227, id.114.06
Publication Date: 01/2016

The Greeks were probably not the first to think of everything, but they were quite often the first to write about it. Thus the first dark matter candidate was the counter-earth of Philolaus (c. 460 BCE), with its illuminated face forever turned away from us. The eclipsing binary interpretation of Algol brought forward the idea (Pigott & Goodricke 1780s) of stars not yet lit up, while the incorporation of thermodynamics into the astronomical tool kit suggested dark, dead stars. Jeans reported a number for these about three times the number of illuminated stars in 1922, the same year that Kapteyn set a comparable limit to what he called dark matter. The phrase appears as an index item in Russell et al.'s 1927 Astronomy</u> and cannot, therefore, have been invented any later. The first extragalactic investigation seems to have been that by Knut Lundmark, writing in German in the Meddelande of the Lund Observatory in 1930. One of the columns of his Tabelle 4 is headed: (Leuchtende + dunkle Materia)/(Leuchtende Materie) and lists values from six up to 100 for six galaxies, e.g. Messier 51 (10), Andromedanebel (20), and NGC 4594 (30). Binary galaxies came from Holmberg (1937), Virgo from Sinclair Smith (1936), and flat rotation curves from Babcock (1939, Andromeda) and Oort (1940, NGC 3115), the latter writing cautiously that the distribution of mass seemed to be very different from that of the light. Then there was a war, but by the time of a 1961 symposium in Santa Barbara focused on the large velocity dispersions in clusters of galaxies, the votes for dark matter slightly outnumbered those for unbound clusters and other alternatives. The idea of a constant of gravity increasing with distance came a smidge later from Arigo Finzi in 1963. The tipping point was arguably 1974 with a pair of short papers summarizing M/L ratios vs. distance scale (which could, of course, have been plotted before WWII). I mention only the slightly earlier and much less often cited one by Einasto, Kaasik, and Saar (published in Nature</u>, in case you are thinking of more Meddelande). I feel enormous respect and affection for Vera Rubin and Fritz Zwicky, but the published papers as are they are.

Interesting and would serve as great fodder for a new DM thread. [hint]

Copernicus
2019-Jan-03, 06:09 PM
The required speeds seem unlikely. Take a redshift of 0.1 (just over a billion light years). That requires a transverse speed of 0.5c.

I haven't done the maths but I'd question whether the required speeds are compatible with observations of Icarus - the star (z 1.5, v/c of 0.91) was lensed by a galaxy cluster(z 0.5, v/c of 0.74) and there was even a microlensing event within this.

Move up to GN-z11 and you are looking at 0.997c.

I know that this thing about measuring red shift in combination with the cosmic distance ladders is very complicated. I was suggesting that if the whole universe was rotating on multiple axes the velocity of galaxies would be proportional to the distance from the center of the universe. I also know that the galaxies are either brighter or dimmer than expected and wider or narrower than expected so adjustments are made because in a radial velocity assumption the galaxies are actually farther away than apparent. I don't really know which way these measurements are adjusted, but I assume that if the universe is actually static, as regard to expansion, and the measurement of cosmic distance ladders would have to be changed based off of the model. This is what I am asking about. I was not suggesting that there would be galaxies rotating around each other at near relativistic velocities.

Ken G
2019-Jan-03, 09:07 PM
Maybe there's a way to keep their relativistic speeds separate from each other so it's always smoothly varying from place to place, but what does that? It would be going back to ancient times where there didn't need to be any reason for the way things behave. That's why I mentioned "just is" models of science-- it doesn't really get you anywhere to assert that the transverse velocities just are, because you can fit most anything by saying that it just is that way. The goal is to establish some kind of unification of myriad phenomena. With the cosmological principle, a single function (the scale factor a(t) ) buys you the CMB, the H/He composition, and the Hubble law, all in one fell swoop. It may turn out to be wrong, but it's good science. But I grant you, it's always good to keep an open mind to questions like, could the redshifts be transverse-- as per Vera Rubin's remark.

Copernicus
2019-Jan-06, 03:16 AM
Lets say, in a sense, space and a light source are moving at nine tenths the speed of light to the left, x direction. If light is emitted perpendicular to this movement, z direction, would the light have a both an x and z component that adds up to the speed of light?

Ken G
2019-Jan-06, 04:00 PM
Lets say, in a sense, space and a light source are moving at nine tenths the speed of light to the left, x direction. If light is emitted perpendicular to this movement, z direction, would the light have a both an x and z component that adds up to the speed of light?It depends on the direction the light is emitted in, so if you state the light is emitted perpendicular to the x direction, then it's moving in the z direction at the speed of light. This follows because it is always moving at c, and you stipulated it is moving in the z direction. You always have to stipulate the direction somehow, and remember that the source doesn't think it's moving at all. Suffice it to say that you have to do Lorentz transformations to transform a direction from one frame to another.

Copernicus
2019-Jan-06, 07:46 PM
It depends on the direction the light is emitted in, so if you state the light is emitted perpendicular to the x direction, then it's moving in the z direction at the speed of light. This follows because it is always moving at c, and you stipulated it is moving in the z direction. You always have to stipulate the direction somehow, and remember that the source doesn't think it's moving at all. Suffice it to say that you have to do Lorentz transformations to transform a direction from one frame to another.
I was just wondering if space it self was moving and the light source was moving with space in the x direction would the light continue to have the same velocity in the x direction as the light source. 0.9 c in the x direction for a total of c.
Further, if the velocity of space was slowing down in the x direction as the light continued to move in the z direction how would that affect the wavelength of the light?

Ken G
2019-Jan-07, 01:09 AM
I was just wondering if space it self was moving and the light source was moving with space in the x direction would the light continue to have the same velocity in the x direction as the light source. 0.9 c in the x direction for a total of c. Oh, I see what you mean, but if space itself is what is moving sideways, then you don't have transverse Doppler shifts. So what we were initially wondering is whether it could be a transverse Doppler shift instead of a cosmological redshift due to expansion.

Further, if the velocity of space was slowing down in the x direction as the light continued to move in the z direction how would that affect the wavelength of the light?Cosmological redshifts don't care about motion, they care about stretching or contracting. If it was all purely transverse and purely due to "what space is doing", I don't think you'd get any frequency change. But beware of any guesses that haven't actually solved Einstein's equations, and purely transverse motions of space might not even be possible, so one should probably stick to transverse Doppler effect.

Copernicus
2019-Jan-07, 01:58 AM
Oh, I see what you mean, but if space itself is what is moving sideways, then you don't have transverse Doppler shifts. So what we were initially wondering is whether it could be a transverse Doppler shift instead of a cosmological redshift due to expansion.Cosmological redshifts don't care about motion, they care about stretching or contracting. If it was all purely transverse and purely due to "what space is doing", I don't think you'd get any frequency change. But beware of any guesses that haven't actually solved Einstein's equations, and purely transverse motions of space might not even be possible, so one should probably stick to transverse Doppler effect.

I guess we will just have to wait until someone can measure the transverse motion at some distance greater than 50 million light years.

Copernicus
2019-Jan-07, 11:03 AM
Are there any curves that show the expansion rate vs time from present, or the Hubble constant vs time from present?

Jean Tate
2019-Jan-07, 07:52 PM
Late to this thread ...

The relative positions of AGNs can be measured with considerable accuracy, as ngc3314 points out. One problem with interpreting any apparent changes in them: the extent to which newly launched jets, or jet "blobs", gives a spurious "motion".

There is the potential to measure some relative motions at very great distances, z>0.2 say: strong gravitational lensing (sgl). In the MW, we can detect MACHOs via sgl, and even some planets; in principle the same applies for very distant sgls. In practice, it's likely nigh on impossible to find and then study any suitable candidates (maybe supernovae?), let alone unambiguously analyze any data.

Copernicus: I think some of your (broader) questions can be easily answered, from existing data, but I think you may need to make them a bit more specific. Ned Wright's CosmoCalc, and associated tutorial, may help you with this.

Copernicus
2019-Jan-07, 11:53 PM
Late to this thread ...

The relative positions of AGNs can be measured with considerable accuracy, as ngc3314 points out. One problem with interpreting any apparent changes in them: the extent to which newly launched jets, or jet "blobs", gives a spurious "motion".

There is the potential to measure some relative motions at very great distances, z>0.2 say: strong gravitational lensing (sgl). In the MW, we can detect MACHOs via sgl, and even some planets; in principle the same applies for very distant sgls. In practice, it's likely nigh on impossible to find and then study any suitable candidates (maybe supernovae?), let alone unambiguously analyze any data.

Copernicus: I think some of your (broader) questions can be easily answered, from existing data, but I think you may need to make them a bit more specific. Ned Wright's CosmoCalc, and associated tutorial, may help you with this.

Thanks Jean Tate

Copernicus
2019-Jan-07, 11:55 PM
Are there any graphs for galaxy collisions vs z value?

Copernicus
2019-Jan-08, 01:23 AM
Are there any graphs for galaxy collisions vs z value?

Or quasars vs z values.