neophyte question about hubble's law

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Mesga dfarr --at-- comcast --dot-- net neophyte question about hubble's law donderdag 17 september 2009 4:32
Mesgb Oh No Re: neophyte question about hubble's law vrijdag 18 september 2009 4:31
Mesgc Phillip Helbig Re: neophyte question about hubble's law dinsdag 22 september 2009 4:43
Mesgd Phillip Helbig Re: neophyte question about hubble's law dinsdag 22 september 2009 4:44
Mesge Nicolaas Vroom Re: neophyte question about hubble's law woensdag 23 september 2009 3:36
Mesgf Phillip Helbig Re: neophyte question about hubble's law donderdag 24 september 2009 3:49
Mesgg Nicolaas Vroom Re: neophyte question about hubble's law donderdag 24 september 2009 3:57
Mesgh Phillip Helbig Re: neophyte question about hubble's law vrijdag 25 september 2009 1:57
Mesgi Jonathan Thornburg Re: neophyte question about hubble's law vrijdag 25 september 2009 2:04
Mesgj Oh No Re: neophyte question about hubble's law zondag 27 september 2009 11:30
Mesgk Nicolaas Vroom Re: neophyte question about hubble's law maandag 28 september 2009 8:52
Mesgl Hans Aberg Re: neophyte question about hubble's law maandag 28 september 2009 13:13
Mesgm Jonathan Thornburg Re: neophyte question about hubble's law dinsdag 29 september 2009 11:36
Mesgn Nicolaas Vroom Re: neophyte question about hubble's law woensdag 7 oktober 2009 9:43
Mesgo Nicolaas Vroom Re: neophyte question about hubble's law woensdag 7 oktober 2009 11:48
Mesgp Phillip Helbig Re: neophyte question about hubble's law maandag 12 oktober 2009 17:14
Mesgq Phillip Helbig Re: neophyte question about hubble's law maandag 12 oktober 2009 17:15
Mesgr Phillip Helbig Re: neophyte question about hubble's law maandag 12 oktober 2009 17:16
Mesgs Nicolaas Vroom Re: neophyte question about hubble's law vrijdag 16 oktober 2009 18:03
Mesgt Phillip Helbig Re: neophyte question about hubble's law maandag 19 oktober 2009 11:03
Mesgu Thomas Smid Re: neophyte question about hubble's law dinsdag 20 oktober 2009 9:59
Mesgv Phillip Helbig Re: neophyte question about hubble's law dinsdag 20 oktober 2009 14:53
Mesgw Hans Aberg Re: neophyte question about hubble's law woensdag 21 oktober 2009 11:21
Mesgx Nicolaas Vroom Re: neophyte question about hubble's law woensdag 21 oktober 2009 15:38
Mesgy Thomas Smid Re: neophyte question about hubble's law donderdag 22 oktober 2009 13:00
Mesgz Stupendous_Man Re: neophyte question about hubble's law donderdag 22 oktober 2009 19:34
Mesga1 Phillip Helbig Re: neophyte question about hubble's law donderdag 22 oktober 2009 21:08
Mesga2 Phillip Helbig Re: neophyte question about hubble's law donderdag 22 oktober 2009 21:09
Mesga3 Phillip Helbig Re: neophyte question about hubble's law donderdag 22 oktober 2009 21:11
Mesga4 Phillip Helbig Re: neophyte question about hubble's law donderdag 22 oktober 2009 21:14
Mesga5 Nicolaas Vroom Re: neophyte question about hubble's law vrijdag 23 oktober 2009 10:03
Mesga6 Phillip Helbig Re: neophyte question about hubble's law vrijdag 23 oktober 2009 13:05
Mesga7 Nicolaas Vroom Re: neophyte question about hubble's law zaterdag 24 oktober 2009 20:35
Mesga8 Phillip Helbig Re: neophyte question about hubble's law zondag 25 oktober 2009 19:35
Mesgb1 Nicolaas Vroom Re: neophyte question about hubble's law donderdag 3 december 2009 16:48
Mesgb2 Phillip Helbig Re: neophyte question about hubble's law zondag 6 december 2009 12:28
Mesgb3 Nicolaas Vroom Re: neophyte question about hubble's law donderdag 10 december 2009 19:03
Mesgb4 Phillip Helbig Re: neophyte question about hubble's law vrijdag 11 december 2009 17:16
Mesgb5 Nicolaas Vroom Re: neophyte question about hubble's law dinsdag 15 december 2009 17:12
Mesgb6 Phillip Helbig Re: neophyte question about hubble's law woensdag 16 december 2009 9:23
Mesgb7 Nicolaas Vroom Re: neophyte question about hubble's law woensdag 16 december 2009 18:47
Mesgb8 Phillip Helbig Re: neophyte question about hubble's law donderdag 17 december 2009 8:45
Mesgb9 Nicolaas Vroom Re: neophyte question about hubble's law vrijdag 18 december 2009 11:00
Mesgba Phillip Helbig Re: neophyte question about hubble's law zaterdag 19 december 2009 10:52
Mesgbb Nicolaas Vroom Re: neophyte question about hubble's law zondag 27 december 2009 21:45
Mesgbc Phillip Helbig Re: neophyte question about hubble's law dinsdag 29 december 2009 20:54

For a critical evaluation of Hubble's Law study this: Comments about Hubble's law - part I
For a slightly more personal approach read this: Comments about Hubble's law - part II


Mesga neophyte question about hubble's law

Van: dfarr --at-- comcast --dot-- net
Onderwerp: neophyte question about hubble's law
Datum: donderdag 17 september 2009 4:32

The 'Hubble's law' Wikipedia article states '...that the velocity at which various galaxies ARE receding from the Earth IS proportional to their distance from us.' (emphasis added)
My question is about the tense of the two verbs in all caps above. Aside from assuming things are orderly, do we have any way of inferring that a galaxy that was moving away from us 12 billion years ago is still doing so? The light from the galaxy which is reaching us now indicates it was moving away, but do we have any way of inferring that it has not slowed down or started to approach us, or disappeared off the 'edge'?
I'm not an astronomer or even a physicist, just an aging isolated mathematics amateur, so go easy on me if this is something all freshmen astronomy students know. Thanks.
[[Mod. note -- The following is quoted with only slight changes from a recent posting of mine in sci.physics.research, and seems relevant here too:
The Earth is roughly 149 million kilometers = 8.5 light-minutes away from the Sun. So, if we look outside during daylight hours, we have observational data that the Sun was shining 8.5 minutes ago. But we have *no* observational data about what the Sun is doing right *now*.

[For present purposes let's ignore the well-known difficulty of defining "right now" for a distant object (a.k.a. the "clock synchronization" problem) in the context of special relativity.]
If you want to ask "does physics say anything meaningful about what the Sun is doing right now?", then I would say that the answer is still "yes": If we combine our observations of what the Sun was doing prior to 8.5 minutes ago, with theoretical models of the Sun's structure,
[note that these *assume* that "things are orderly", i.e., that the laws of quantum mechanics, atomic & nuclear structure, thermodynamics, electromagnetism, and many other aspects of physics work "properly" in the Sun right now, even though there can be no causal contact between the Sun-right-now and any observation we have ever made, or will make any sooner than 8.5 minutes from now]
then we can infer with (*very*) high certainty that the Sun is still shining right now, with a total luminosity which is very close to what it was 8.5 minutes ago.
This same sort of reasoning is necessary in cosmology: we only directly observe things at places/times such that their light or other signals can get to us, so aside from assuming that "things are orderly", we don't know directly what a distant galaxy is doing *now*.
[We can observationally test some cases of whether "things are orderly", i.e. whether "physics works the same way everywhere":
For example, we can verify that the spectrum of hydrogen observed at high redshift looks just like that observed in Earth-bound laboratories except for an overall redshift. We can also observationally test these assumptions for (some) events which are *closer* to us than their light-distance. For example, we can measure isotope ratios of the Oklo uranium deposits http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor to check that nuclear reactions and energy levels were the same on Earth 2 billion years ago as they are today.

With the exception of some quite-controversial claims of very small variations in the fine-structure constant, so far all these tests have come out supporting the assumptions that things are indeed "orderly". This makes the extension of these assumptions to not-directly-observable things, e.g., the Sun and/or distant galaxies right now, at least plausible.]

For much more (very clear and insightful) about what Hubble's law does and doesn't say, see

Edward R. Harrison
"Cosmology: The Science of the Universe", 2nd Edition
Cambridge U.P., 2000,
hardcover ISBN 0-521-66148-X

As Phillip Helbig said later in the same sci.physics.research thread from which I quoted above, "EVERYONE interested in cosmology should read this book at least twice.". -- jt]]

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Mesgb neophyte question about hubble's law

Van: Oh No
Onderwerp: neophyte question about hubble's law
Datum: vrijdag 18 september 2009 4:31 In
posting Mesga dfarr --at-- comcast --dot-- net writes: > The 'Hubble's law' Wikipedia article states '...that the velocity at > which various galaxies ARE receding from the Earth IS proportional to > their distance from us.' (emphasis added) That is correct. This is the only possible velocity-distance law for a universe which is expanding homogeneously and isotropically. However, the distance is the proper distance and the velocity is the temporal derivative of the proper distance. Neither of these distances is a distance which is useful in observational cosmology (examples of the latter are luminosity distance and angular-size distance). > My question is about the tense of the two verbs in all caps above. > Aside from assuming things are orderly, > do we have any way of inferring that a galaxy that was moving away > from us 12 billion years ago is still doing so? The light from the > galaxy which is reaching us now indicates it was moving away, but do > we have any way of inferring that it has not slowed down or started to > approach us, or disappeared off the 'edge'? You are confused. Hubble's Law as stated above is correct, but describes unobservable quantities. If a galaxy which was moving away from us 12 billion years ago is now approaching us, then nearby galaxies would be approaching us as well. Another point: the only thing the light from the galaxy indicates is the ratio of the scale factor of the universe compared to the time when the light was emitted. It says nothing about distance, velocity etc. To convert the observed redshift into such quantities, we need to know the cosmological parameters.
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Mesgc neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: dinsdag 22 september 2009 4:43 In
posting Mesga dfarr --at-- comcast --dot-- net writes: > The 'Hubble's law' Wikipedia article states '...that the velocity at > which various galaxies ARE receding from the Earth IS proportional to > their distance from us.' (emphasis added) That is correct. This is the only possible velocity-distance law for a universe which is expanding homogeneously and isotropically. However, the distance is the proper distance and the velocity is the temporal derivative of the proper distance. Neither of these distances is a distance which is useful in observational cosmology (examples of the latter are luminosity distance and angular-size distance). > My question is about the tense of the two verbs in all caps above. > Aside from assuming things are orderly, > do we have any way of inferring that a galaxy that was moving away > from us 12 billion years ago is still doing so? The light from the > galaxy which is reaching us now indicates it was moving away, but do > we have any way of inferring that it has not slowed down or started to > approach us, or disappeared off the 'edge'? You are confused. Hubble's Law as stated above is correct, but describes unobservable quantities. If a galaxy which was moving away from us 12 billion years ago is now approaching us, then nearby galaxies would be approaching us as well. Another point: the only thing the light from the galaxy indicates is the ratio of the scale factor of the universe compared to the time when the light was emitted. It says nothing about distance, velocity etc. To convert the observed redshift into such quantities, we need to know the cosmological parameters.
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Mesgd neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: dinsdag 22 september 2009 4:44 In
posting Mesgb Oh No writes: > Thus spake dfarr --at-- comcast --dot-- net > >The 'Hubble's law' Wikipedia article states '...that the velocity at > >which various galaxies ARE receding from the Earth IS proportional to > >their distance from us.' (emphasis added) > > Bear in mind that this applies only for small cosmological distances That depends on how one defines Hubble's Law. See my other post in this thread and the recent thread in sci.physics.research. > Perhaps. It's just a pity Harrison's ideas about the expansion of space > time are somewhat inaccurate. Care to elabourate?
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Mesge neophyte question about hubble's law

Van: Nicolaas Vroom
Onderwerp: neophyte question about hubble's law
Datum: woensdag 23 september 2009 3:36 In
posting Mesga dfarr --at-- comcast --dot-- net writes: [[Mod. note -- 79 excessively-quoted lines snipped. -- jt]]

[[Mod. note -- > > For much more (very clear and insightful) about what Hubble's law does > and doesn't say, see
> Edward R. Harrison
> "Cosmology: The Science of the Universe", 2nd Edition
> Cambridge U.P., 2000, > hardcover ISBN 0-521-66148-X > > As Phillip Helbig said later in the same sci.physics.research thread > from which I quoted above, "EVERYONE interested in cosmology should > read this book at least twice.". > -- jt]]

I think this picture is too simple. We will all agree that the sun is shining right "now" based on current observations. And we will also all agree that all our planets will be there 100 years from now, because they were be there 100 years ago. On the other hand the Andromeda Galaxy M31 is moving towards us which is in disagreement with Hubble's Law. In fact this shows that Hubble's Law is only an approximation. [[Mod. note -- Yes, galaxies have random velocities about the large-scale Hubble flow, not to mention non-random gravitational motions due to the mass of superclusters. This is well-known to all cosmologists. Give or take a bit, Hubble's law refers to the overall *average* velocity (redshift) of galaxies at a given distance. For a more precise definition, see the book by Harrison, or his paper http://adsabs.harvard.edu/abs/1993ApJ...403...28H -- jt]] However there is more.
This document by Hilton Ratcliffe http://vixra.org/pdf/0907.0003v1.pdf also discusses the validity of Hubble's Law. The question to what extend his objections are true requires thoroughly investigation. [[Mod. note -- Alas, Ratcliffe's paper is very badly flawed. I'll comment further on it in a following posting. -- jt]]

Nicolaas Vroom
http://users.pandora.be/nicvroom/

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Mesgf neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: donderdag 24 september 2009 3:49 In
posting Mesge , "Nicolaas Vroom" writes: > On the other hand the Andromeda Galaxy M31 is moving towards us > which is in disagreement with Hubble's Law. > In fact this shows that Hubble's Law is only an approximation. Yes, it is an approximation. If the universe were exactly homogeneous and isotropic, it would hold exactly. (In that case, though, there would be no galaxies. We can imagine "test particles", though, which essentially just serve as markers for position.) In reality, galaxies have their own so-called peculiar motions, which are combined with the "Hubble flow". For nearby galaxies, the former dominate; for high-redshift galaxies, the latter dominates. In other words, the fact that the Andromeda galaxy is approaching us no more and no less contradicts Hubble's Law than the fact that a person approaches me in the street. (In the ideal case, Hubble's Law applies to every particle, whatever its distance. In practice, it applies only at distances large enough that other velocities are negligible.)
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Mesgg neophyte question about hubble's law

Van: Nicolaas Vroom
Onderwerp: neophyte question about hubble's law
Datum: donderdag 24 september 2009 3:57 "Phillip Helbig" schreef in
posting Mesge > However, > the distance is the proper distance and the velocity is the temporal > derivative of the proper distance. Neither of these distances is a > distance which is useful in observational cosmology (examples of the > latter are luminosity distance and angular-size distance). I do not understand this. As far as I know v = c * z and z is caculated via z = d labda/labda which both are measured by means of observations.
Why the distiction between proper distance and Luminosity distance ? None of the books I have studied (Hoyle, Silk, Kaufmann, and the book Galactic Astronomy Chapter 7) make this distinction.
The last book uses the concept: Luminosity function as a distance indicator 415-418.
Basically the distance is calculated bij using the formula: L = 4 * pi * d *d * f (f = flux, d = distance, L = luminosity).
Using those measured and or observed values H is calculated. Finally if only z is measured the distance d can be inferred. >> Aside from assuming things are orderly, >> do we have any way of inferring that a galaxy that was moving away >> from us 12 billion years ago is still doing so? > > You are confused. Hubble's Law as stated above is correct, but > describes unobservable quantities. I expect you mean an unobservable situation right now. > If a galaxy which was moving away > from us 12 billion years ago is now approaching us This seems highly unlikely. IMO 6 billion years ago that same galaxy was also moving away from us. or am I wrong. The question is did the speed increase or decrease between those two events. > , then nearby galaxies would be approaching us as well. > From a local point of view they can move in any direction. > Another point: the only thing the > light from the galaxy indicates is the ratio of the scale factor of the > universe compared to the time when the light was emitted. It says > nothing about distance, velocity etc. Is that true ? Again the books I have tell a different story. > To convert the observed redshift > into such quantities, we need to know the cosmological parameters. Is that not the Hubble constant ? Why not mentioned ?
What amazes me the most is if you look at galaxys at very large distances their shape seems to be much more develloped than you should expected solely based on their early age. Or are they much older ? In fact almost all galaxys look like M31 (What you should expect is much more small elliptical than large spirals) Nicolaas Vroom [[Mod. note -- The distinction between proper and luminosity distances is because logically they're different quantities, so it's clearer to use different names for them. It is not the case that "amost all galaxies look like M31", either for nearby galaxies or for very distant galaxies. -- jt]]
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Mesgh neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: vrijdag 25 september 2009 1:57 In
posting Mesgg , "Nicolaas Vroom" writes: > "Phillip Helbig" > schreef in posting Mesgf > > > However, > > the distance is the proper distance and the velocity is the temporal > > derivative of the proper distance. Neither of these distances is a > > distance which is useful in observational cosmology (examples of the > > latter are luminosity distance and angular-size distance). > > I do not understand this. > As far as I know v = c * z and z is caculated via z = d labda/labda > which both are measured by means of observations. What is "both"? Only the wavelength of a distant object is observed, and compared to the wavelength in the laboratory. Everything else is inferred. (I'm assuming we all agree on what c is.)
There is a velocity-distance law and there is a redshift-distance law. But only at low redshift can one combine them to get a straightforward relationship between velocity and redshift. What is v? Velocity. That is distance per time. Which distance (in cosmology there are several, which at high redshift are different, because a) the universe can be non-Euclidean and b) non-static)? Which time? We can assume cosmic time, that measured by someone at rest relative to the CMB. But there is no distance which is otherwise used in cosmology (luminosity distance, angular-size distance) whose derivative with respect to cosmic time (or any other time, except perhaps one specially defined so that the desired result is achieved) result in a velocity related to the redshift by the equation above. (And no, at high redshifts it doesn't help to use the relativistic Doppler formula. Since it contains no cosmological parameters, that would imply that the velocity---whatever it is---of an object at high redshift is independent of the cosmological model.)
(It IS possible to view cosmological redshifts as Doppler redshifts, but neither the familiar formula nor familiar distances are involved, so this seems more trouble than it is worth.) > Why the distiction between proper distance and Luminosity distance ? > None of the books I have studied (Hoyle, Silk, Kaufmann, > and the book Galactic Astronomy Chapter 7) make this distinction. At low redshift, no distinction is necessary. The luminosity distance is (1+z)^2 times as large as the angular-size distance. > The last book uses the concept: > Luminosity function as a distance indicator 415-418. > Basically the distance is calculated bij using the formula: > L = 4 * pi * d *d * f (f = flux, d = distance, L = luminosity). Right. This defines the luminosity distance. But it is not the same as the distance one would measure with a rigid ruler, neither now nor at the time when the light was emitted. Nor is it the same as distance derived from angular size (objects farther away look smaller) nor the distance derived from parallax nor the distance from the light-travel time. To convert one type of distance to the other, one needs to know at least the redshift (for some distances) and perhaps the cosmological parameters (for other distances). > Using those measured and or observed values H is calculated. Yes, but the redshifts at which H is calculated are so small that the distances all agree. > Finally if only z is measured the distance d can be inferred. Assuming one knows H, and if the redshift is small. > >> Aside from assuming things are orderly, > >> do we have any way of inferring that a galaxy that was moving away > >> from us 12 billion years ago is still doing so? > > > > You are confused. Hubble's Law as stated above is correct, but > > describes unobservable quantities. > I expect you mean an unobservable situation right now. While your statement is true, I meant unobservable distances. The distances involved can be calculated from others, if one knows the cosmological parameters. > > If a galaxy which was moving away > > from us 12 billion years ago is now approaching us > This seems highly unlikely. > IMO 6 billion years ago that same galaxy was also moving away from us. > or am I wrong. > The question is did the speed increase or decrease between those two events. Depends on the cosmological parameters. > > Another point: the only thing the > > light from the galaxy indicates is the ratio of the scale factor of the > > universe compared to the time when the light was emitted. It says > > nothing about distance, velocity etc. > Is that true ? Again the books I have tell a different story. Yes, it is true. All else can be inferred, IF one knows the cosmological parameters. Or one can calculate other quantities for different sets of cosmological parameters and compare them to observations. This is in practice how the cosmological parameters are measured. > > To convert the observed redshift > > into such quantities, we need to know the cosmological parameters. > Is that not the Hubble constant ? Why not mentioned ? That's one of them, but there is also Omega (the density parameter) and lambda (the cosmological constant). Also, the clumpiness of matter between ourselves and a distant object can affect some measures of distance. > It is not the case that "amost all galaxies look like M31", either for > nearby galaxies or for very distant galaxies. > -- jt]] Once Richard Ellis was showing some strangely shaped galaxies observed with HST. He remarked that were Gerard de Vaucouleurs in the audience, he could name some similarly looking nearby galaxies.
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Mesgi neophyte question about hubble's law

Van: Jonathan Thornburg
Onderwerp: neophyte question about hubble's law
Datum: vrijdag 25 september 2009 2:04 Nicolaas Vroom wrote in
posting Mesge: > This document by Hilton Ratcliffe > http://vixra.org/pdf/0907.0003v1.pdf > also discusses the validity of Hubble's Law. Unfortunately, this paper has major flaws, and should not be relied on to convey what is and isn't known about any given research field.
Here are a few flaws in Ratcliffe's paper which I noticed in a brief perusal:
Ratcliffe (section 5) discusses (favorably) Tifft's work on galaxy redshift periodicities, and argues that these are a significant challenge to standard cosmological models.
[For those who haven't seen it, Tifft claimed that if one looks at binary galaxies, and for each pair tabulates the *difference* in redshift of the two members of the pair, the resulting distribution is strongly periodic with a period of around cz = 72 km/sec. If this were true, it would indeed be a huge challenge to standard cosmological models.]
But Ratcliffe makes no mention of the refutation of this work by Newman, Haynes, and Terzian http://adsabs.harvard.edu/abs/1989ApJ...344..111N who showed that Tifft's statistical analysis was horribly flawed: it would find "periodicities" even in Gaussian random noise! Ratcliffe also makes no mention of the later work by Chengalur, Salpeter, and Terzian http://adsabs.harvard.edu/abs/1993ApJ...419...30C or Tang & Zhang's study of quasar-galaxy--pair redshift differences http://adsabs.harvard.edu/abs/2005ApJ...633...41T In section 2, Ratcliffe writes
| Thus, we may assume that there is something anomalous about the redshift of an astrophysical object if:
1.1. There is a prevalence of high redshift objects near the nucleus of nearby galaxies, or high redshift galaxy-like systems associated with low redshift clusters;
The key phrase there is "a prevalence of high redshift objects". This (of course) only considers *known* high-redshift objects. The question is, are known high-redshift objects a random sample of all high-redshift objects? Of course, the answer is "no": known objects comprise only those which are (among other criteria) * which are in a part of the sky which has been observed, and * bright enough to have been observed Thus you can easily create a spurious apparent prevalence of [known] high redshift objects in some part of the sky, simply by observing that part of of the sky a lot. And the sky around nearby galaxies and low redshift clusters does get observed a lot, probably more than less "interesting" parts of the sky. The only way to figure out whether there is a true prevalence of high redshift objects on a certain part of the sky, is to do a careful statistical analysis of the selection criteria of whatever catalogs you're using. Ratcliffe does not discuss this issue. Indeed, the word "selection" or the phrase "selection bias" doesn't seem to appear anywhere in his paper! In section 3.2, Ratcliffe writes:
| If one plots quasars' redshift against apparent brightness, as Hubble did for galaxies, one gets a wide scatter, as compared with a smooth curve for the same plot done for galaxies. This seems to indicate that quasars do not follow the Hubble law, and there is no direct indication that they are at their proposed redshift distance.
There are several obvious flaws with this argument:
* First, there seems to be a misunderstanding of just what Hubble's law is. See http://adsabs.harvard.edu/abs/1993ApJ...403...28H for a very clear account, including refutation of some common misconceptions. For present purposes, the key point is that Hubble's law (at least as the term is usually used in cosmology) connectes some measure of distance to either redshift or recessional velocity. It does *not* say that the brightness of galaxies, quasars, or any other objects has any necessary relation to their redshift!
* Second, the author seems to think that if one plots galaxies' redshift against apparent brightness, one gets a tight correlation. This is only true if one pre-selects the galaxies to be relatively homogeneous in intrinsic brightness. ["intrinsic brightness" = brightness as measured at some fixed distance away from the object = often just called "luminosity"] Without such a pre-selection, galaxies vary by (plural) orders of magnitude in intrinsic brightness.
* Third, the author makes no mention of the obvious alternative hypothesis: quasars' intrinsic brightnesses vary over a wide range (even wideer than those of galaxies). Later in section 3.2, Ratcliffe writes:
| Even more onerous was the precision measurement of radial expansion rate [[of quasars]] by very long baseline radio interferometry. Quasars appeared to be expanding at up to ten times the speed of light, with obviously serious implications for underlying theory and Einsteinian physics.
However, Ratcliffe doesn't mention the well-known special-relativity optical illusion that can readily explain such apparent "superluminal" motions. For a nice brief explanation of how this works, see http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/Superluminal/superluminal.html This examples are unfortunately all too typical of Ratcliffe's paper: he points out apparent problems, without critiquing or even *mentioning* well-known alternative hypotheses or resolutions of the problems. This makes his paper a seriously unreliable source of information.
For a much more reliable brief introduction to some of the controversies (mis-)described by Ratcliffe, see Bill Keel's web page http://www.astr.ua.edu/keel/galaxies/arp.html (This is a few years old, but still good.) -- -- "Jonathan Thornburg" Dept of Astronomy, Indiana University, Bloomington, Indiana, USA "Washing one's hands of the conflict between the powerful and the powerless means to side with the powerful, not to be neutral."
-- quote by Freire / poster by Oxfam
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Van: Oh No
Onderwerp: neophyte question about hubble's law
Datum: zondag 27 september 2009 11:30 In
posting Mesgd "Phillip Helbig" wrote: >In posting Mesgb Oh No > writes: > >> Thus spake dfarr --at-- comcast --dot-- net >> >The 'Hubble's law' Wikipedia article states '...that the velocity at >> >which various galaxies ARE receding from the Earth IS proportional to >> >their distance from us.' (emphasis added) >> >> Bear in mind that this applies only for small cosmological distances > >That depends on how one defines Hubble's Law. See my other post in this >thread and the recent thread in sci.physics.research. Hubble's law is defined as a linear relationship between redshift and distance. This relationship only holds for small cosmological distances. For large cosmological distances such a relationship doesn't even make sense unless you first define what you mean by distance, and there certainly isn't a natural definition of distance which would give a linear relationship. You would only get a linear relationship if you defined distance from Hubble's law - which is tautology, and certainly an unhelpful measure of large scale structure. > >> Perhaps. It's just a pity Harrison's ideas about the expansion of space >> time are somewhat inaccurate. > >Care to elabourate? I have only dipped into the book, so cannot comment on much of it, but the discussion of the Hubble sphere on p281 struck me as particularly misleading. It is absolutely not meaningful to talk about the recession velocity of a star in the early universe with respect to ourselves now. Think of the balloon analogy. Cosmological redshift is the consequence of cosmological expansion, not recession velocities. It just happens that, for small cosmological distances, cosmological expansion looks like recession velocity. You can't take that concept too far. Regards -- Charles Francis
moderator sci.physics.foundations.
< charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and braces)> http://www.rqgravity.net
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Mesgk neophyte question about hubble's law

Van: Nicolaas Vroom
Onderwerp: neophyte question about hubble's law
Datum: maandag 28 september 2009 8:52 "Phillip Helbig" wrote in
posting Mesgh : > In posting Mesgg "Nicolaas Vroom" > writes: > >> I do not understand this. >> As far as I know v = c * z and z is caculated via z = d labda/labda >> which both are measured by means of observations. > > What is "both"? Only the wavelength of a distant object is observed, > and compared to the wavelength in the laboratory. Everything else is > inferred. (I'm assuming we all agree on what c is.) The main problem with c is it also affected by an expanding universe ? The question is is the distance of one lightyear going in the expanding direction equal as going in the opposite direction ?
Is forward equal as backward ?
Is this distance the same as using a rigid ruler ? >> The last book uses the concept: >> Luminosity function as a distance indicator 415-418. >> Basically the distance is calculated bij using the formula: >> L =4 * pi * d *d * f (f = flux, d = distance, L = luminosity). > > Right. This defines the luminosity distance. But it is not the same as > the distance one would measure with a rigid ruler, neither now nor at > the time when the light was emitted. Nor is it the same as distance > derived from angular size (objects farther away look smaller) nor the > distance derived from parallax nor the distance from the light-travel > time. To convert one type of distance to the other, one needs to know > at least the redshift (for some distances) and perhaps the cosmological > parameters (for other distances). > >> Using those measured and or observed values H is calculated. > > Yes, but the redshifts at which H is calculated are so small that the > distances all agree. > >> IMO 6 billion years ago that same galaxy was also moving away from us. >> or am I wrong. >> The question is did the speed increase or decrease between those two >> events. > > Depends on the cosmological parameters. Anyway what is the answer ? >> > Another point: the only thing the >> > light from the galaxy indicates is the ratio of the scale factor of the >> > universe compared to the time when the light was emitted. It says >> > nothing about distance, velocity etc. >> Is that true ? Again the books I have tell a different story. > > Yes, it is true. All else can be inferred, IF one knows the > cosmological parameters. Or one can calculate other quantities for > different sets of cosmological parameters and compare them to > observations. This is in practice how the cosmological parameters are > measured. > The more I read the less I understand. When you observe the distance to the Sun what you measure is not the true/present position/distance but the historic/past position/distance roughly 5 minutes in the past. Based from that position using for example Newton's Law you can calculate the true or present position. The method used is for example parallax. (trigonometric distances) You can do that also for stars. You measure a distance in the past and when you know the speed (direction) you can calculate the true/present position.
A different way to measure the distance is by measuring the flux but than You use the assumption that stars of the same type all have the same Luminosity. This introduces an error but again what you measure is the past distance. IMO for the same star the distance should be identical. The same can be done by using redshifts and the following three equations: V = H*d d=c*z and z = d labda/labda. The problem is those 3 equations can not be used around M31. The H constant calculated is not correct. In order to calculate H by two independent measurements of V(speed) and d(distance) much further galaxies should be used. How is this done in practice ? I expect this is very difficult specific for the speed. (You need two distances) Any way what you calculate is the past distance. You need the present distance. How is this done. ? What is v going from past to present ? Is v a constant ? There is an aditional problem. What you observe is the present value of d labda. The question is assuming expanding space that that value is not identical as when it was emitted. ie it was smaller. This inturn means that both z and d are much smaller or is this wrong. Nicolaas Vroom
http://users.telenet.be/nicvroom/
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Mesgl neophyte question about hubble's law

Van: Hans Aberg
Onderwerp: neophyte question about hubble's law
Datum: maandag 28 september 2009 13:13 Nicolaas Vroom wrote in
posting Mesgk: > The main problem with c is it also affected by an expanding universe ? The idea is that the universe itself expands without changing c. So the observable universe is at least 47 Gyr in radius, but the expansion theory claims that the distant objects did not arrive there by actual traveling on their own, but surfing on the universe expansion, thus explaining away why one does not see the Big Bang by looking 14 Gyr out. (Also see the link below.) The idea comes from GR, but I recall that in original GR claiming that an object moving by its own or by universe expansion are equivalent. So the expansion theory would depend on some alteration of GR that destroys that equivalence - but I could not find any reference for that. So it would be nice if the experts here would clarify. Hans http://en.wikipedia.org/wiki/Universe
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Mesgm neophyte question about hubble's law

Van: Jonathan Thornburg
Onderwerp: neophyte question about hubble's law
Datum: dinsdag 29 september 2009 11:36 Nicolaas Vroom wrote in
posting Mesgk: > When you observe the distance to the Sun what you measure > is not the true/present position/distance but the historic/past > position/distance roughly 5 minutes in the past. > Based from that position using for example Newton's Law > you can calculate the true or present position. > The method used is for example parallax. > (trigonometric distances) > You can do that also for stars. You measure a distance > in the past and when you know the speed (direction) > you can calculate the true/present position. > A different way to measure the distance is by measuring > the flux but than You use the assumption that stars of the > same type all have the same Luminosity. > This introduces an error but again what you measure is > the past distance. > IMO for the same star the distance should be identical. I'm afraid you're mistaken: while they're very close for stars in our galaxy, they would only be *identical* if we lived in a flat spacetime (one where Newton's laws hold for slowly-moving objects).... but we don't live in a flat spacetime.
To be specific, let's consider the simple case of measuring the mean Earth-Sun distance. Suppose you measure this distance in the following ways:
(a) Measure this distance by parallax, i.e., by observing how much the Sun's angular position on the sky shifts when you move (say) 100 kilometers (measured by lining up meter sticks end-to-end, sighting along them to make sure they're in a straight line) sideways on the Earth.
(b) Same as (a), but with a larger baseline, say the Earth-Moon distance (i.e., observe how much the Sun's angular position on the sky shifts when you move from the Earth to the Moon at a time when the Sun-Earth-Moon angle is roughly 90 degrees)
(c) Measure the distance by radar, i.e., send a radar signal from the Earth to the Sun, let some of it bounce off the Sun, and time how long it takes the echo to get back to the Earth.
(d) Line up meter sticks (sighting along them to make they're all in a straight line) from the Earth to the Sun, and count how many meter sticks it takes to span the distance.
(e) Construct a circle centered on the Sun of radius matching the mean Earth-Sun distance, and measure the circumference of this circle by lining up meter sticks around the circle and counting how many meter sticks it takes to go all the way around. Then compute a radius by dividing that circumference by 2 pi.
[Of course, these are all gedanken measurements; here we're treating each of the Earth and Sun as point masses, and neglecting assorted other complications which aren't relevant to the point at hand. For actual measurements, what we really care about is the mean distance between the Earth-center-of-mass and the Sun-center-of-mass.]
Contrary to what you might expect, in general these 5 measurements will give slightly *different* results, typically differing by a few kilometers. This is because in a curved spacetime, each of these measures something fundamentally *different* (they use different trajectories in spacetime). For close-enough objects these measurements all agree, but the Earth and Sun aren't "close enough" if you care about a level of accuracy of a few kilometers.
[In case you're wondering, that "a few kilometers" is really some O(1) constant (in general different for each measurement method) times the spacetime-curvature scale near the Sun.] [Switching from gedanken to real experiments for a moment, modern experimental measurements of the mean Earth-Sun center-of-mass distance (the "astronomical unit") via radar-like techniques are accurate to a few *meters*! Of course, at that level of accuracy the analysis needs to be done very carefully, including using a careful curved-spacetime definition of just what it is that's being measured, and an analysis of the observational data that takes curved-spacetime effects into account.]
If we now move to the original poster's example of stars within our galaxy, then it's certainly true that these curved-spacetime effects are (*much*) smaller than the other errors involved in astronomical distance measurements. So for this case, you might reasonably decide to ignore curved-spacetime effects. But as we'll see, the conceptual distinction between different notions of "distance" is still important to keep in mind as preparation for the cosmological case. In general, all reasonable definitions of "distance" agree for objects which are much closer than the spacetime curvature scale.
[More precisely, if you Taylor-expand any one reasonable definition of "distance" in terms of any other one, you'll get something which looks like the identity function plus higher order terms in the distance, i.e., D = d + k_2 d^2 + k_3 d^3 + k_4 d^4 + k_5 d^5 + ... where d and D are the two distances, and the k_i are coefficients depending on how the distances are defined, and what the local spacetime looks like, but not on the numerical values of d or D. (I suspect, but haven't bothered to check right now, that in fact k_2 is always zero.) So, if d is small enough, we can neglect all the higher order terms in the Taylor series and approximate D = d.]
For cosmology, that scale is around 4000 megaparsecs, so (given the accuracy scales common in astrophysics) for objects closer than (say) a few hundred megaparsecs, it's generally reasonable to neglect spacetime curvature and say that "distance" is (within the approximations we're making) uniquely defined.
[That is, for objects within a few hundred megaparsecs, the percentage difference between d and D is generally much smaller than our experimental/observational uncertainties in measuring d, D, the object's redshift, or most other properties of the object. Of course, this "a few hundred megaparsecs" is really a somewhat fuzzy boundary, depending on just how large or small our experimental/observational uncertainties are.]
For such objects (distance < 200 megaparsecs or so, or equivalently redshift z < 0.05 or so, or equivalently cz < 15,000 km/sec or so), Hubble's law is observed to hold to a good approximation.
[The main deviations from it are the random motions of galaxies and smaller objects, and the large-scale gravitational-flow effects of large clusters of galaxies; both of these effects introduce scatter on the order of 1000 km/sec or less.]
Now let's move to cosmology, i.e., let's consider observing objects with redshifts of order unity or larger. Now it's *not* reasonable to neglect spacetime curvature: there's no simply no unique concept of "distance" for cosmologically-distant objects. Rather, there are a bunch of different "distances" (e.g., luminosity distance, angular-diameter or parallax distance, the distance you'd measure if you lined up meter sticks in a straight line from us to the object, the distance you get from measuring the circumference of a big circle and dividing by 2 pi, etc etc), which can easily differ a lot (factors of 2 or more) from each other.
It's precisely because of the lack of a unique "distance" that astronomers almost always use redshift when describing such objects: redshift is what can be directly observed, and it's readily compared across objects (& between observations & theoretical models).
For cosmologically-distant objects it's *not* valid to interpret redshift as simply a flat-spacetime Doppler shift (even with the special-relativity Doppler-shift formula).
And finally, since we don't have a reasonable definition of "distance" for cosmologically-distant objects, Hubble's law isn't meaningful for them. --
-- "Jonathan Thornburg" Dept of Astronomy, Indiana University, Bloomington, Indiana, USA
"Washing one's hands of the conflict between the powerful and the powerless means to side with the powerful, not to be neutral." -- quote by Freire / poster by Oxfam
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Mesgn neophyte question about hubble's law

Van: Nicolaas Vroom
Onderwerp: neophyte question about hubble's law
Datum: woensdag 7 oktober 2009 9:43 "Jonathan Thornburg" schreef in
posting Mesgm > Nicolaas Vroom wrote in posting Mesgk: >> A different way to measure the distance is by measuring >> the flux but than You use the assumption that stars of the >> same type all have the same Luminosity. > > I'm afraid you're mistaken: while they're very close for stars in > our galaxy, Fred Hoyle also uses that concept. He also uses that for other galaxies. > To be specific, let's consider the simple case of measuring the mean > Earth-Sun distance. Suppose you measure this distance in the following > ways: > (a),(b),(c),(d),(e) > > Contrary to what you might expect, in general these 5 measurements > will give slightly *different* results, typically differing by a few > kilometers. I also would expect they are all different > This is because in a curved spacetime, each of these > measures something fundamentally *different* (they use different > trajectories in spacetime). IMO if you are not carefull you measure the distance at different moments. You have to take the position of the Earth and the Sun into account. > If we now move to the original poster's example of stars within our > galaxy, then it's certainly true that these curved-spacetime effects > are (*much*) smaller than the other errors involved in astronomical > distance measurements. So for this case, you might reasonably decide > to ignore curved-spacetime effects. What you might not ignore that infact you measure the distance in the past. When you use parallax for a star and you measure the angle now what you measure is not angle based on the present position but the position approx d/c earlier (d = distance) The same for the angle a half year from now and the same for the calculated distance. (for practical reasons you can ignore this)
If you know the Limunosity of a star and you can measure the flux than based on the relation L=4*pi*d*d*f you can calculate the distance but again you calculate the past distance. > For such objects (distance < 200 megaparsecs or so, or equivalently > redshift z < 0.05 or so, or equivalently cz < 15,000 km/sec or so), > Hubble's law is observed to hold to a good approximation. What you need here is curve to show measured z versus measured Luminosity distance d The book by Fred Hoyle at page 617 shows such a curve and the impression is that Hubles Law is at least valid between z = 0.05 and z = 0.5 > It's precisely because of the lack of a unique "distance" that > astronomers almost always use redshift when describing such objects: > redshift is what can be directly observed, and it's readily compared > across objects (& between observations & theoretical models). The issue is what does this red shift mean. > For cosmologically-distant objects it's *not* valid to interpret > redshift as simply a flat-spacetime Doppler shift (even with the > special-relativity Doppler-shift formula). > > And finally, since we don't have a reasonable definition of "distance" > for cosmologically-distant objects, Hubble's law isn't meaningful for > them. What is than the meaning of Hubble's Law ?
IMO if Hubble's Law is true between z=0.05 and z = 0.5 and that there are no exceptions than you can reasonably assume that this relation is also true for z = 1 . However that is not the most important issue. The issue is that z=0.5 indicates that the star had in the past a certain distance and speed. The question is what was the speed of the star when the light was emitted that we observe now. If you assume space expansion than that speed was much lower as c*z.
Even more interesting is the question what is the speed of that star now (i.e the one of which we measure z=0.5 now) Maybe the speed is zero.
IMO this are all difficult issues Nicolaas Vroom
http://users.pandora.be/nicvroom/
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Mesgo neophyte question about hubble's law

Van: Nicolaas Vroom
Onderwerp: neophyte question about hubble's law
Datum: woensdag 7 oktober 2009 11:48 "Hans Aberg" schreef in
posting Mesgl > Nicolaas Vroom wrote: >> The main problem with c is it also affected by an expanding universe ? > > The idea is that the universe itself expands without changing c. So the > observable universe is at least 47 Gyr in radius, > > Hans > > http://en.wikipedia.org/wiki/Universe I do not understand this claim. IMO the concept of expanding Universe is based on two concepts: First we measure L and f and than we calculate the distance based on the equation: L = 4 * pi * d * d * f Secondly we measure d(labda) and labda and we calculate z = d(labda)/labda = "redshift" Fig 14.13 in the book by Hoyle shows a lineair relation between those two for z between 0.05 and 0.5. This implies that that concept can be used to calculate a distance for much larger values of z.
The next step is to multiply z with c and than you get the recession velocity v of a star based on the measured values L and f.
The problem I have with this approach is that when you use Luminosity as a measurement you calculate a distance in the past implying that it is very difficult to claim what the present distant (position) is. The same problem you have for the total size of the Universe. The Second problem is with the equation v=c*z. If you assume an expanding Univerese than d labda is also expanded meaning that both z and v (of the origin in the past when light was emitted) are much smaller. Nicolaas Vroom
http://users.pandora.be/nicvroom/
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Mesgp neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: maandag 12 oktober 2009 17:14 In
posting Mesgj Oh No writes: > Hubble's law is defined as a linear relationship between redshift and > distance. Some people define it as the linear relationship between velocity and distance, i.e. between the present derivative, with respect to cosmic time, of the proper distance and the proper distance of the object. (Hubble OBSERVED a relation between redshift and luminosity, which can be taken as a proxy for distance. At small redshifts, none of these distinctions matters. > >> Perhaps. It's just a pity Harrison's ideas about the expansion of space > >> time are somewhat inaccurate. > > > >Care to elabourate? > > I have only dipped into the book, so cannot comment on much of it, but > the discussion of the Hubble sphere on p281 struck me as particularly > misleading. It is absolutely not meaningful to talk about the recession > velocity of a star in the early universe with respect to ourselves now. > Think of the balloon analogy. Cosmological redshift is the consequence > of cosmological expansion, not recession velocities. It just happens > that, for small cosmological distances, cosmological expansion looks > like recession velocity. You can't take that concept too far. Yes, but it does go this far. Assuming a homogeneous and isotropic expansion, the recession velocity is proportional to distance---otherwise the homogeneity and/or isotropy are destroyed. At large redshifts, though, these aren't observable distances and observable velocities. (Knowing the cosmological parameters, though, they can still be calculated.)

Mesgq neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: maandag 12 oktober 2009 17:15 In
posting Mesgk Nicolaas Vroom writes: > The same can be done by using redshifts and the following > three equations: > V = H*d d=c*z and z = d labda/labda. > The problem is those 3 equations can not be used around > M31. The H constant calculated is not correct. No one should ever even think of thinking of redshifts of nearby galaxies as being cosmological redshifts. There is a component, yes, but it is swamped by peculiar motion. > In order to calculate H by two independent measurements > of V(speed) and d(distance) much further galaxies > should be used. And are, in practice. > How is this done in practice ? There is a HUGE literature on this. Many people have devoted their lives to it. > I expect this is very difficult > specific for the speed. (You need two distances) The speed is not calculated by a change in distance with time. It is inferred, assuming a cosmological model.
One observes the redshift. One observes something such as brightness which can be used as a proxy for distance. The relationship between them depends on the cosmological parameters.
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Mesgr neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: maandag 12 oktober 2009 17:16 In
posting Mesgo Nicolaas Vroom writes: > Secondly we measure d(labda) and labda > and we calculate z = d(labda)/labda = "redshift"
> Fig 14.13 in the book by Hoyle shows a lineair relation between > those two for z between 0.05 and 0.5. This implies that that concept > can be used to calculate a distance for much larger values of z. Non sequitur. Why does the former imply the latter?

Mesgs neophyte question about hubble's law

Van: Nicolaas Vroom
Onderwerp: neophyte question about hubble's law
Datum: vrijdag 16 oktober 2009 18:03
"Phillip Helbig" schreef in posting Mesgq > In posting Mesgk Nicolaas Vroom > writes: > >> The same can be done by using redshifts and the following >> three equations: >> V=H*d d=c*z and z =d labda/labda. >> In order to calculate H by two independent measurements >> of V(speed) and d(distance) much further galaxies >> should be used. >> I expect this is very difficult >> specific for the speed. (You need two distances) > > The speed is not calculated by a change in distance with time. It is > inferred, assuming a cosmological model. Should that not be based on a physical model of the universe ? > One observes the redshift. One observes something such as brightness > which can be used as a proxy for distance. The relationship between > them depends on the cosmological parameters. Accordingly to the Book by Hoyle it goes like this: page 640
"For the relation v=H*d, with v obtained from c*z, the quantity z being given by d labda/labda (See chapter 14) and with d obtained (See chapter 8). Hence by determining v and d values for a number of nearby galaxys the Hubble constant H is obtained "
I do not know if this description is accordingly to what you call the cosmological parameters.
Besides that I have a problem with the method explained by Hoyle. What we are observing is a number of galaxies (in the past) which are moving away from us in an expanding Universe.
The question is can we use v=c*z in order to calculate the speed (in the past) by measuring d labda now (frequency shift) which IMO is influenced by the expanding universe itself. IMO the expanding universe is partly the cause of the frequency shift, the futher away the more. That means that the v of the source (in the past) is much smaller as calculated based from c*z.
A second problem is what is the v of those galaxies now ?
IMO you need at least an answer on both questions in order to calculate the present size of the Universe. Nicolaas Vroom
http://users.pandora.be/nicvroom/neophyte.htm
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Mesgt neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: maandag 19 oktober 2009 11:03 In
posting Mesgs Nicolaas Vroom writes: > > The speed is not calculated by a change in distance with time. It is > > inferred, assuming a cosmological model. > > Should that not be based on a physical model of the universe ? By "cosmological model", what I mean IS a physical model of the universe, for example General Relativity. In particular, a universe described by General Relativity with certain values for the cosmological parameters. One could, of course, also compare other models which are not based on GR to observations. > Accordingly to the Book by Hoyle it goes like this: page 640 > "For the relation v=H*d, with v obtained from c*z, > the quantity z being given by d labda/labda (See chapter 14) > and with d obtained (See chapter 8). Hence by determining v > and d values for a number of nearby galaxys the Hubble > constant H is obtained " Fine, no problem. Unstated (apparently by Hoyle) assumptions: the distance has to be great enough that cosmological redshift is large compared to that from peculiar motion but not so high that higher-order effects (depending on the other cosmological parameters but not on the Hubble constant) need to be taken into account.
(I don't know if that applies here, but Hoyle was a believer in the steady-state model of the universe and might be deliberately setting up a straw-man version of more conventional cosmology in order to knock it down more easily.) > I do not know if this description is accordingly to what you call the > cosmological parameters. In the quote above, the redshift is so low that the velocity can be calculated as given with only a small error. This doesn't work at larger redshifts. > Besides that I have a problem with the method explained by Hoyle. > What we are observing is a number of galaxies (in the past) which > are moving away from us in an expanding Universe.
> The question is can we use v=c*z in order to calculate > the speed (in the past) by measuring d labda now (frequency shift) > which IMO is influenced by the expanding universe itself. At the low redshifts involved, this is not a problem. > IMO the expanding universe is partly the cause of the frequency > shift, the futher away the more. > That means that the v of the source (in the past) is much smaller > as calculated based from c*z. > > A second problem is what is the v of those galaxies now ? > > IMO you need at least an answer on both questions in order > to calculate the present size of the Universe. Trust me, it is all very clear and easy, but a bit too much for a usenet post before breakfast.
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Mesgu neophyte question about hubble's law

Van: Thomas Smid
Onderwerp: neophyte question about hubble's law
Datum: dinsdag 20 oktober 2009 9:59 On 17 Sep, 02:32, in
posting Mesga dfarr --at-- comcast --dot-- net wrote: > The 'Hubble's law' Wikipedia article states '...that the velocity at > which various galaxies are receding from the Earth are proportional to > their distance from us.' This is at least historically incorrect (so Wikipedia shouldn't be writing that): what Hubble discovered was the linear redshift/ distance relationship; the association of the redshift with a recession velocity was made by others and only adopted by Hubble as a kind of working hypothesis. Hubble himself believed in the possibility of a different cause for the redshift (see http://home.pacbell.net/skeptica/edwinhubble.html for more regarding the historical facts). Thomas
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Mesgv neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: dinsdag 20 oktober 2009 14:53 In
posting Mesgu, Thomas Smid writes: > On 17 Sep, 02:32, in posting Mesga dfarr --at-- comcast --dot--net > wrote: > > The 'Hubble's law' Wikipedia article states '...that the velocity at > > which various galaxies are receding from the Earth are proportional to > > their distance from us.' > > This is at least historically incorrect (so Wikipedia shouldn't be > writing that): what Hubble discovered was the linear redshift/ > distance relationship; To be pedantic, he discovered a linear relationship between redshift and apparent magnitude. One can interpret apparent magnitude as distance and redshift as velocity, at least at the low redshifts Hubble was working at. Then one has a relationship between velocity and distance.
The linear relationship between velocity and distance applies at all distances and for all velocities (even those greater than the speed of light) and the constant of proportionality is the Hubble constant, so some call this Hubble's Law. However, at large redshifts one can't simply calculate the velocity from the redshift, and the distance involved is not a "directly observable" distance.
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Mesgw neophyte question about hubble's law

Van: Hans Aberg
Onderwerp: neophyte question about hubble's law
Datum: woensdag 21 oktober 2009 11:21 Phillip Helbig in
posting Mesgv wrote: > To be pedantic, he discovered a linear relationship between redshift and > apparent magnitude. One can interpret apparent magnitude as distance > and redshift as velocity, at least at the low redshifts Hubble was > working at. Then one has a relationship between velocity and distance. > > The linear relationship between velocity and distance applies at all > distances and for all velocities (even those greater than the speed of > light) and the constant of proportionality is the Hubble constant, so > some call this Hubble's Law. However, at large redshifts one can't > simply calculate the velocity from the redshift, and the distance > involved is not a "directly observable" distance. What formula is used to compute velocity from redshift? Hans
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Mesgx neophyte question about hubble's law

Van: Nicolaas Vroom
Onderwerp: neophyte question about hubble's law
Datum: woensdag 21 oktober 2009 15:38 "Phillip Helbig" schreef in bericht
posting Mesgt > In posting Mesgs, Nicolaas Vroom > writes: > >> Accordingly to the Book by Hoyle it goes like this: page 640 >> "For the relation v=H*d, with v obtained from c*z, >> the quantity z being given by d labda/labda (See chapter 14) >> and with d obtained (See chapter 8). Hence by determining v >> and d values for a number of nearby galaxys the Hubble >> constant H is obtained " etc > (I don't know if that applies here, but Hoyle was a believer in the > steady-state model of the universe and might be deliberately setting up > a straw-man version of more conventional cosmology in order to knock it > down more easily.) Strange sentence..... >> Besides that I have a problem with the method explained by Hoyle. >> What we are observing is a number of galaxies (in the past) which >> are moving away from us in an expanding Universe. >> The question is can we use v=c*z in order to calculate >> the speed (in the past) by measuring d labda now (frequency shift) >> which IMO is influenced by the expanding universe itself. > > At the low redshifts involved, this is not a problem. Where do you draw this border line ? At 1% of the speed of light ?
If you start from Andromeda Galaxy with speed of 2,2 Myr (From the book Universe Box 26.1) you get using H=70 from: http://en.wikipedia.org/wiki/Hubble's_law v = H*d = 154 km/sec (Using box 26.2)
(This should cause a redshift, but in reality it has a blue shift resulting in a relative/peculiar velocity of roughly 300 km/sec towards the Sun)
I have no problem using v=c*z if we lived in an Universe with no expansion in order to measure the peculiar velocity of far away galaxies. I have a problem using that formula as soon as space expansion becomes involved to measure based on a speed measured here now to calculate the speed over there in the past. IMO the speed over there is smaller. >> IMO the expanding universe is partly the cause of the frequency >> shift, the futher away the more. etc > Trust me, it is all very clear and easy, but a bit too much for a usenet > post before breakfast. For high speeds relativistic redshift equations has to be used. z = sqrt(c+v)/(c-v) -1 (See Box 27.1). Is that the solution ?
I already see a problem at much lower speeds. Nicolaas Vroom
http://users.pandora.be/nicvroom/
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Mesgy neophyte question about hubble's law

Van: Thomas Smid
Onderwerp: neophyte question about hubble's law
Datum: donderdag 22 oktober 2009 13:00 On 20 Oct, 12:53, Phillip Helbig in
posting Mesgv wrote: > To be pedantic, he discovered a linear relationship between redshift and > apparent magnitude. The Big-Bang model of the universe rests solely on the interpretation of the redshift as being due to recessional velocities, so you can hardly call this issue pedantic. The point is that Hubble's work has nothing to do with this interpretational step. The latter is an ad-hoc assumption made by others, so with the formulation as in the Wikipedia article (and many other publications), Hubble's name and work has effectively been hijacked to promote this ad-hoc interpretation of the galactic redshifts. >One can interpret apparent magnitude as distance > and redshift as velocity, Whether one 'can' or not is not the point here. The question here is whether one *has to*. Only if one could answer this unambiguously with yes, would this justify the interpretation of the redshifts as recessional velocities. [Mod. note: in science, one rarely 'has to' interpret anything as anything, as Descartes pointed out some time ago -- mjh] Thomas
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Mesgz neophyte question about hubble's law

Van: Stupendous_Man
Onderwerp: neophyte question about hubble's law
Datum: donderdag 22 oktober 2009 19:34 Comments from:
posting Mesgy
If you actually read Hubble's work for yourself (here's a copy of his 1929 paper, for example) http://spiff.rit.edu/classes/phys240/lectures/expand/hub_1929.html you'll see that he discusses a relationship between distance and radial velocity. Note the title of the paper, for example:
"A RELATION BETWEEN DISTANCE AND RADIAL VELOCITY AMONG EXTRA-GALACTIC NEBULAE"
Hubble used several methods involving stars (including Cepheids and luminous blue stars) to estimate distances to other galaxies. He converted the shift in apparent wavelength of their spectra into radial velocities.
It is true that he offered two explanations for the shift in wavelengths, one of which is motion (radial velocity) and the other some sort of scattering.
I recommend that people who argue about the work of old-timey astronomers actually read those old-timey papers themselves, rather than reading an interpretation of those papers on someone's website.
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Mesga1 neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: donderdag 22 oktober 2009 21:08 In
posting Mesgw, Hans Aberg writes: > Phillip Helbig in posting Mesgv wrote: > > To be pedantic, he discovered a linear relationship between redshift and > > apparent magnitude. One can interpret apparent magnitude as distance > > and redshift as velocity, at least at the low redshifts Hubble was > > working at. Then one has a relationship between velocity and distance. > > > > The linear relationship between velocity and distance applies at all > > distances and for all velocities (even those greater than the speed of > > light) and the constant of proportionality is the Hubble constant, so > > some call this Hubble's Law. However, at large redshifts one can't > > simply calculate the velocity from the redshift, and the distance > > involved is not a "directly observable" distance. > > What formula is used to compute velocity from redshift? For small redshifts, the Doppler formula. Since you're a mathematician, I'm sure you understand that all things are linear to first order. :-)
For larger redshifts, the easy part is v = H*D. This is why has the dimensions of inverse time, or km/s/Mpc. The hard part is calculating D from the redshift. What you want is the proper distance. This, in the general case, is rather tricky and involves elliptic integrals. See, for example,
http://www.astro.multivax.de:8000/helbig/research/publications/info/angsiz.html
The paper is mainly concerned with a general numerical method for calculating certain distances in the case of a locally inhomogeneous universe, but for questions like this there is an appendix which explains the relationships between redshifts and various distances.

Mesga2 neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: donderdag 22 oktober 2009 21:09 In
posting Mesgx, Nicolaas Vroom writes: > > (I don't know if that applies here, but Hoyle was a believer in the > > steady-state model of the universe and might be deliberately setting up > > a straw-man version of more conventional cosmology in order to knock it > > down more easily.) > > Strange sentence..... Hoyle was a strange character. Once, he presented a rather over-simplified model, emphasising its simplicity. Some in the audience thought it unrealistically simplified, and someone remarked "You'd look pretty simple, too, Fred, at a distance of 10 parsecs.". > >> Besides that I have a problem with the method explained by Hoyle. > >> What we are observing is a number of galaxies (in the past) which > >> are moving away from us in an expanding Universe. > >> The question is can we use v=c*z in order to calculate > >> the speed (in the past) by measuring d labda now (frequency shift) > >> which IMO is influenced by the expanding universe itself. > > > > At the low redshifts involved, this is not a problem. > > Where do you draw this border line ? > At 1% of the speed of light ? Depends on the cosmological parameters. For some models, the relationship is exact at all redshifts. For others (though perhaps not within the framework of GR), it might not even be close in the limit of small redshifts (but in all GR models, at small redshifts the approximation is OK). > I have no problem using v=c*z if we lived in an Universe > with no expansion in order to measure the peculiar velocity of > far away galaxies. > I have a problem using that formula as soon as space expansion > becomes involved to measure based on a speed measured here now > to calculate the speed over there in the past. > IMO the speed over there is smaller. Depends on the cosmological model. However, for any model in which the approximation v=cz is valid, the difference between here and now and there and then is negligible. > For high speeds relativistic redshift equations has to be used. > z = sqrt(c+v)/(c-v) -1 (See Box 27.1). Is that the solution ? Definitely not. To paraphrase Wolfgang Pauli, it is wrong to use the Doppler formula at low redshifts, it is very wrong to say that the cosmological expansion is caused by the Doppler effect, and it is not even wrong to say that the relativistic Doppler formula is appropriate at high redshifts. Simple argument: This formula makes no reference to the cosmological parameters, so it will give the same result no matter what their values are.
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Mesga3 neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: donderdag 22 oktober 2009 21:11 In
posting Mesgy, Thomas Smid writes: > On 20 Oct, 12:53, Phillip Helbig in posting Mesgv > wrote: > > > To be pedantic, he discovered a linear relationship between redshift and > > apparent magnitude. > > The Big-Bang model of the universe rests solely on the interpretation > of the redshift as being due to recessional velocities, so you can > hardly call this issue pedantic. But Hubble's discovery came first, the big-bang model as a model for the real universe came later.

Mesga4 neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: donderdag 22 oktober 2009 21:14 In
posting Mesgz, Stupendous_Man writes: > If you actually read Hubble's work for yourself > (here's a copy of his 1929 paper, for example) > > http://spiff.rit.edu/classes/phys240/lectures/expand/hub_1929.html > > you'll see that he discusses a relationship between > distance and radial velocity. Note the title of the > paper, for example: > > "A RELATION BETWEEN DISTANCE AND RADIAL VELOCITY > AMONG EXTRA-GALACTIC NEBULAE" OK, but it is still an interpretation, even if it is Hubble's. I was merely pointing out that if on the one hand one is discussing whether to interpret the redshift as velocity, one should or could also discuss---and I did use the word pedantic---whether to interpret the magnitude as distance. The latter is actually non-trivial, since it relies on a "standard candle". Only within the last 10--15 years have reliable standard candles been found and used to accurately measure the Hubble constant.
As Mach said: "Every statement in physics has to state relations between observable quantities." What is observed are magnitude and redshift, distance and velocity are derived. (One could be even more pedantic and talk about what is actually recorded by the photographic emulsion, which in practice actually has to be taken into account.)

Mesga5 neophyte question about hubble's law

Van: Nicolaas Vroom
Onderwerp: neophyte question about hubble's law
Datum: vrijdag 23 oktober 2009 10:03 "Thomas Smid" schreef in
bericht Mesgy news:mt2.0-19629-1256209237@hercules.herts.ac.uk... > On 20 Oct, 12:53, Phillip Helbig posting Mesgv wrote: > >>One can interpret apparent magnitude as distance >> and redshift as velocity, > > Whether one 'can' or not is not the point here. The question here is > whether one *has to*. Only if one could answer this unambiguously with > yes, would this justify the interpretation of the redshifts as > recessional velocities. > I have no problem with the statement one can interpret redshift as velocity. IMO the issue is how. The current point of view is that for values of z << 1 one has to use the equation v = c*z (Also called the nonrelativistic equation for the Doppler shift)
I have a problem with that equation. Suppose a galaxy at a far distance in the past is receding from us with a speed of 0.01c resulting in a value of z of 0.01. Light from that galaxy in an expanding universe is travelling towards us at a speed c and is stretched. Suppose we receive it now. Is it not possible in principle that we measure a value of z=0.02 implying a speed of v=0.02*c ? My point is what we measure is not the true speed of the source at the point of emission. This speed is much lower because the waves are stretched.
Even if we measure a z=2 it does not mean that the source in the past was travelling at a speed higher than c.
The overall implication is that maybe there is no reason to use the relativistic equation for the Doppler shift.
A second implication in principle is that the true speed, of a galaxy with z=2 measured now here, could be zero over there.
A third implication is that the size of the Observable Universe is much smaller than 47 Gyr. See the posting by Hans Aberg. Nicolaas Vroom http://users.pandora.be/nicvroom/neophyte.htm
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Mesga6 neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: vrijdag 23 oktober 2009 13:05 In article , Nicolaas Vroom writes: > I have no problem with the statement one can interpret > redshift as velocity. > IMO the issue is how. > The current point of view is that for values of z << 1 one has to > use the equation v = c*z > (Also called the nonrelativistic equation for the Doppler shift) You don't have to use it, but you CAN use it and get the same result as a more detailed analysis. > I have a problem with that equation. > Suppose a galaxy at a far distance in the past is receding from us > with a speed of 0.01c resulting in a value of z of 0.01. > Light from that galaxy in an expanding universe is travelling towards > us at a speed c and is stretched. OK. > Suppose we receive it now. Is it not possible in principle that we measure > a value of z=0.02 implying a speed of v=0.02*c ? Why should that happen? > My point is what we measure is not the true speed of the source at the point > of emission. This speed is much lower because the waves are stretched. Again, for the low redshifts at which one can use the Doppler formula, the change in speed between the time of emission and the time of absorption is negligible. > Even if we measure a z=2 it does not mean that the source in the past > was travelling at a speed higher than c. But it could be. > The overall implication is that maybe there is no reason to > use the relativistic equation for the Doppler shift. Right. > A second implication in principle is that the true speed, of a galaxy > with z=2 measured now here, could be zero over there. Possible. But now you are in the high-redshift regime, where you can't get a useful answer from the Doppler formula. > A third implication is that the size of the Observable Universe > is much smaller than 47 Gyr. See the posting by Hans Aberg. I don't think that anyone claims that the size of the Observable Universe is as large as 47 Gyr.

Mesga7 neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: zaterdag 24 oktober 2009 20:35
"Phillip Helbig" schreef in bericht Mesga2 > In article posting Mesgx, Nicolaas Vroom > writes : > >> I have no problem using v=c*z if we lived in an Universe >> with no expansion in order to measure the peculiar velocity of >> far away galaxies. >> I have a problem using that formula as soon as space expansion >> becomes involved to measure based on a speed measured here now >> to calculate the speed over there in the past. >> IMO the speed over there is smaller. > > Depends on the cosmological model. However, for any model in which the > approximation v=cz is valid, the difference between here and now and > there and then is negligible. > Finally I found a document which describes the problem. See http://wapedia.mobi/en/Hubble's_law Specific paragraph 2. 2. "Observability of parameters"
Here we read:
"For relatively nearby galaxies (redshift z much less than unity), v and D will not have changed much, and v can be estimated using the formula v=zc where c is the speed of light. This gives the empirical relation found by Hubble."
The book Universe Box 26.2 has an example: for NGC 4889.
Normally wavelength 3933A measured 4018A z = (4018-3933)/3933= 65/3933= 0,0216
Next we read:
The galaxy is therefore moving away from us with a speed of: v=zc= 6500 km/sec
Also at this small value of z there is an issue exactly what is the definition of v? Is this a speed measured over here now or is this the speed of the galaxy over there in the past.
To write this slightly different. What was the difference in wavelength at the moment of transmission at the source:
1) 0 A or close to zero
2) inbetween 0A and 65A for example close to 32A
3) 65 A or close to 65

People who believe in the retarded light concept are in favour of option 1 (ie no space expansion)
I place the above sentence > the difference between here and now and > there and then is negligible. as option 3
I have a problem with this. IMO the difference in wave length can have two causes:
By a speed of the galaxy away from us at the moment of emission.
By space expansion during its traveling from source to destination.
The question is how much is this and how do we know ?
How much is this when d labda = 65 A
How much is this when d labda = 3933 A resulting z=1
Why can not it be 50% already in the case of 65 A
Using H = 70, NGC 4889 is roughly 90 million lightyears away from us. Nicolaas Vroom http://users.pandora.be/nicvroom/neophyte.htm
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Mesga8 neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: zondag 25 oktober 2009 19:35 In article
posting Mesga7, Nicolaas Vroom writes: > The galaxy is therefore moving away > from us with a speed of: v=zc= 6500 km/sec > > Also at this small value of z there is an issue exactly > what is the definition of v? > Is this a speed measured over here now > or > is this the speed of the galaxy over there in the past. Again, for the redshifts involved, the difference is negligible. > To write this slightly different. > What was the difference in wavelength at the moment > of transmission at the source: > 1) 0 A or close to zero
> 2) inbetween 0A and 65A for example close to 32A
> 3) 65 A or close to 65 0. The wavelength gets stretched out as the universe expands.

Mesgb1 neophyte question about hubble's law

Van: Nicolaas Vroom
Onderwerp: neophyte question about hubble's law
Datum: donderdag 3 december 2009 16:48 "dfarr --at-- comcast --dot-- net" schreef in bericht
posting Mesga: > The 'Hubble's law' Wikipedia article states '...that the velocity at > which various galaxies ARE receding from the Earth IS proportional to > their distance from us.' (emphasis added) > > My question is about the tense of the two verbs in all caps above. > Aside from assuming things are orderly, > do we have any way of inferring that a galaxy that was moving away > from us 12 billion years ago is still doing so? The light from the > galaxy which is reaching us now indicates it was moving away, but do > we have any way of inferring that it has not slowed down or started to > approach us, or disappeared off the 'edge'? > > > [[Mod. note -- The following is quoted with only slight changes > from a recent posting of mine in sci.physics.research, and seems > relevant here too: > > The Earth is roughly 149 million > kilometers = 8.5 light-minutes away from the Sun. So, if we look > outside during daylight hours, we have observational data that the > Sun was shining 8.5 minutes ago. But we have *no* observational > data about what the Sun is doing right *now*. > The more I read the less I understand. By measuring the parallax of an object we can calculate the past distance of that object. Q: Is it also possible to calculate the present position and velocity of that object? Within our solar system the answer is yes because we can perform a sequence of observations and use Newton's law. Within our Galaxy the answer is also Yes. Outside Our Galaxy the Answer is No
For individual stars (Cepheids) at that distance we use luminosity to calculate the past distance.
By measuring the redshift (z) we can calculate the velocity of an object in the past. Q: Can we use redshift also the calculate the past distance and what about the present distance and present velocity ?. Using redshift we can calculate the past velocity of stars within Andromeda Galaxy but we cannot use z to calculate the past position nor the present position and present velocity.
Galaxy NGC 4258 (at a larger distance) the same problems exists.
For a Galaxy like UGC 3789 (again at a larger distance) the problems become worse. At the moment of emission this Galaxy has a radial speed away from us resulting in a certain value of z. However that is not the measured value of z, which will be larger, being caused by the expansion of space. The question is now: Which part is caused by movement of source (in the past) and which part by expansion of space (going from past to present) Assuming we can solve that we are left with the Question: What is the current position and velocity of UGC 3789 ? IMO we cannot answer that. (Only that its position will be further away)
For more details and documents studied read: http://users.telenet.be/nicvroom/Hubble-Faq.htm Nicolaas Vroom
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Mesgb2 neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: zondag 6 december 2009 12:28 In article
posting Mesgb1, "Nicolaas Vroom" writes: > "dfarr --at-- comcast --dot-- net" schreef in bericht > news:645be291-d735-4aa7-a15c-b1d6eb89b15d@l13g2000yqb.googlegroups.com... > > The 'Hubble's law' Wikipedia article states '...that the velocity at > > which various galaxies ARE receding from the Earth IS proportional to > > their distance from us.' (emphasis added) True, though keep in mind that some people use the term "Hubble's Law" to mean something different.
See http://adsabs.harvard.edu/abs/1993ApJ...403...28H
In the sense in which you use it above, this means that the velocity (defined as the change in proper distance (which is the distance you could measure instantaneously with a rigid ruler) per cosmic time (essentially time as measured by someone at rest with respect to the microwave background) as measured now) is proportional to the proper distance. However, these are not quantities which can be "directly observed" This law is a trivial consequence of homogeneous and isotropic expansion. > > My question is about the tense of the two verbs in all caps above. In the sense above, it refers to the present value of all the quantities, at this moment in cosmic time. > > Aside from assuming things are orderly, > > do we have any way of inferring that a galaxy that was moving away > > from us 12 billion years ago is still doing so? Yes (see below). > The light from the > > galaxy which is reaching us now indicates it was moving away, Why do you say this? You uppercased the terms referring to the present tense. > but do > > we have any way of inferring that it has not slowed down or started to > > approach us, or disappeared off the 'edge'? Yes (see below). > > The Earth is roughly 149 million > > kilometers = 8.5 light-minutes away from the Sun. So, if we look > > outside during daylight hours, we have observational data that the > > Sun was shining 8.5 minutes ago. But we have *no* observational > > data about what the Sun is doing right *now*. Strictly speaking, true. But we have good reason to believe that we can extrapolate 8 minutes into the future. > The more I read the less I understand. > By measuring the parallax of an object we can calculate the past > distance of that object. If you mean parallax in the traditional sense, then this is true only for objects which are quite close, well within our galaxy, not at cosmological distances, with light-travel times of a few years. > Q: Is it also possible to calculate the present position and velocity > of that object? Yes (see below). > Within our solar system the answer is yes because we can perform > a sequence of observations and use Newton's law. Right. > Within our Galaxy the answer is also Yes. Right. > Outside Our Galaxy the Answer is No You are answering your own question here. Why do you say "no"? The answer is "yes". > For individual stars (Cepheids) at that distance we use luminosity > to calculate the past distance. Distances which can be measured using Cepheids are extremely small, cosmologically, so all distances are equivalent. > By measuring the redshift (z) we can calculate the velocity of an object > in the past. At low redshift this is true, but not at high redshift, unless you have a really bizarre definition of "velocity". See the paper by Harrison mentioned above. > Q: Can we use redshift also the calculate the past distance > and what about the present distance and present velocity ?. Yes (see below). > Using redshift we can calculate the past velocity of stars within > Andromeda Galaxy but we cannot use z to calculate the past > position nor the present position and present velocity. The Andromeda galaxy is so close that the cosmological redshift is negligible. The redshift is due to its peculiar velocity and hence can't be used to calculate the past velocity. > At the moment of emission this Galaxy has a radial speed away from us > resulting in a certain value of z. However that is not the measured > value of z, which will be larger, being caused by the expansion of space. Wrong. If the redshift is large enough to be cosmologically interesting, then you cannot use it to infer the velocity. There is a small range where cosmological redshifts are larger than those due to other causes (primarily peculiar velocity) but still small enough to use a linear approximation and calculate the velocity as if the redshift were due to a Doppler effect. In this case, the difference between the distance now and distance then (at the time of light emission), or velocity now and velocity then, is negligible. > The question is now: Which part is caused by movement of source > (in the past) and which part by expansion of space > (going from past to present) The cosmological redshift is due only to the expansion of space. The galaxy might have a peculiar velocity which will make an additional contribution. > Assuming we can solve that we are left with the Question: > What is the current position and velocity of UGC 3789 ? > IMO we cannot answer that. > (Only that its position will be further away) If it is close enough so that one can approximate its velocity from measuring the redshift, then the difference between these two distances is negligible.
In the general case (arbitrarily large redshift), we can calculate the distance (however it is defined) both now and at the time the light was emitted.
The redshift, by itself, tells us the ratio of the scale factor now to that at the time the light was emitted. It tells us NOTHING ELSE. At low redshift, one can show that one can approximate the velocity by using the Doppler formula. At high redshift this is not possible (and don't even think about the relativistic Doppler formula; it is irrelevant here). To get further, one has to know the cosmological parameters. If they are known, then one can calculate any distance at any time from the redshift for the given cosmological parameters.
See, for example, http://www.astro.multivax.de:8000/helbig/research/p/abstracts/angsiz.html
next posting Mesgb3

Mesgb3 neophyte question about hubble's law

Van: Nicolaas Vroom
Onderwerp: neophyte question about hubble's law
Datum: donderdag 10 december 2009 19:03 "Phillip Helbig---undress to reply" schreef in bericht
posting Mesgb2 > In article posting Mesgb1, "Nicolaas Vroom" > writes: > >> "dfarr --at-- comcast --dot-- net" schreef in bericht >> news:645be291-d735-4aa7-a15c-b1d6eb89b15d@l13g2000yqb.googlegroups.com... >> > The 'Hubble's law' Wikipedia article states '...that the velocity at >> > which various galaxies ARE receding from the Earth IS proportional to >> > their distance from us.' (emphasis added) > > True, though keep in mind that some people use the term "Hubble's Law" > to mean something different. See > > http://adsabs.harvard.edu/abs/1993ApJ...403...28H > > In the sense in which you use it above, this means that the velocity > (defined as the change in proper distance (which is the distance you > could measure instantaneously with a rigid ruler) per cosmic time > (essentially time as measured by someone at rest with respect to the > microwave background) as measured now) is proportional to the proper > distance. However, these are not quantities which can be "directly > observed" This law is a trivial consequence of homogeneous and > isotropic expansion. I fully agree that there two Hubble Laws. See also: http://users.telenet.be/nicvroom/Hubble-Faq.htm The first law describes the relation between Distance and redshift (z) Expressed as D = c/H * z The second law describes the relation between Distance and speed and uses the relation v = c * z and then you get: D = v/H or v = H *d
I have a problem with this second law specific with the definition of what means v ? a) Is v the speed of the Galaxy in the past at moment of emission. or b) is the v the present speed of the Galaxy ?
For example what means the speed of 7772 km/sec for NGC 6323 calculated using z = 0.026 and H = 72 km/sec/Mpc ?
I have no problem with the relation v = c * z only at very small distances, implying that the second law does not apply. >> The more I read the less I understand.
>> Q: Can we use redshift also the calculate the past distance >> and what about the present distance and present velocity ?. > > Yes (see below). >> At the moment of emission this Galaxy has a radial speed away from us >> resulting in a certain value of z. However that is not the measured >> value of z, which will be larger, being caused by the expansion of space. > > Wrong. If the redshift is large enough to be cosmologically > interesting, then you cannot use it to infer the velocity. I agree if you mean the velocity of the galaxy in the past at emission.
The question then remains what does z at those distance represent ? Does z then represent distance ? what is this relation ? based on which observations is this relation demonstrated ? and what means large enough ? z =0.023 ?
If z = 0.023 is the minimal boundary than you need indepent measurments in order to establish this relation. >> The question is now: Which part is caused by movement of source >> (in the past) and which part by expansion of space >> (going from past to present) > > The cosmological redshift is due only to the expansion of space. The > galaxy might have a peculiar velocity which will make an additional > contribution. The issue is that contribution will be larger the further the galaxy is and the further you go back in time. Making it more and more difficult to calculate its distance. >> Assuming we can solve that we are left with the Question: >> What is the current position and velocity of UGC 3789 ? >> IMO we cannot answer that. >> (Only that its position will be further away) > > If it is close enough so that one can approximate its velocity from > measuring the redshift, then the difference between these two distances > is negligible. UGC 3789 has a redshift of 0.011 i.e. below 0.023
That means you can not use it to establish the first Hubble Law (ie the z versus distance relation) > The redshift, by itself, tells us the ratio of the scale factor now to > that at the time the light was emitted. It tells us NOTHING ELSE. At > low redshift, one can show that one can approximate the velocity by > using the Doppler formula. At high redshift this is not possible (and > don't even think about the relativistic Doppler formula; it is > irrelevant here). To get further, one has to know the cosmological > parameters. Based on which observations ?
Is one of those parameters density ? > If they are known, then one can calculate any distance at > any time from the redshift for the given cosmological parameters. >
> See, for example, > http://www.astro.multivax.de:8000/helbig/research/p/abstracts/angsiz.html I can not read that document. angsiz.tar-gz shows an error message: It does not appesr to be a valid zip file etc Nicolaas Vroom
next posting Mesgb4

Mesgb4 neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: vrijdag 11 december 2009 17:16 In article
posting Mesgb3, "Nicolaas Vroom" writes: > The first law describes the relation between Distance and redshift (z) > Expressed as D = c/H * z Fine for low redshift. > The second law describes the relation between Distance and speed > and uses the relation v = c * z Fine for low redshift. > and then you get: D = v/H or v = H *d Fine for all redshifts, except you have to keep in mind that this D is not necessarily the same as the one above. > I have a problem with this second law specific with the definition > of what means v ?
> a) Is v the speed of the Galaxy in the past at moment of emission. > or b) is the v the present speed of the Galaxy ? In the form in which you present it, it is valid only in the limit of low redshifts, so it doesn't matter. If the redshift is high enough that it does matter, the equation isn't valid. > The question then remains what does z at those distance represent ? > Does z then represent distance ? > what is this relation ? Without any additional information, 1+z is the ratio of the size of the universe now to the size of the universe when the light was emitted. > > The redshift, by itself, tells us the ratio of the scale factor now to > > that at the time the light was emitted. It tells us NOTHING ELSE. At > > low redshift, one can show that one can approximate the velocity by > > using the Doppler formula. At high redshift this is not possible (and > > don't even think about the relativistic Doppler formula; it is > > irrelevant here). To get further, one has to know the cosmological > > parameters. > Based on which observations ? Have a look for "cosmological parameters" att arXiv.org. You will get hundreds or thousands of papers from within the last 15 years. > Is one of those parameters density ? Yes. > > http://www.astro.multivax.de:8000/helbig/research/p/abstracts/angsiz.html > > I can not read that document. > angsiz.tar-gz shows an error message: > It does not appesr to be a valid zip file etc It's not a ZIP file, it's a gzipped tar file. You should be able to get a PDF of the paper from ArXiv.org, though.
next posting Mesgb5

Mesgb5 neophyte question about hubble's law

Van: Nicolaas Vroom
Onderwerp: neophyte question about hubble's law
Datum: dinsdag 15 december 2009 17:12 "Phillip Helbig" schreef in bericht
posting Mesgb4 > In article posting Mesgb3, "Nicolaas Vroom" > writes: > >> The first law describes the relation between Distance and redshift (z) >> Expressed as D = c/H * z > > Fine for low redshift. > >> The second law describes the relation between Distance and speed >> and uses the relation v = c * z > > Fine for low redshift. > >> and then you get: D = v/H or v = H *d > > Fine for all redshifts, except you have to keep in mind that this D is > not necessarily the same as the one above. Do you mean d as trigonometric distance (parallax) versus d as luminosity distance ? >> I have a problem with this second law specific with the definition >> of what means v ?
>> a) Is v the speed of the Galaxy in the past at moment of emission. >> or b) is the v the present speed of the Galaxy ? > > In the form in which you present it, it is valid only in the limit of > low redshifts, so it doesn't matter. What do you mean with: it does not matter ? Does it matter in the case of NGC 6323 ?
In the case of NGC 6323 we get a speed of 7772 km/sec using z = 0.026 and H = 72 km/sec/Mpc. The distance is 110 Mpc.
The question is what does this speed of 7772 km/sec mean ?
1. Is this the speed of NGC 6323 in the past, when light was emitted ?
2. Is this the speed of NGC 6323 now ?
3. Or is it something else ?
See also: http://users.telenet.be/nicvroom/Hubble-Faq.htm This document shows you the litterature where you can find more detail information. Nicolaas Vroom http://users.telenet.be/nicvroom/
next posting Mesgb6

Mesgb6 neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: woensdag 16 december 2009 9:23 In article
posting Mesgb5, "Nicolaas Vroom" writes: > "Phillip Helbig" > schreef in bericht posting Mesgb4 > > In article posting Mesgb3, "Nicolaas Vroom" > > writes: > > > >> The first law describes the relation between Distance and redshift (z) > >> Expressed as D = c/H * z > > > > Fine for low redshift. > > > >> The second law describes the relation between Distance and speed > >> and uses the relation v = c * z > > > > Fine for low redshift. > > > >> and then you get: D = v/H or v = H *d > > > > Fine for all redshifts, except you have to keep in mind that this D is > > not necessarily the same as the one above. > > Do you mean d as trigonometric distance (parallax) versus > d as luminosity distance ? Neither. The D is proper distance, i.e. the distance which one could theoretically measure at the present instant of cosmic time with a rigid ruler. It is, in general, not the same as the luminosity distance, nor the parallax distance, nor the distance from light-travel time, nor the proper-motion distance, nor the angular-size distance. > In the case of NGC 6323 we get a speed of 7772 km/sec > using z = 0.026 and H = 72 km/sec/Mpc. > The distance is 110 Mpc. > The question is what does this speed of 7772 km/sec mean ? > 1. Is this the speed of NGC 6323 in the past, when light was > emitted ?
> 2. Is this the speed of NGC 6323 now ?
> 3. Or is it something else ? A "typical" value for the peculiar velocity of a galaxy is 600 km/s. So there is a substantial contamination from a non-cosmological redshift. This effect is much greater than the difference between the speed now and the speed at the time the light was emitted. Assume no contamination, i.e. the ideal case. The Doppler formula is exact as the redshift approaches zero, i.e. it is a limit. For larger redshift, it gives NEITHER the speed now NOR the speed when the light was emitted. Assuming we know the Hubble constant, and we know the distance, then we get the velocity NOW. This holds at any redshift. But the distance is the proper distance (not something "directly observable" like luminosity distance) (see above) and the velocity is its derivative with respect to cosmic time as measured now.
next posting Mesgb7

Mesgb7 neophyte question about hubble's law

Van: Nicolaas Vroom
Onderwerp: neophyte question about hubble's law
Datum: woensdag 16 december 2009 18:47 "Phillip Helbig" schreef in bericht
posting Mesgb6 > In article posting Mesgb5, "Nicolaas Vroom" > writes: > >> "Phillip Helbig" >> posting Mesgb4 >> > In article posting Mesgb3, "Nicolaas Vroom" >> > writes: >> > >> >> and then you get: D = v/H or v = H *d >> > >> > Fine for all redshifts, except you have to keep in mind that this D is >> > not necessarily the same as the one above. >> >> Do you mean d as trigonometric distance (parallax) versus >> d as luminosity distance ? > > Neither. The D is proper distance, i.e. the distance which one could > theoretically measure at the present instant of cosmic time with a rigid > ruler. Okay. In fact the way I see it there are two proper distances in volved: 1. The proper distance in the past at the moment of emission.
2. The proper distance now. This is the distance we are looking for. Suppose I call the proper distance: D, the parallax distance pd and the luminisity distance: ld The law above then becomes: D=v/H or v=H*D The first low, how should it be defined ?
1) D = c/H*z or 2) pd= c/H*z or 3) pl = c/H*z ? I expect either 2 or 3. If I am correct then Hubble constant H describes the relation between z and pd or pl. The important question is: (assuming that the second law uses the proper distance D) are the two Hubble constants H the same ? ( are both Hubble relations the same ?) >> In the case of NGC 6323 we get a speed of 7772 km/sec >> using z = 0.026 and H = 72 km/sec/Mpc. >> The distance is 110 Mpc. > >> The question is what does this speed of 7772 km/sec mean ?
>> 1. Is this the speed of NGC 6323 in the past, when light was >> emitted ?
>> 2. Is this the speed of NGC 6323 now ?
>> 3. Or is it something else ? > > A "typical" value for the peculiar velocity of a galaxy is 600 km/s. So > there is a substantial contamination from a non-cosmological redshift. I expect you mean contamation caused by the expansion of space. > This effect is much greater than the difference between the speed now > and the speed at the time the light was emitted. > > Assume no contamination, i.e. the ideal case. That is the situation within the Milky Way or at maximum between us and Andromeda Galaxy. > The Doppler formula is > exact as the redshift approaches zero, i.e. it is a limit. For larger > redshift, it gives NEITHER the speed now NOR the speed when the light > was emitted. > Assuming we know the Hubble constant, and we know the distance, then we > get the velocity NOW. Does that mean that the speed of 7772 km/sec is the present speed NOW ? Nicolaas Vroom
next posting Mesgb8

Mesgb8 neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: donderdag 17 december 2009 8:45 In article
posting Mesgb7, "Nicolaas Vroom" writes: > In fact the way I see it there are two proper distances in volved: Yes. > 1. The proper distance in the past at the moment of emission.
> 2. The proper distance now. This is the distance we are looking for. OK. > Suppose I call the proper distance: D, the parallax distance pd > and the luminisity distance: ld > The law above then becomes: D=v/H or v=H*D Right. If you think about it, this is trivial. There is no physics involved. This law HAS TO hold as long as the universe expands homogeneously and isotropically; any other velocity-distance law would not be compatible with such an expansion. > The first low, how should it be defined ?
> 1) D = c/H*z or 2) pd= c/H*z or 3) pl = c/H*z ? If the redshift is low enough so that the distance is (nearly) proportional to it, then the differences between the various definitions of distance are small enough to be ignored (especially considering the fact that the redshift has a non-cosmological component as well which at low redshift might not be negligible. However, in general NONE of your equations is correct. Look at it this way. All your equations have a LINEAR relationship between distance and redshift. However, in general distance is NOT linear with redshift; it is a more complicated function of redshift. This is true for all distances. However, at low redshifts, all the distances are roughly equal, and all your equations are roughly right. > I expect either 2 or 3. > If I am correct then Hubble constant H describes the relation between > z and pd or pl. No; see above. > The important question is: (assuming that the second law uses > the proper distance D) are the two Hubble constants H the same ? > > ( are both Hubble relations the same ?) There is but one Hubble constant. However, it might not be appropriate to use it in all contexts. > > A "typical" value for the peculiar velocity of a galaxy is 600 km/s. So > > there is a substantial contamination from a non-cosmological redshift. > > I expect you mean contamation caused by the expansion of space. No; the cosmological redshift is caused by the expansion of space. However, in addition, the galaxy can be moving through space, which also produces a redshift (or blueshift). > > This effect is much greater than the difference between the speed now > > and the speed at the time the light was emitted. > > > > Assume no contamination, i.e. the ideal case. > That is the situation within the Milky Way or at maximum > between us and Andromeda Galaxy. Quite the opposite. Within the Milky Way, there is no cosmological redshift. The peculiar velocity of the Andromeda galaxy dwarfs its cosmological redshift (it actually has a net blueshift). > > Assuming we know the Hubble constant, and we know the distance, then we > > get the velocity NOW. > Does that mean that the speed of 7772 km/sec is the present speed NOW ? Yes, if a) we are talking about the proper distance now and its derivative with respect to cosmic time as measured now and b) if this redshift is due only to the cosmological redshift.
next posting Mesgb9

Mesgb9 neophyte question about hubble's law

Van: Nicolaas Vroom
Onderwerp: neophyte question about hubble's law
Datum: vrijdag 18 december 2009 11:00 "Phillip Helbig" schreef in bericht
posting Mesgb8 > In article posting Mesgb7, "Nicolaas Vroom" > writes: >> Suppose I call the proper distance: D, the parallax distance pd >> and the luminisity distance: ld >> The law above then becomes: D=v/H or v=H*D > > Right. If you think about it, this is trivial. There is no physics > involved. This law HAS TO hold as long as the universe expands > homogeneously and isotropically; It is 100 % physics.
The question is: Is this law a correct description of the physical reality ?
The physical reality being the state over there now. Not the state over there in the past which we can observe. >> The first low, how should it be defined ? >> 1) D = c/H*z or 2) pd= c/H*z or 3) pl = c/H*z ? > > If the redshift is low enough so that the distance is (nearly) > proportional to it, then the differences between the various definitions > of distance are small enough to be ignored The proper distance D versus the parallax distance (and ld) are fundamental different. The first being defined as the distance using rigid rulers (ie between two present positions) and the second as the distance between the present and the past. > (especially considering the > fact that the redshift has a non-cosmological component as well which at > low redshift might not be negligible. However, in general NONE of your > equations is correct. This are not my equations.
See for example the book "Astronomy and Cosmology" by Fred Hoyle page 617 which discusses the relation between redshift z and distance d >> > A "typical" value for the peculiar velocity of a galaxy is 600 km/s. >> > So >> > there is a substantial contamination from a non-cosmological redshift. >> >> I expect you mean contamation caused by the expansion of space. > > No; the cosmological redshift is caused by the expansion of space. > However, in addition, the galaxy can be moving through space, which also > produces a redshift (or blueshift). Okay. The real issue is how typical is your example of 600 km/s. If you go towards larger distances could this typical value not be much larger implying that contamination increases with distance ? (in time) Secondly how do you measure this so called contamination ? >> > Assuming we know the Hubble constant, and we know the distance, then we >> > get the velocity NOW. >> Does that mean that the speed of 7772 km/sec is the present speed NOW ? > > Yes, if a) we are talking about the proper distance now and its > derivative with respect to cosmic time as measured now and b) if this > redshift is due only to the cosmological redshift. And what is the verdict ? Are both a and b correct ? I have great problems with both, but ofcourse my opinion is of no importance. http://users.telenet.be/nicvroom/Hubble-Faq.htm#ol9 See comments near Document 9 Nicolaas Vroom
next posting Mesgba

Mesgba neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: zaterdag 19 december 2009 10:52 In article
posting Mesgb9, "Nicolaas Vroom" writes: > "Phillip Helbig" posting Mesgb8 > > In article posting Mesgb7, "Nicolaas Vroom" > > writes: > > >> Suppose I call the proper distance: D, the parallax distance pd > >> and the luminisity distance: ld
> >> The law above then becomes: D=v/H or v=H*D > > > > Right. If you think about it, this is trivial. There is no physics > > involved. This law HAS TO hold as long as the universe expands > > homogeneously and isotropically; > > It is 100 % physics. > The question is: Is this law a correct description > of the physical reality ? > The physical reality being the state over there now. > Not the state over there in the past which we can observe. We can only observe what is happening around us now. Everything else might have ceased to exist. Actually, our brain only responds to signals---they might be generated by something other than external reality. Or we might be dreaming. However, if we talk about cosmology the way we talk about day-to-day life, we have a model (an expanding homogeneous and isotropic universe) and we can observe some things and infer others. > > (especially considering the > > fact that the redshift has a non-cosmological component as well which at > > low redshift might not be negligible. However, in general NONE of your > > equations is correct. > This are not my equations.
> See for example the book "Astronomy and Cosmology" by Fred Hoyle > page 617 which discusses the relation between redshift z and distance d Yes, but it is JUST AN APPROXIMATION FOR REDSHIFT. Even if it's Fred's and not yours, it's still just an approximation. > > No; the cosmological redshift is caused by the expansion of space. > > However, in addition, the galaxy can be moving through space, which also > > produces a redshift (or blueshift). > Okay. > The real issue is how typical is your example of 600 km/s. > If you go towards larger distances could this typical value not be much > larger > implying that contamination increases with distance ? (in time) It might have been larger in the past, but at large redshift the RELATIVE contribution is much less. (If that weren't the case, then the framework of a universe which is homogeneous and isotropic at large scales wouldn't be valid.) > Secondly how do you measure this so called contamination ? All we measure is the redshift; we don't know, without further assumptions, how much of it is cosmological and how much due to peculiar motion. > >> > Assuming we know the Hubble constant, and we know the distance, then we > >> > get the velocity NOW. > >> Does that mean that the speed of 7772 km/sec is the present speed NOW ? > > > > Yes, if a) we are talking about the proper distance now and its > > derivative with respect to cosmic time as measured now and b) if this > > redshift is due only to the cosmological redshift. > > And what is the verdict ? > Are both a and b correct ? A is something we can choose to talk about. B is an approximation which is not very useful at low redshift.
next posting Mesgbb

Mesgbb neophyte question about hubble's law

Van: Nicolaas Vroom
Onderwerp: neophyte question about hubble's law
Datum: zondag 27 december 2009 21:45 "Phillip Helbig---undress to reply" schreef in bericht
posting Mesgba... > In article posting Mesgb9, "Nicolaas Vroom" > writes: > >> It is 100 % physics. >> The question is: Is this law a correct description >> of the physical reality ? >> The physical reality being the state over there now. >> Not the state over there in the past which we can observe. > > We can only observe what is happening around us now. > However, if we talk about cosmology > the way we talk about day-to-day life, we have a model (an expanding > homogeneous and isotropic universe) and we can observe some things and > infer others. Correct The issue is between observe and infer. IMO the law V = H * D is inferred assuming you mean proper speed and proper distances. See below. >> The real issue is how typical is your example of 600 km/s. >> If you go towards larger distances could this typical value not be much >> larger >> implying that contamination increases with distance ? (in time) > > It might have been larger in the past, I agree and most probably it is. This is in line which the concept of the Big Bang which allows for much higher speeds in the past compared to the present. > but at large redshift the > RELATIVE contribution is much less. (If that weren't the case, then the > framework of a universe which is homogeneous and isotropic at large > scales wouldn't be valid.) > >> Secondly how do you measure this so called contamination ? > > All we measure is the redshift; we don't know, without further > assumptions, how much of it is cosmological and how much due to peculiar > motion. That is correct. >> >> > Assuming we know the Hubble constant, and we know >> >> > the distance, then we get the velocity NOW >> >> Does that mean that the speed of 7772 km/sec >> >> is the present speed NOW ? >> > Yes, if a) we are talking about the proper distance now and its >> > derivative with respect to cosmic time as measured now and >> > b) if this redshift is due only to the cosmological redshift. >> >> And what is the verdict ? >> Are both a and b correct ? > > A is something we can choose to talk about. B is an approximation which > is not very useful at low redshift. I want to talk about this. As I already remarked there are two Hubble Laws. The first Hubble Law establishes a relation between redshift z and distance d with d the distance between the observer and the Galaxy in the past. This Law is expressed as z = (H/c) * d (1) d can be measured as parallax distance or luminosity distance. If I'am correct than we can measure d for the following galaxies: M31, NGC 4258 (z=0.002), UGC 3789 (z=0.011) and NGC 6323 (z =0.026) Using that information we can find the relation H/c. Using the equation v = z * c (2) and by multiplying both sides of (1) with c we get the second Hubble's law: v = H * d (3) There is also a second version of this law: V = H * D (4) In equation (3) v and d are the past speed and the past distance. In equation( 4) V and D are the present speed and the proper distance Equation (4) is the equation that is used to calculate the "proper speed" of 7772 km/sec of NGC 6323 Equation (2) is standard used to calculate Galaxy rotation curves by observing the red shift value z. For example of M31 It is important to note that in this case v represents the past speed of a certain region of M31. In equation (3) the v is also past speed. The question is what does it physical represents ? In equation (4) the speed and the distance represent the speed and the distance of the Galaxy NOW.. But here we have a new problem: Is the relation between in equation 3 and 4 the same ? Assuming that the realation is linear, the problem is: Is the Hubble constant H in both equations the same ? I have great doubts. To read more see: http://users.telenet.be/nicvroom/Hubble-Faq-PH.htm There are two problems: 1) first neither V nor D (present values) can be measured directly. 2) What is the physical meaning of z. Let us go back to equation (1) what does the measured value in z represent ? Exactly as written above: > All we measure is the redshift; we don't know, without further > assumptions, how much of it is cosmological and how much due to peculiar > motion. That means z represents partly the peculiar velocity of the Galaxy in the past and partly space expansion. What the ratio is between those two numbers I do not know. For M31 this can easily be 90 to 10 For NGC 6323 for example 30 to 70 The important question to answer is: what is the evolution of the speed of a Galaxy after emission of light that we can observe now ? IMO assuming a big bang (which implies space expansion) the speed became smaller in time and the distance between the galaxies became larger. However the frequency of the light changed and became larger. This increase, observed as a redshift is only a function of distance (travel time) but does not reflect the actual speed of the Galaxy. IMO there is not something like: "action at a distance involved" which allows me to calculate the present speeds based on redshifts. As such I disagree with the idea that 7772 km/sec is the proper speed of NGC 6323. This picture is in line with the idea that the Universe is homogeneous. Nicolaas Vroom
http://users.telenet.be/nicvroom/
next posting Mesgbc

Mesgbc neophyte question about hubble's law

Van: Phillip Helbig
Onderwerp: neophyte question about hubble's law
Datum: dinsdag 29 december 2009 20:54 In article
posting Mesgbb, "Nicolaas Vroom" writes: > > We can only observe what is happening around us now. > > However, if we talk about cosmology > > the way we talk about day-to-day life, we have a model (an expanding > > homogeneous and isotropic universe) and we can observe some things and > > infer others. > > Correct > The issue is between observe and infer. > IMO the law V = H * D is inferred assuming you mean proper speed > and proper distances. See below. Yes. However, it follows trivially from isotropic and homogeneous expansion. If you don't grant that, then much of standard cosmological theory isn't valid. (It's not a matter of belief; there is good observational evidence for homogeneous and isotropic expansion.) > >> >> > Assuming we know the Hubble constant, and we know > >> >> > the distance, then we get the velocity NOW > >> >> Does that mean that the speed of 7772 km/sec > >> >> is the present speed NOW ? > >> > Yes, if a) we are talking about the proper distance now and its > >> > derivative with respect to cosmic time as measured now and > >> > b) if this redshift is due only to the cosmological redshift. > >> > >> And what is the verdict ? > >> Are both a and b correct ? > > > > A is something we can choose to talk about. B is an approximation which > > is not very useful at low redshift. > > I want to talk about this. > > As I already remarked there are two Hubble Laws. > The first Hubble Law establishes a relation between redshift z > and distance d with d the distance between the observer > and the Galaxy in the past. > This Law is expressed as z = (H/c) * d (1) > d can be measured as parallax distance or luminosity distance.
> If I'am correct than we can measure d for the following galaxies: > M31, NGC 4258 (z=0.002), UGC 3789 (z=0.011) > and NGC 6323 (z =0.026) > Using that information we can find the relation H/c. Yes, but there is scatter because in practice only luminosity distances can be used at this distance, but the absolute luminosity is not precisely known. > Using the equation v = z * c (2) > and by multiplying both sides of (1) with c we get > the second Hubble's law: v = H * d (3) Yes, valid in the limit of low redshifts. > There is also a second version of this law: V = H * D (4) > In equation (3) v and d are the past speed and the past distance. NO. In (3) it is an approximation valid in the limit of low redshifts; at such low redshifts, any difference between various distances, or distance now and distance then, is lost in the uncertainties due to other factors. > In equation( 4) V and D are the present speed and the proper distance > Equation (4) is the equation that is used to calculate the > "proper speed" of 7772 km/sec of NGC 6323 Assuming that the redshift is purely cosmological. > Equation (2) is standard used to calculate Galaxy rotation curves > by observing the red shift value z. For example of M31 > It is important to note that in this case v represents the past > speed of a certain region of M31. This is not a cosmological redshift. Still, the equation is valid in the limit. Again, in this case it doesn't make any practical difference if you say it is the past speed or the present speed. Such distinctions are important only at large cosmological redshift. > In equation (3) the v is also past speed. The question is what > does it physical represents ? Velocity. What else? > In equation (4) the speed and the distance represent the speed > and the distance of the Galaxy NOW.. OK. > But here we have a new problem: > Is the relation between in equation 3 and 4 the same ? > Assuming that the realation is linear, the problem is: > Is the Hubble constant H in both equations the same ? Yes, it is the same number. However, it doesn't have the same "function" in that in one case it is an exact theoretical quantity and in the other it is a constant of proportionality in an approximately linear relation. > I have great doubts. > To read more see: > http://users.telenet.be/nicvroom/Hubble's Law part 2.htm > There are two problems: > 1) first neither V nor D (present values) can be measured directly.
> 2) What is the physical meaning of z. I think you're making a mountain out of a molehill. All you need to know is here:
@ARTICLE      {EHarrison93a,
               AUTHOR       = "Edward R. Harrison",
               TITLE        = "The Redshift-Distance and Velocity-Distance
                               Laws",
               JOURNAL      = APJ,
               YEAR         = "1993",
                 VOLUME       = "403",
                 NUMBER       = "1",
                 PAGES        = "28",
                 MONTH        = jan
              }
Please read it.
Created: 14 Oktober 2009
Modified 20 Februari 2016

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