neophyte question about hubble's law

In article , Nicolaas Vroom
writes:

I have no problem with the statement one can interpret
redshift as velocity.
IMO the issue is how.
The current point of view is that for values of z 1 one has to
use the equation v = c*z
(Also called the nonrelativistic equation for the Doppler shift)

You don't have to use it, but you CAN use it and get the same result as
a more detailed analysis.

I have a problem with that equation.
Suppose a galaxy at a far distance in the past is receding from us
with a speed of 0.01c resulting in a value of z of 0.01.
Light from that galaxy in an expanding universe is travelling towards
us at a speed c and is stretched.

OK.

Suppose we receive it now. Is it not possible in principle that we measure
a value of z=0.02 implying a speed of v=0.02*c ?

Why should that happen?

My point is what we measure is not the true speed of the source at the point
of emission. This speed is much lower because the waves are stretched.

Again, for the low redshifts at which one can use the Doppler formula,
the change in speed between the time of emission and the time of
absorption is negligible.

Even if we measure a z=2 it does not mean that the source in the past
was travelling at a speed higher than c.

But it could be.

The overall implication is that maybe there is no reason to
use the relativistic equation for the Doppler shift.

Right.

A second implication in principle is that the true speed, of a galaxy
with z=2 measured now here, could be zero over there.

Possible. But now you are in the high-redshift regime, where you can't
get a useful answer from the Doppler formula.

A third implication is that the size of the Observable Universe
is much smaller than 47 Gyr. See the posting by Hans Aberg.

I don't think that anyone claims that the size of the Observable
Universe is as large as 47 Gyr.

Phillip Helbig---remove CLOTHES to reply wrote:
But it is this redshift-velocity interpretation that results in speeds
exceeding c?

Yes, but that's not a problem.

But it would appear as though you arrive at two different, non-unified
concepts of velocity: of relativity which excludes FLT for massive
objects, and of universe expansion of unclear independent verification.

See
.....

@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
}

This one is avilable he
http://tinyurl.com/ygq7aq2
expanded might appear as broken:
http://articles.adsabs.harvard.edu/c...&clas sic=YES

It does not seem to discuss redshift-velocity formulas, though, only
formulas depending on distance.

And my guess there is no experimental verification of such a formula at
high speeds. Suppose a particle at speed close to c emits a photon, what
is the measured wavelength shift?

I'm sure this happens all the time in particle accelerators which
produce synchrotron radiation.

So how are the redshift-velocity formulas used in astronomy derived from
that?

Hans

On 22 Oct, 17:34, Stupendous_Man wrote:
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.../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.


If you had fully read the web reference http://home.pacbell.net/skeptica/edwinhubble.html
, you would have found there

"To the best of my knowledge Hubble’s 1929 paper (3) is the only
published paper where the reader is left with the view by Hubble, and
now apparently universally adopted, that the linear law of redshifts
applies only as a velocity-distance relation. It is no wonder that
this is the paper that is usually cited by itself in astronomy
textbooks."

and also the quote from Hubble's book 'The Realm of the Nebulae':

"Meanwhile, red-shifts may be expressed on a scale of velocities as a
matter of convenience. They behave as velocity-shifts behave and they
are very simply represented on the same familiar scale, regardless of
the ultimate interpretation. The term “apparent velocity” may be used
in carefully considered statements, and the adjective always implied
where it is omitted in general usage. --pp. 122-123"

As is well known, the interpretation of the redshift in terms of
velocities was already suggested much earlier by Vesto Slipher, and
even he considered other causes possible. I quote from from
http://en.wikipedia.org/wiki/Vesto_Slipher : Slipher first reports on
the making the first Doppler measurement on September 17, 1912 in The
radial velocity of the Andromeda Nebula in the inaugural volume of the
Lowell Observatory Bulletin, pp.2.56-2.57. In his report Slipher
writes: "The magnitude of this velocity, which is the greatest
hitherto observed, raises the question whether the velocity-like
displacement might not be due to some other cause, but I believe we
have at present no other interpretation for it."

Thomas

Hans Aberg wrote:
Phillip Helbig---remove CLOTHES to reply wrote:
But it is this redshift-velocity interpretation that results in speeds
exceeding c?
Yes, but that's not a problem.

But it would appear as though you arrive at two different, non-unified
concepts of velocity: of relativity which excludes FLT for massive
objects, and of universe expansion of unclear independent verification.

Actually, I noticed after making the post that
http://en.wikipedia.org/wiki/Redshif...nsion_of_space
indicates it is a metric expansion of space.

And my guess there is no experimental verification of such a formula at
high speeds. Suppose a particle at speed close to c emits a photon, what
is the measured wavelength shift?
I'm sure this happens all the time in particle accelerators which
produce synchrotron radiation.

So how are the redshift-velocity formulas used in astronomy derived from
that?

And the link above also indicates different redshift formulas, for
example, a gravitational one.

Hans

In article , Hans Aberg
writes:

Phillip Helbig---remove CLOTHES to reply wrote:
But it is this redshift-velocity interpretation that results in speeds
exceeding c?

Yes, but that's not a problem.

But it would appear as though you arrive at two different, non-unified
concepts of velocity: of relativity which excludes FLT for massive
objects,

Within the constraints of special relativity, i.e. Minkowski space-time.

And my guess there is no experimental verification of such a formula at
high speeds. Suppose a particle at speed close to c emits a photon, what
is the measured wavelength shift?

I'm sure this happens all the time in particle accelerators which
produce synchrotron radiation.

So how are the redshift-velocity formulas used in astronomy derived from
that?

There are two possibilities. One, imagine the expansion of space
stretching the wavelength of light. This provides a quantitatively
correct interpretation, and underlines the fact that, without further
assumptions, the redshift tells us ONLY the ratio of the scale factor of
the universe now to that at the time the light was emitted. Or, do the
the full-scale GR derivation, which is too much for a usenet post.

On 25 Oct, 09:36, Thomas Smid wrote:

As is well known, the interpretation of the redshift in terms of
velocities was already suggested much earlier by Vesto Slipher, and
even he considered other causes possible. I quote from fromhttp://en.wikipedia.org/wiki/Vesto_Slipher: Slipher first reports on
the making the first Doppler measurement on September 17, 1912 in The
radial velocity of the Andromeda Nebula in the inaugural volume of the
Lowell Observatory Bulletin, pp.2.56-2.57. In his report Slipher
writes: "The magnitude of this velocity, which is the greatest
hitherto observed, raises the question whether the velocity-like
displacement might not be due to some other cause, but I believe we
have at present no other interpretation for it."

Thomas

Just an add-on to this: in case of the Andromeda Nebula (and some
other close galaxies), the line shift is of course actually a blue-
shift, and thus is not representable by the Hubble law. But
nonetheless, it seems that both Slipher and Hubble potentially
questioned the interpretation of all these shifts in terms of radial
velocities.

Thomas

In article , Thomas Smid
writes:

Just an add-on to this: in case of the Andromeda Nebula (and some
other close galaxies), the line shift is of course actually a blue-
shift, and thus is not representable by the Hubble law. But
nonetheless, it seems that both Slipher and Hubble potentially
questioned the interpretation of all these shifts in terms of radial
velocities.

Hubble did so, IIRC, due to a misunderstanding of the K-correction.
Alan Sandage detailed this in his Saas-Fee lecture notes from the 1993
summer school. I have the proceedings, but haven't unpacked them since
having recently moved house, but if there is interest I can post a short
summary in a couple of weeks. Sandage, of course, was Hubble's
assistant (at the same time that he was Baade's doctoral student).
Ironically, when Hubble died he probably doubted that the expansion of
the universe was real.

(A note of caution to readers of historical literatu the modern
definitions are often different than the ones used by authors in the
first half of the 20th century. Caveat lector! (A similar situation
exists with regard to 17th- and 18th-century physics literature.))

Phillip Helbig---remove CLOTHES to reply wrote:
But it is this redshift-velocity interpretation that results in speeds
exceeding c?
Yes, but that's not a problem.
But it would appear as though you arrive at two different, non-unified
concepts of velocity: of relativity which excludes FLT for massive
objects,

Within the constraints of special relativity, i.e. Minkowski space-time.

As in the link of my other post (which probably did not appear before
you made your post), it is called metric expansion of space.
http://en.wikipedia.org/wiki/Redshift
This is perfectly logical, but falls in the same category as for example
MOND: one has incomplete information as to why something is happening,
and adjust the questions so that known data fits. This is the problem of
retrodiction without genuine prediction.

And my guess there is no experimental verification of such a formula at
high speeds. Suppose a particle at speed close to c emits a photon, what
is the measured wavelength shift?
I'm sure this happens all the time in particle accelerators which
produce synchrotron radiation.
So how are the redshift-velocity formulas used in astronomy derived from
that?

There are two possibilities. One, imagine the expansion of space
stretching the wavelength of light. This provides a quantitatively
correct interpretation, and underlines the fact that, without further
assumptions, the redshift tells us ONLY the ratio of the scale factor of
the universe now to that at the time the light was emitted. Or, do the
the full-scale GR derivation, which is too much for a usenet post.

Metric expansion is easy to understand from the mathematical point of
view: just let it depend on time, as here
http://en.wikipedia.org/wiki/FLRW_metric
But then sort of the very physics principles change, because the metric
is not a directly measurable quantity, but an intermediate used to
explain connections between physically measurable quantities.

I like the gravitational redshift more. If background radiation is
produced by black hole, that would explain the heavy redshift there. I
think such possibilities were in the past excluded because the GR
predict the universe cannot be stable. But at that time, one did not
know that only a small fraction of the mass is not visible.

Hans

In article , Hans Aberg
writes:

As in the link of my other post (which probably did not appear before
you made your post), it is called metric expansion of space.
http://en.wikipedia.org/wiki/Redshift
This is perfectly logical, but falls in the same category as for example
MOND: one has incomplete information as to why something is happening,
and adjust the questions so that known data fits. This is the problem of
retrodiction without genuine prediction.

Historically, it was exactly the opposite. The universe was believed to
be static, Einstein saw that this was not what his theory predicted. So
first the expanding space, and then, more than 10 years later, the
observations to support it. (Einstein modified his theory---not just by
introducing the cosmological constant, which someone else might have
included from the outset, but by introducing it with a special, finely
tuned value (the distinction is important)---to allow a static universe,
but disowned this when observations confirmed his original prediction.

I like the gravitational redshift more. If background radiation is
produced by black hole, that would explain the heavy redshift there.

It's not just the redshift. Cosmology, today, is a data-driven science.
(This wasn't the case until quite recently.) There are a lot of data to
explain. Any alternative theory has to explain the CMB observations in
detail.

Phillip Helbig---remove CLOTHES to reply wrote:
As in the link of my other post (which probably did not appear before
you made your post), it is called metric expansion of space.
http://en.wikipedia.org/wiki/Redshift
This is perfectly logical, but falls in the same category as for example
MOND: one has incomplete information as to why something is happening,
and adjust the questions so that known data fits. This is the problem of
retrodiction without genuine prediction.

Historically, it was exactly the opposite. The universe was believed to
be static, Einstein saw that this was not what his theory predicted. So
first the expanding space, and then, more than 10 years later, the
observations to support it.

Did his analysis say that the universe must have a metric expansion? - I
thought it just said that it could not be stable.

I like the gravitational redshift more. If background radiation is
produced by black holes, that would explain the heavy redshift there.

It's not just the redshift. Cosmology, today, is a data-driven science.
(This wasn't the case until quite recently.) There are a lot of data to
explain. Any alternative theory has to explain the CMB observations in
detail.

The problem is that there is only retrofitting of data, and the theory
seems designed so that it can't be refuted. For example, HE 1523-0901 is
a 13.2 Gy old generation two star in the Milky Way and the BB universe
13.73 Gy. Suppose one would find a star older that this theoretical age,
would the BB theory be judged wrong and scrapped? If not, what is the
litmus test of this theory?

Hans