Re: "Tired" light
From: Bjoern Feuerbacher (feuerbac_at_thphys.uni-heidelberg.de)
Date: 07/02/04
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Date: Fri, 02 Jul 2004 16:03:27 +0200
Marcel Luttgens wrote:
> Bjoern Feuerbacher <feuerbac@thphys.uni-heidelberg.de> wrote in message news:<cc0svv$7v9$1@news.urz.uni-heidelberg.de>...
>
>>Marcel Luttgens wrote:
>>
>>>Bjoern Feuerbacher <feuerbac@thphys.uni-heidelberg.de> wrote in message news:<cbu5re$jom$1@news.urz.uni-heidelberg.de>...
>>>
>>>
>>>
>>>>Marcel Luttgens wrote:
>>>>
>>>>
>>>>>In an expanding Euclidian universe, the reddening of light
>>>>>emitted at a distance d is given by
>>>>>
>>>>>z = d/(Ro - d), with Ro = c/Ho.
>>>>
>>>>Where did you get this from?
>>>
>>>
>>>It is based on an observable universe of radius c/Ho
>>
>>Why do you think this is the "observable universe radius"?
>
>
> Because nothing physically goes faster than light.
Apparent motion due to the expansion of space *can* be faster
than light. I.e. we can see the light from things which
are *now* *more* light years away from us than the age of the universe
in years.
>>>and the Gauss theorem.
>>
>>How do you use Gauss's theorem to obtain this?
>>
>
>
> It is explained in
> http://perso.wanadoo.fr/mluttgens/Universe.htm
Your sentence "We isolate from this universe a sphere of radius R.",
directly before you inoke Gauss, makes little sense. Such a calculation
would give a gravitational field of the universe which strength
depends on the distance to the center of this sphere. However, this
contradicts your earlier claim that you talk about a homogeneous
and isotropic universe!
>>>>Observations show that red shift z is proportional to distance d
>>>>(neglecting the accelerated expansion here - you also don't
>>>>mention it). Your formula contradicts that.
>>>>
>>>
>>>
>>>It is proportional to distance for small recession velocities.
>>
>>The red shift has been measured for distances which are *not* small
>>comparable to Ro. So the deviations from proportionality
>>predicted by your formula should be quite clear. Hint: this is not
>>observed.
>>
>
>
> Could you give some clear examples?
Are you familiar with the supernova results?
>>>As v = Ho*d, the above formula can be written z = v/(c-v), whose
>>>results are close to those given by the classical formula
>>>z = v/c when v is small.
>>>But you recognize this (more or less) in your last message.
>>
>>You ignore everything else I wrote in that follow-up message, I see.
>>
>>
>>
>>>Otoh, the accelerated expansion is a mere hypothesis.
>>
>>A hypothesis which is consistent with multiple lines of evidence: not only
>>the supernova observations, but also observations of the X-ray radiation
>>of gas clouds recently, and the results of the WMAP satellite.
>>
>
>
> It is still a hypothesis.
And what is *your* stuff, if not a hypothesis?
[snip]
>>>Steve Weinberg claimed it.
>>
>>1) Steven Weinberg works in particle physics, not in cosmology.
[snip]
> This is a poor argument.
Not at all. There is a *reason* for specialization in science.
>>>>>This is wrong, because expansion is symmetrical. In other words,
>>>>>a clock situated on a galaxy moving at v from an observer
>>>>>situated on Earth will show the same time as an Earth clock,
>>>>>which moves at -v wrt the galaxy. The two relativistic effects
>>>>>cancel each other.
>>>>
>>>>This makes no sense at all. How could the two effects cancel???
>>>>
>>>>Try reading up on the "twin paradox".
>>>
>>>
>>>This has nothing to do with the twin paradox.
>>
>>It is quite similar to it: in both, there is a relative motion
>>between two observers.
>
>
> Motion is always relative.
Agreed. But try to tell that to Ken Seto in sci.physics. ;-)
> A "triplets" thought experiment is much more similar:
> Terence sits at home on Earth. Galaxy (yes, it's her name)
> flies off in a space ship at a velocity v/2. Simultaneously,
> Terra (also a name) flies off in the opposite direction at -v/2.
> After a while, Terra, a SR adept who considers that Galaxy
> flies away from her at a velocity v, claims that Galaxy is now
> younger than her, exactly like the GRists claim that time
> goes slower on SN because of space expansion.
No, that is *not* exactly like the claims in GR. A crucial difference is
that the time dilation in GR has nothing to do with the relative motion.
> According to Terence, both Terra and the GRists are wrong,
> because Terra's clock and Galaxy's clock tick at the same
> rate.
> Are you claiming that Terence is wrong? Then why?
I am claiming that you confuse the time dilation in SR with
the cosmological time dilation.
>>But OTOH, you are right, since the cosmological red shift has nothing
>>to do with time dilation due to the relative motion.
>>
>
>
> You seem to imply that expansion has no effect on SN clocks.
I imply nothing like that. I only point out that the time dilation
due to expansion has nothing to do with the SR time dilation due
to relative motion.
[snip]
>>You confuse apparent motion due to the expansion of space with real motiom.
>>
>
>
> This is new for me.
Yes, that's quite apparent.
> Are you claiming that galaxies are physically at rest
> wrt each other?
Depends on what you mean by "physically" at rest.
They do not move wrt each other - that the distance between them
increases is due to the fact that the space between them expands.
> Then how do you explain the observed redshift?
By the expansion of space. That's the *standard* explanation in cosmology.
> Is it due to an apparent, no a real, motion?
It is due to the expansion of space.
>> > If SR claims that this is irrelevant, SR is simply physically
>> > wrong, because Terence's clock and Stella's clock will tick at the
>> > same rate.
>>
>>SR has nothing to do with the expansion of the universe.
>>
>
>
> Could you elaborate?
The expansion of the universe is described by General, not Special
Relativity, if you haven't noticed.
[snip]
>>I wanted to have a reference for your assertion that GR ignores this
>>gravitational time dilation. (you are quite wrong on this: one can argue
>>that the cosmological red shift *is* essentially due to such a
>>gravitational time dilation)
>>
>
>
> My point was that the gravitational field of the universe (whose
> expression is the negative acceleration a_H) affects the frequency
> of light, and could be the only significant factor in a stable
> universe.
And my point was already in my first post in this thread that for
a homogenous, isotropic universe, the gravitational field is zero.
Do you know how the following equation for the gravitational potential Phi?
Laplace Phi = 4 pi G rho
("Laplace" denotes the Laplace operator, G is the gravitational
constant, and rho the mass density)
If yes, do you also know that the gravitational field strength is
the gradient of Phi?
If yes, do you also know how to solve this differential equation?
If you know all of that, then please solve the equation for a
homogeneous universe (i.e. rho = const.)
>>>Let's appreciate the great insight of Weinberg.
>>
>>That's by no means a new insight. And Weinberg is simply wrong in saying
>>that the red shift in cosmology is interpreted as being due to the
>>Doppler effect. As I said, he works in particle physics, not cosmology.
>>
>
>
> And his book, "Gravitation and Cosmology" is no more than gibberish.
Nice straw man. I said nothing like that.
However, why don't you try reading what *real* cosmologists have to say
about that topic? I recommend "The early universe" by Kolb & Turner,
two well-known cosmologists.
>>>>And: What do you even mean by "gravitational field of the universe"? The
>>>>local gravitational acceleration? If yes: In a homogenous, isotropic
>>>>universe, the local gravitational acceleration is zero everywhere.
>>>>
>>>
>>>
>>>You should ask Steven Weinberg.
>>
>>Why don't you?
>>
>
>
> Why should the gravitational acceleration be locally zero?
Do the calculation above.
> What is the limit of "local"?
If the universe is *totally* homogeneous, the gravitational field
*everywhere* is *exactly* zero. Do the calculation above.
[snip]
>>>Note that some of your buddies call now H a parameter,
>>
>>"now"? AFAIK, this wording has been in use for decades.
>>
>>
>>
>>>implying that it is no more a constant.
>>
>>It is *spatially* constant, but varies with time. That's why many
>>people (including me) think that it is unfortunate to call it a constant.
>>
>
>
> Are you referring to inflation?
> Or to the hypothetic acceleration of the space expansion.
No, not at all! Why on earth do you think so???
During quite ordinary expansion, H changes with time!
> Then, how can you claim that we know the
> exact (almost exact?) value of Ho?
Err, I did claim nowhere that we know the "exact" or "almost exact"
value. Only that the value you propose is ruled out.
Ever heard of the WMAP satellite?
<http://lambda.gsfc.nasa.gov/product/map/pub_papers/firstyear/parameters/wmap_parameters.pdf>
> Does it correspond to exactly 13.7 Gly?
Obviously not "exactly" - but the margin of error is quite small.
See the paper above.
>>>As for me, it is still a cosmological constant.
>>
>>You *do* know that the term "cosmological constant" refers to something
>>other than the Hubble constant / parameter, don't you?
>>
>
>
> We weren't argueing about dark energy.
Well, you brought up the term "cosmological constant", which
is a term with a well-defined meaning.
>>And I notice that you ignored my point above that that value for H is
>>now ruled out by observations.
>>
>
>
> Which value for H are you referring to?
You quoted from John D. Anderson and al. (arXiv: gr- qc/ 0104064 19
April 2001):
"As a number of people have noted, a_H = cH, or 8E-8 cm/s2 if
H=82 km/s/Mpc."
That value is ruled out (at almost 3 standard deviations). See the paper
above.
[snip]
>>>>>By replacing g by cHo in the formula d = (c^2/g) * z/(1+z),
>>>>
>>>>Why should one do that?
>>>>
>>>
>>>
>>>The light sent vertically from the ground reddens according
>>>to z = gd/(c^2-gd),
>>
>>Approximately, since g depends on d itself!
>>
>
>
> I clearly postulated that g is constant.
That postulate makes little sense for any actual physical situation.
>>>where g is a negative acceleration.
>>>a_H is also a negative acceleration.
>>
>>Huh? Why do you think so?
Hello?
>>>The formula is general,
>>
>>No, it isn't. It was derived for the special case of g = constant,
>>for starters.
>>
>
>
> Do you imply that a_H is not constant?
No, I question why on earth one should insert a_H here.
And I pointed out that your formula is *not* "general". That has
little to do with the question if a_H is constant or not.
> Of course, you are free to claim it.
Nice straw man again.
>>>hence it can be rightly used with a_H = cHo.
>>
>>You still did not tell me why one should insert that.
>>
>
>
> Do you know the meaning of "mutatis mutandis"?
Essentially it means here that you have made "necessary changes".
<http://dictionary.reference.com/search?q=mutatis%20mutandis>
That does not answer my question *why* one should make
these changes.
[snip]
>>>I did. The SNe Ia data can be analysed in terms of my formula.
>>
>>Please present your analysis.
>>
>
>
> Please look at http://perso.wanadoo.fr/mluttgens/SNpaper2.htm
There, I don't see a graphic which directly compares z and d.
And the graphic you *do* present has no error bars on it, apparently
(or do the tiny little crosses represent the errors?), and only
the very last point seems to agree better with your model than
with the standard one (assuming here that you got the description
of the standard model right...) That does not look like compelling
evidence for your ideas.
> But I agree that many formulae could nicely fit such data,
> in vue of their high variability. This is one of the reason why
> I am so reluctant to accept hypothesis like those of "accelerated
> expansion", or "time slowing on SN".
As I mentioned above, there are other lines of evidence for the
accelerated expansion than only the supernovae.
> And unlike GRists, I don't claim that mere hypothesis are
> the ultimate Truth, or present them as such.
I am not aware of any "GRist" who presents the acceleration of
the universe as the ultimate Truth".
> I'm just trying to find alternatives to the present paradigm.
Since you don't even know that H changes even during the
quite ordinary expansion, that no inflation or accelerated
expansion is needed for that, I sincerely doubt that you
even understand the present paradigm.
[snip]
Bye,
Bjoern
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