| 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
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)
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
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]]
[[Mod. note --
>
> For much more (very clear and insightful) about what Hubble's law does
> and doesn't say, see
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.
Nicolaas Vroom
Back to my home page Contents of This Document
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
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*.
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,
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*.
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.
"Cosmology: The Science of the Universe", 2nd Edition
Cambridge U.P., 2000,
hardcover ISBN 0-521-66148-X
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
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
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
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
> 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]]
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]]
https://www.nicvroom.be/
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
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"
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]]
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
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.
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
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.
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
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:
|
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;
There are several obvious flaws with this argument:
|
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.
* 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:
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.
|
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.
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"
-- quote by Freire / poster by Oxfam
Mesgj neophyte question about hubble's law
Van: Oh No
Onderwerp: neophyte question about hubble's law
Datum: zondag 27 september 2009 11:30
In posting Mesgd "Phillip Helbig"
moderator sci.physics.foundations.
< charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and
braces)>
http://www.rqgravity.net
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"
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
https://www.nicvroom.be/
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
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
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:
[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.]
(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.
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.
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.
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.
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.
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"
"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
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"
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
https://www.nicvroom.be/
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"
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
https://www.nicvroom.be/
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
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
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.
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
> 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"
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
https://www.nicvroom.be/neophyte.htm
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
(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.
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
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
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.
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
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
>
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
https://www.nicvroom.be/
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
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.
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
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
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
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
As Mach said: "Every statement in physics has to state relations between
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"
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
https://www.nicvroom.be/neophyte.htm
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
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
>
Here we read:
Normally wavelength 3933A measured 4018A
z = (4018-3933)/3933= 65/3933= 0,0216
Next we read:
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:
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
https://www.nicvroom.be/neophyte.htm
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
> 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"
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:
https://www.nicvroom.be/Hubble-Faq.htm
Nicolaas Vroom
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"
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
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"
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
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"
> 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.
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"
>> 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: https://www.nicvroom.be/Hubble-Faq.htm
This document shows you the litterature where you can find
more detail information.
Nicolaas Vroom
https://www.nicvroom.be/
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"
> 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.
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"
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
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"
> 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.
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"
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.
https://www.nicvroom.be/Hubble-Faq.htm#ol9
See comments near Document 9
Nicolaas Vroom
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"
> >> 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.
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"
https://www.nicvroom.be/
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"
> 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:
> https://www.nicvroom.be/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