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neophyte question about hubble's law



 
 
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  #1  
Old September 17th 09, 03:32 AM posted to sci.astro.research
dfarr --at-- comcast --dot-- net
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Posts: 7
Default neophyte question about hubble's law

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...ission_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]]
  #2  
Old September 18th 09, 03:31 AM posted to sci.astro.research
Oh No
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Default neophyte question about hubble's law

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


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.


Basically we have general relativity, plus a bit of common sense.
General relativity is itself based on the common sense principle that
the laws of physics are locally the same everywhere, and if we can't be
sure of that principle we cannot be sure of anything.

Under the assumption that matter is reasonably uniformly distributed we
can solve the equations of general relativity, and show that if the
universe is expanding now, then it has always been expanding (since the
big bang).


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.".


Perhaps. It's just a pity Harrison's ideas about the expansion of space
time are somewhat inaccurate. I would recommend everyone should read
some real general relativity also.


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
  #3  
Old September 22nd 09, 03:43 AM posted to sci.astro.research
Phillip Helbig---remove CLOTHES to reply
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Posts: 198
Default neophyte question about hubble's law

In article
,
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.
  #4  
Old September 22nd 09, 03:44 AM posted to sci.astro.research
Phillip Helbig---remove CLOTHES to reply
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Posts: 198
Default neophyte question about hubble's law

In article , 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?
  #5  
Old September 23rd 09, 02:36 AM posted to sci.astro.research
Nicolaas Vroom
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Posts: 216
Default neophyte question about hubble's law

"dfarr --at-- comcast --dot-- net" schreef in bericht
...

[[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/
  #6  
Old September 24th 09, 02:49 AM posted to sci.astro.research
Phillip Helbig---remove CLOTHES to reply
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Posts: 198
Default neophyte question about hubble's law

In article , "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.)
  #7  
Old September 24th 09, 02:57 AM posted to sci.astro.research
Nicolaas Vroom
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Posts: 216
Default neophyte question about hubble's law

"Phillip Helbig---remove CLOTHES to reply"
schreef in bericht ...

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]]
  #8  
Old September 25th 09, 12:57 AM posted to sci.astro.research
Phillip Helbig---remove CLOTHES to reply
external usenet poster
 
Posts: 198
Default neophyte question about hubble's law

In article , "Nicolaas Vroom"
writes:

"Phillip Helbig---remove CLOTHES to reply"
schreef in bericht ...

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.
  #9  
Old September 25th 09, 01:04 AM posted to sci.astro.research
Jonathan Thornburg [remove -animal to reply][_3_]
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Posts: 137
Default neophyte question about hubble's law

Nicolaas Vroom wrote:
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/physic...erluminal.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 [remove -animal to reply]"
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
  #10  
Old October 20th 09, 08:59 AM posted to sci.astro.research
Thomas Smid
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Posts: 151
Default neophyte question about hubble's law

On 17 Sep, 02:32, 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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