Question about dark matter, dark energy, and MOND

Max

For Modified Newtonian Dynamics proposed by Milgrom, there is a
deviation from the classical gravitational force (or a deviation from
the classical Newtonian dynamics, depending on your perspective) once
the acceleration of a particle is less than a0. Thus one can account
for the star motion on the outer fringe of galaxies without invoking
dark matter.

One interesting observation is that a0 is also the acceleration scale
where dark energy should play a significant role. My question is why
dark energy is not usually considered for intra-galactic (non-
cosmological) dynamics with less-than-a0 acceleration? Would dark
energy change the predictions of MOND?

-Max

eric gisse

On Sun, 14 Dec 2008 21:59:07 +0100 (CET), Max <awsg02@yahoo.com>
wrote:

>For Modified Newtonian Dynamics proposed by Milgrom, there is a
>deviation from the classical gravitational force (or a deviation from
>the classical Newtonian dynamics, depending on your perspective) once
>the acceleration of a particle is less than a0. Thus one can account
>for the star motion on the outer fringe of galaxies without invoking
>dark matter.

But not in galaxies involved in collisions, eg: Bullet cluster, MACS
J0025.4-1222.

>
>One interesting observation is that a0 is also the acceleration scale
>where dark energy should play a significant role. My question is why
>dark energy is not usually considered for intra-galactic (non-
>cosmological) dynamics with less-than-a0 acceleration? Would dark
>energy change the predictions of MOND?
>
>-Max

You ask the question as if the answer matters.

Putting aside the arguments about MOND's applicability in galaxies,
MOND does not explain the large scale structure of the universe in
either the structuring of matter [eg, results from the Millenium
simulation...] or the power spectrum of the CMB.

My explanation is that dark energy is not usually considered since the
proponents of MOND seem to reject not just dark matter but dark
energy, and it would be ... odd ... to include dark energy.

Regardless, returning to dark energy. It does not appear to have any
influence on galaxies as the galaxies are gravitationally bound and
dark energy isn't strong enough to have a significant impact.

Max

On Dec 15, 12:26 pm, eric gisse <jowr.pi.nos...@gmail.com> wrote:
> Regardless, returning to dark energy. It does not appear to have any
> influence on galaxies as the galaxies are gravitationally bound and
> dark energy isn't strong enough to have a significant impact.

On the outer fringe of a galaxy, the negative pressure of dark energy
(cosmological constant, quintessence, or whatever it's called) would
be larger than the gravitational pull (MOND or Newtonian). And stars
would be unbound, zipping away from the galaxy. It is certainly not
what is observed. The absence of dark energy for intra-galaxy dynamics
is so baffling for me.

[[Mod. note -- While in widespread use, the term "dark energy" is
somewhat unfortunate; I have seen several suggestions that "smooth
tension" would be a more suitable term. With that in mind, the key
point is that this "smooth tension" is *smooth* and is only significant
on a cosmological scale -- an individual galaxy (or matter on its
outskirts) isn't going to feel any significant forces from the smooth
tension.
-- jt]]

Phillip Helbig---remove CLOTHES to reply

In article <gi3s2r$3ev$1@fb07-hees.theo.physik.uni-giessen.de>, Max
<awsg02@yahoo.com> writes:

> For Modified Newtonian Dynamics proposed by Milgrom, there is a
> deviation from the classical gravitational force (or a deviation from
> the classical Newtonian dynamics, depending on your perspective) once
> the acceleration of a particle is less than a0. Thus one can account
> for the star motion on the outer fringe of galaxies without invoking
> dark matter.
>
> One interesting observation is that a0 is also the acceleration scale
> where dark energy should play a significant role. My question is why
> dark energy is not usually considered for intra-galactic (non-
> cosmological) dynamics with less-than-a0 acceleration? Would dark
> energy change the predictions of MOND?

Reason 1: The motivation for MOND was to avoid dark matter. Not
believing in dark matter but believing in dark energy is not really an
option for someone who doesn't want to believe in dark matter. Reason
2: No-one knows what dark energy is or what its behaviour is like in
detail, especially at small (intra-galactic) scales. (We see
unambiguous effects only at large (cosmological) scales.) Reason 3:
MOND was developed mainly to explain the observed flat rotation curves
of galaxies, which is not what one expects if the mass distribution
follows the light distribution. At least phenomenologically, MOND
offers a simple explanation. An explanation involving dark energy would
be contrived.

Phillip Helbig---remove CLOTHES to reply

In article <gi640m$8pm$1@fb07-hees.theo.physik.uni-giessen.de>, eric
gisse <jowr.pi.nospam@gmail.com> writes:

> On Sun, 14 Dec 2008 21:59:07 +0100 (CET), Max <awsg02@yahoo.com>
> wrote:
>
> Putting aside the arguments about MOND's applicability in galaxies,
> MOND does not explain the large scale structure of the universe in
> either the structuring of matter [eg, results from the Millenium
> simulation...] or the power spectrum of the CMB.

True, in a sense. On the other hand, I think it's fair to say that we
don't know if that statement would still hold if the same amount of
research was put into MOND as has been put into "standard" cosmology.

> My explanation is that dark energy is not usually considered since the
> proponents of MOND seem to reject not just dark matter but dark
> energy, and it would be ... odd ... to include dark energy.

Right; that's probably the main reason.

> Regardless, returning to dark energy. It does not appear to have any
> influence on galaxies as the galaxies are gravitationally bound and
> dark energy isn't strong enough to have a significant impact.

In the conventional form (and there is no evidence that this is not the
case), dark energy is distributed completely smoothly. In that case, I
believe (someone correct me if I am wrong) that the only effect it could
have would be to change the size of a gravitationally bound structure.
To notice this, we would have to compare expected and observed sizes,
probably at a variety of scales. The data are too messy to note any
deviations, at least for the value of the cosmological constant which we
believe we know from cosmological observations.

Melroy

See also http://arxiv.org/abs/0809.1183 which explains
how dark energy, why MOND acceleration ~ Hubble scale
and other mysteries can be explained by backreaction,
On Dec 15, 4:14 pm, hel...@astro.multiCLOTHESvax.de (Phillip Helbig---
remove CLOTHES to reply) wrote:
> In article <gi3s2r$3e...@fb07-hees.theo.physik.uni-giessen.de>, Max
>
> <aws...@yahoo.com> writes:
> > For Modified Newtonian Dynamics proposed by Milgrom, there is a
> > deviation from the classical gravitational force (or a deviation from
> > the classical Newtonian dynamics, depending on your perspective) once
> > the acceleration of a particle is less than a0. Thus one can account
> > for the star motion on the outer fringe of galaxies without invoking
> > dark matter.
>
> > One interesting observation is that a0 is also the acceleration scale
> > where dark energy should play a significant role. My question is why
> > dark energy is not usually considered for intra-galactic (non-
> > cosmological) dynamics with less-than-a0 acceleration? Would dark
> > energy change the predictions of MOND?
>
> Reason 1: The motivation for MOND was to avoid dark matter. Not
> believing in dark matter but believing in dark energy is not really an
> option for someone who doesn't want to believe in dark matter. Reason
> 2: No-one knows what dark energy is or what its behaviour is like in
> detail, especially at small (intra-galactic) scales. (We see
> unambiguous effects only at large (cosmological) scales.) Reason 3:
> MOND was developed mainly to explain the observed flat rotation curves
> of galaxies, which is not what one expects if the mass distribution
> follows the light distribution. At least phenomenologically, MOND
> offers a simple explanation. An explanation involving dark energy would
> be contrived.

juanREMOVE@canonicalscience.com

Max wrote on Sun, 14 Dec 2008 21:59:07 +0100:

> For Modified Newtonian Dynamics proposed by Milgrom, there is a
> deviation from the classical gravitational force (or a deviation from
> the classical Newtonian dynamics, depending on your perspective)

The pictures are equivalent regarding usual MOND applications.

> once
> the acceleration of a particle is less than a0. Thus one can account
> for the star motion on the outer fringe of galaxies without invoking
> dark matter.

MOND also is useful for some aspects of cosmology. E.g. WMAP data
(second peak?) was predicted by one MOND theoretician before
observation, whereas dark matter theorists were forced to change their
wrong initial estimations once the data was known.

You may find histories as this and lot of other useful information in

http://www.astro.umd.edu/~ssm/mond/

> One interesting observation is that a0 is also the acceleration scale
> where dark energy should play a significant role.

Hum, not still sure about this.

> My question is why
> dark energy is not usually considered for intra-galactic (non-
> cosmological) dynamics with less-than-a0 acceleration?

Because due to small value of cosmological constant the effects can be
ignored for both regimes a >> a0 and a << a0.

> Would dark energy
> change the predictions of MOND?

Yes.

Notice that MOND reduces to Newtonian theory for a >> a0. However, the
presence of a cosmological constant changes the equations, for example
the Poisson equation for the Newtonian potential may be modified in
presence of cosmological constant.

MOND was not designed for cosmology but for galactic scale.

--
http://www.canonicalscience.org/

Usenet Guidelines:
http://www.canonicalscience.org/en/m...uidelines.html

Phillip Helbig---remove CLOTHES to reply

In article <pan.2008.12.20.12.47.31@canonicalscience.com>,
juanREMOVE@canonicalscience.com writes:

> MOND also is useful for some aspects of cosmology. E.g. WMAP data
> (second peak?) was predicted by one MOND theoretician before
> observation, whereas dark matter theorists were forced to change their
> wrong initial estimations once the data was known.

I think this is stretching it a bit. Yes, on the surface the claim is
true. However, IIRC there was no "MOND cosmology" involved in
calculating the power spectrum, but rather a standard calculation was
done using a value for Omega consistent with what MOND folks believe
(i.e. no dark matter). Whether one NEEDS MOND cosmology to do a
comparison between CMB theory and observations is a different question.
My point is simply that the "predicted" power spectrum (i.e. the second
peak) is a standard one---there are so many parameters that there is
more than one way to get it---which happens to agree with MOND in a
superficial sense.

It's also too harsh to say "dark matter theorists were forced to change
their wrong initial estimations once the data was known". This is
almost a definition of good (perhaps not canonical :-) ) science: revise
one's view of the world as more and better data come in. Yes, the WMAP
data aren't compatible with cardboard models, but it wasn't like
everyone (except MOND folks) believed these cardboard models, then WMAP
caused "the cosmology textbooks to be rewritten". One CAN explain WMAP
within the context of standard cosmology. Yes, the reason for this is
that there are many parameters one can adjust (the fact that MOND just
has one parameter makes it a good theory, since it makes clear
predictions), but the parameter combination which agrees with WMAP also
agrees with essentially everything else.

> You may find histories as this and lot of other useful information in
>
> http://www.astro.umd.edu/~ssm/mond/

Yes, a good source of information.

juanREMOVE@canonicalscience.com

Max wrote on Sun, 14 Dec 2008 21:59:07 +0100:

> For Modified Newtonian Dynamics proposed by Milgrom, there is a
> deviation from the classical gravitational force (or a deviation from
> the classical Newtonian dynamics, depending on your perspective)

The pictures are equivalent regarding usual MOND applications.

> once
> the acceleration of a particle is less than a0. Thus one can account
> for the star motion on the outer fringe of galaxies without invoking
> dark matter.

MOND also is useful for some aspects of cosmology. E.g. WMAP data
(second peak?) was predicted by one MOND theoretician before
observation, whereas dark matter theorists were forced to change their
wrong initial estimations once the data was known.

You may find histories as this and lot of other useful information in

http://www.astro.umd.edu/~ssm/mond/

> One interesting observation is that a0 is also the acceleration scale
> where dark energy should play a significant role.

Hum, not still sure about this.

> My question is why
> dark energy is not usually considered for intra-galactic (non-
> cosmological) dynamics with less-than-a0 acceleration?

Because due to small value of cosmological constant the effects can be
ignored for both regimes a >> a0 and a << a0.

> Would dark energy
> change the predictions of MOND?

Yes.

Notice that MOND reduces to Newtonian theory for a >> a0. However, the
presence of a cosmological constant changes the equations, for example
the Poisson equation for the Newtonian potential may be modified in
presence of cosmological constant.

MOND was not designed for cosmology but for galactic scale.

--
http://www.canonicalscience.org/

Usenet Guidelines:
http://www.canonicalscience.org/en/m...uidelines.html

Juan R. González-Álvarez

Phillip Helbig---remove CLOTHES to reply wrote on Tue, 23 Dec 2008
11:37:28 +0100:

> In article <pan.2008.12.20.12.47.31@canonicalscience.com>,
> juanREMOVE@canonicalscience.com writes:
>
>> MOND also is useful for some aspects of cosmology. E.g. WMAP data
>> (second peak?) was predicted by one MOND theoretician before
>> observation, whereas dark matter theorists were forced to change
>> their wrong initial estimations once the data was known.
>
> I think this is stretching it a bit. Yes, on the surface the claim is
> true. However, IIRC there was no "MOND cosmology" involved in
> calculating the power spectrum, but rather a standard calculation was
> done using a value for Omega consistent with what MOND folks believe
> (i.e. no dark matter).

You say is not accurate, lacking a satisfactory relativistic MOND
theory, the "MOND cosmological solution" was approximated by a
conventional model with no CDM. It was claimed this *approximated* MOND
would fail at some level but would be enough for an estimation for the
amplitude ratios of the peaks

http://xxx.lanl.gov/abs/astro-ph/9907409

http://xxx.lanl.gov/abs/astro-ph/0008188

> Whether one NEEDS MOND cosmology to do a comparison between CMB theory
> and observations is a different question. My point is simply that the
> "predicted" power spectrum (i.e. the second peak) is a standard
> one---there are so many parameters that there is more than one way to
> get it---which happens to agree with MOND in a superficial sense.
>
> It's also too harsh to say "dark matter theorists were forced to
> change their wrong initial estimations once the data was known".

A beatiful aspect of science is that claims and prediction are archived.

The *prediction* done by dark matter theorists using the 1999 Lambda-CDM
model is reproduced in the left image at

http://www.astro.umd.edu/~ssm/mond/CMB1.html

At the right you can see the prediction done by a MOND theorist using a
purely baryonic (MOND) model.

Both predictions were done before the data was known.

The data (green dots) showed excellent agreement with MOND theoreticians
prediction. That is already part of history of science :-)

Once the data was known, dark matter theoreticians changed the original
parameters in the Lambda-CDM model to *fit* the observed data.

Two essential remarks:

i)
MOND theoreticians predicted the right data, *before* observation. Dark
matter theoreticians prediction failed and now they only fit the right
data, *after* observation, using a many parameters model.

ii)
Why do dark matter theoreticians failed at their prediction? You seems
to ignore this important point.

Initially they used standard values for the parameters in the Lambda-CDM
model. After this failed they assumed nonstandard values for parameters
for fitting the observed data.

For instance, the barion density used in Lambda-CDM models is open to
objections (inconsistencies with densities given by other tests).

Whereas MOND solution uses a more consistent barion density.

http://xxx.lanl.gov/abs/astro-ph/0312570

Taking into account that MOND was *not* initially developed for
cosmology but for galactic phenomena, its partial aplicability to
cosmology is really surprising, at least for some of us.


--
http://www.canonicalscience.org/

Usenet Guidelines:
http://www.canonicalscience.org/en/m...uidelines.html

Phillip Helbig---remove CLOTHES to reply

In article <pan.2008.12.23.18.11.59@canonicalscience.com>, "Juan R.
González-Álvarez" <juanREMOVE@canonicalscience.com> writes:

> Phillip Helbig---remove CLOTHES to reply wrote on Tue, 23 Dec 2008
> 11:37:28 +0100:
>
> > In article <pan.2008.12.20.12.47.31@canonicalscience.com>,
> > juanREMOVE@canonicalscience.com writes:
> >
> >> MOND also is useful for some aspects of cosmology. E.g. WMAP data
> >> (second peak?) was predicted by one MOND theoretician before
> >> observation, whereas dark matter theorists were forced to change
> >> their wrong initial estimations once the data was known.
> >
> > I think this is stretching it a bit. Yes, on the surface the claim is
> > true. However, IIRC there was no "MOND cosmology" involved in
> > calculating the power spectrum, but rather a standard calculation was
> > done using a value for Omega consistent with what MOND folks believe
> > (i.e. no dark matter).
>
> You say is not accurate, lacking a satisfactory relativistic MOND
> theory, the "MOND cosmological solution" was approximated by a
> conventional model with no CDM. It was claimed this *approximated* MOND
> would fail at some level but would be enough for an estimation for the
> amplitude ratios of the peaks

I don't really see the difference between our two statements.

> > Whether one NEEDS MOND cosmology to do a comparison between CMB theory
> > and observations is a different question. My point is simply that the
> > "predicted" power spectrum (i.e. the second peak) is a standard
> > one---there are so many parameters that there is more than one way to
> > get it---which happens to agree with MOND in a superficial sense.
> >
> > It's also too harsh to say "dark matter theorists were forced to
> > change their wrong initial estimations once the data was known".
>
> A beatiful aspect of science is that claims and prediction are archived.
>
> The *prediction* done by dark matter theorists using the 1999 Lambda-CDM
> model is reproduced in the left image at

Yes, but even in 1999 there were many other predictions, namely for
different values of the parameters entering into the calculation, which
one can vary within the known uncertainties.

> http://www.astro.umd.edu/~ssm/mond/CMB1.html

Of course, the author has an axe to grind (and would be the first to
admit it), so of course a prediction from standard cosmology was chosen
which highlights its shortcomings.

> At the right you can see the prediction done by a MOND theorist using a
> purely baryonic (MOND) model.

Which differs not at all from a standard cosmological prediction with no
CDM. Calling it MOND is stretching it. Yes, MOND (usually) has no
(need for) CDM, but not everything with no CDM is MOND. In particular,
there is no "MOND physics" involved in this "MOND prediction". I'm not
saying that this is unjustified; maybe it is a good approximation to
what MOND cosmology would offer, but until there is a MOND cosmology,
no-one can say.

> Both predictions were done before the data was known.

Yes, otherwise they wouldn't be predictions.

> Once the data was known, dark matter theoreticians changed the original
> parameters in the Lambda-CDM model to *fit* the observed data.

This is what scientists do. The whole point of fitting models to CMB
data is to find out what the parameters are. If they were known for
certain beforehand, we wouldn't need to do CMB observations to determine
them.

> i)
> MOND theoreticians predicted the right data, *before* observation. Dark
> matter theoreticians prediction failed and now they only fit the right
> data, *after* observation, using a many parameters model.

First, as already noted, the MOND prediction has little to do with MOND
directly. Second, it's in a sense unfair to disadvantage a theory just
because it has more free parameters. Yes, I think it is impressive that
MOND, with only one free parameter, isn't ruled out, but as always one
can't prove a theory, only disprove it. It's also unfair to compare the
MOND prediction to a standard prediction which turned out to be wrong;
after all, there were other standard predictions (and no, there aren't
enough parameters that one can fit everything). As Bill Press said a
few years ago, someone knows the value of the Hubble constant exactly;
we just don't know who that person is. :-)

> ii)
> Why do dark matter theoreticians failed at their prediction? You seems
> to ignore this important point.

There were many predictions; some fit the data, others don't. Thus,
some are eliminated, others are still around. Where is the failure?
Failure would be if NO standard models could fit the data, or if ad-hoc
ideas (i.e. ideas which no-one had mentioned before) had to be introduced
to fit the data. Neither is the case.

> Initially they used standard values for the parameters in the Lambda-CDM
> model. After this failed they assumed nonstandard values for parameters
> for fitting the observed data.

Yes, in a sense, but the "standard values" were partly chosen because,
mathematically, they were easy to work with. (Yes, a few people took
them as gospel, but they were in a minority even back in the 1990s.) As
long as they fit the then-known data, why use others? But the very fact
that other parameter combinations were seen to fit the data shows that
these standard values were just a working hypothesis (and predictions
for all kinds of parameter combinations were done long before the data
were in).

Chalky

On Dec 16, 7:42=A0pm, hel...@astro.multiCLOTHESvax.de (Phillip Helbig---
remove CLOTHES to reply) wrote:
> In article <gi640m$8p...@fb07-hees.theo.physik.uni-giessen.de>, eric giss=
e <jowr.pi.nos...@gmail.com> writes:

> > Regardless, returning to dark energy. It does not appear to have any
> > influence on galaxies as the galaxies are gravitationally bound and
> > dark energy isn't strong enough to have a significant impact.
>
> In the conventional form (and there is no evidence that this is not the
> case), dark energy is distributed completely smoothly. =A0In that case, I
> believe (someone correct me if I am wrong) that the only effect it could
> have would be to change the size of a gravitationally bound structure. =
=A0
> To notice this, we would have to compare expected and observed sizes,
> probably at a variety of scales. =A0The data are too messy to note any
> deviations, at least for the value of the cosmological constant which we
> believe we know from cosmological observations.

Hmm, the "gravitationally bound" argument is traditionally used to
explain (away) our failure to observe expansion, as opposed to
acceleration in that expansion, on the scales of both the solar system
and the Milky Way. This would seem to imply that the data _would_ be
clean enough to detect that, if it actually happened on these scales.

Since Anderson et al. used NASA radiometric ranging data to
demonstrate that the Pioneer acceleration anomaly is not also
experienced by planets, I would guess that this is also the preferred
method for demonstrating that Hubble expansion does not actually
happen on the scale of the solar system. However, I can't see that
working on the scale of our local galaxy, so would appreciate
clarification on what the data actually is, for the galaxy, if anyone
can fill me in on this.

It seems to me that there are similarities in the considerations for
observing expansion and its acceleration, locally (except, of course,
that the effects of Hubble expansion should be much, much easier to
detect).

It also seems to me that, unless the gravitationally bound argument
can predict the physics in the transition region, and where that
region actually is, one might be justified in viewing that argument
with a modicum of healthy scepticism.

Phillip Helbig---remove CLOTHES to reply

In article
<d7128508-74d1-48ec-93cb-fd955953ccc6@f33g2000vbf.googlegroups.com>,
Chalky <chalkyspam@bleachboys.co.uk> writes:

> On Dec 16, 7:42 pm, hel...@astro.multiCLOTHESvax.de (Phillip Helbig---
> remove CLOTHES to reply) wrote:
> > In article <gi640m$8p...@fb07-hees.theo.physik.uni-giessen.de>,
> > eric gisse <jowr.pi.nos...@gmail.com> writes:
>
> > > Regardless, returning to dark energy. It does not appear to have any
> > > influence on galaxies as the galaxies are gravitationally bound and
> > > dark energy isn't strong enough to have a significant impact.
> >
> > In the conventional form (and there is no evidence that this is not the
> > case), dark energy is distributed completely smoothly. In that case, I
> > believe (someone correct me if I am wrong) that the only effect it could
> > have would be to change the size of a gravitationally bound structure.
>
> > To notice this, we would have to compare expected and observed sizes,
> > probably at a variety of scales. The data are too messy to note any
> > deviations, at least for the value of the cosmological constant which we
> > believe we know from cosmological observations.
>
> Hmm, the "gravitationally bound" argument is traditionally used to
> explain (away) our failure to observe expansion, as opposed to
> acceleration in that expansion, on the scales of both the solar system
> and the Milky Way.

Yes.

> This would seem to imply that the data _would_ be
> clean enough to detect that, if it actually happened on these scales.

In principle, yes, but in practice the "expected size" mentioned above
is not known precisely enough. The real universe is messy. There is a
paper by Lineweaver et al. which discusses this---I don't have the
reference right now, but can dig it up if necessary. They show that
while the expansion of the universe doesn't cause bound objects to
expand, it does change their equilibrium size (very slightly). A change
in the rate of expansion would in turn change this. But the effect is
too small to be observed.

(Note that the cosmological redshift of an object, in general, is not
constant with time. Spectrographs are now becoming so precise that the
change in the cosmological redshift can be noticed on a timescale of
years or decades. Not long ago, this was also an effect which was there
in principle but not measurable in practice.)

> Since Anderson et al. used NASA radiometric ranging data to
> demonstrate that the Pioneer acceleration anomaly is not also
> experienced by planets, I would guess that this is also the preferred
> method for demonstrating that Hubble expansion does not actually
> happen on the scale of the solar system. However, I can't see that
> working on the scale of our local galaxy, so would appreciate
> clarification on what the data actually is, for the galaxy, if anyone
> can fill me in on this.

Again, the real universe is messy. Assuming the galaxy DID expand with
the Hubble flow, this would mean a typical difference in stellar
positions of the order of one-tenth the size of the Earth's orbit over
the course of a year. Thus, a direct measurement doesn't seem possible,
since the effect would be swamped by other effects.

> It also seems to me that, unless the gravitationally bound argument
> can predict the physics in the transition region, and where that
> region actually is, one might be justified in viewing that argument
> with a modicum of healthy scepticism.

Why do you think there is any confusion about the transition region?

Chalky

On Dec 28, 7:19 pm, hel...@astro.multiCLOTHESvax.de (Phillip Helbig---
remove CLOTHES to reply) wrote:

> (Note that the cosmological redshift of an object, in general, is not
> constant with time.

It is certainly nothing like constant within the context of GR, that's
for sure. That is one reason why the relevant calculations are
sufficiently tedious and long winded, to need a computer.

> Spectrographs are now becoming so precise that the
> change in the cosmological redshift can be noticed on a timescale of
> years or decades.

Are you talking here about changes in the cosmological redshift of the
same object, when viewed years or decades later? If so, this
development sounds particularly interesting.

> > It also seems to me that, unless the gravitationally bound argument
> > can predict the physics in the transition region, and where that
> > region actually is, one might be justified in viewing that argument
> > with a modicum of healthy scepticism.

> Why do you think there is any confusion about the transition region?

1) Because it wasn't that long ago, relative to the lifetime of GR
(and of myself), when many physicists thought the entire universe
might be gravitationally bound, in the sense of debating whether it
would eventually collapse on itself, under the influence of gravity.

2) Because I have never seen or heard of such an analysis, and am
myself confused about how one would approach that particular problem
in dynamics.

Phillip Helbig---remove CLOTHES to reply

In article
<76a8cd14-7687-478f-a8a9-c502c261e48e@o40g2000prn.googlegroups.com>,
Chalky <chalkyspam@bleachboys.co.uk> writes:

> On Dec 28, 7:19 pm, hel...@astro.multiCLOTHESvax.de (Phillip Helbig---
> remove CLOTHES to reply) wrote:
>
> > (Note that the cosmological redshift of an object, in general, is not
> > constant with time.
>
> It is certainly nothing like constant within the context of GR, that's
> for sure. That is one reason why the relevant calculations are
> sufficiently tedious and long winded, to need a computer.

Actually, almost all calculations assume it is constant in time. It is
calculated for one time, NOW. I'm not referring to objects at different
distances.

> > Spectrographs are now becoming so precise that the
> > change in the cosmological redshift can be noticed on a timescale of
> > years or decades.
>
> Are you talking here about changes in the cosmological redshift of the
> same object, when viewed years or decades later? If so, this
> development sounds particularly interesting.

Yes.

> > > It also seems to me that, unless the gravitationally bound argument
> > > can predict the physics in the transition region, and where that
> > > region actually is, one might be justified in viewing that argument
> > > with a modicum of healthy scepticism.
>
> > Why do you think there is any confusion about the transition region?
>
> 1) Because it wasn't that long ago, relative to the lifetime of GR
> (and of myself), when many physicists thought the entire universe
> might be gravitationally bound, in the sense of debating whether it
> would eventually collapse on itself, under the influence of gravity.

This is not really the same as being gravitationally bound in the
traditional sense. Is the universe a black hole if a sufficient density
is reached? Not in any meaningful sense. If you put in the numbers, it
might look like that, but that's more a consequence of dimensional
analysis.

Chalky

On Dec 30, 10:00=A0am, hel...@astro.multiCLOTHESvax.de (Phillip Helbig---
remove CLOTHES to reply) wrote:

> > > Why do you think there is any confusion about the transition region?
>
> > 1) Because it wasn't =A0that long ago, relative to the lifetime of GR
> > (and of myself), when many physicists thought the entire universe
> > might be gravitationally bound, in the sense of debating whether it
> > would eventually collapse on itself, under the influence of gravity.
>
> This is not really the same as being gravitationally bound in the
> traditional sense.

Then, perhaps, you should explain what this "traditional sense" is.

To be precise, what is the traditional time limit for deciding that,
if a receding object has not returned in that time, it will never
return /and/or is not gravitationally bound?

Jonathan Thornburg [remove -animal to reply]

Phillip Helbig---remove CLOTHES to reply <helbig@astro.multiclothesvax.de>
wrote:
[[bound systems don't participate in the overall Hubble expansion]]
> In principle, yes, but in practice the "expected size" mentioned above
> is not known precisely enough. The real universe is messy. There is a
> paper by Lineweaver et al. which discusses this---I don't have the
> reference right now, but can dig it up if necessary. They show that
> while the expansion of the universe doesn't cause bound objects to
> expand, it does change their equilibrium size (very slightly). A change
> in the rate of expansion would in turn change this. But the effect is
> too small to be observed.

You may be referring to the (excellent) paper

Tamara M. Davis, Charles H. Lineweaver, John K. Webb
"Solutions to the tethered galaxy problem in an expanding universe
and the observation of receding blueshifted objects"
http://arxiv.org/abs/astro-ph/0104349
American J. Physics 71 (2003) 358-364

My personal favorite paper along these lines is

Richard H. Price
"In an expanding universe, what doesn't expand?"
http://arxiv.org/abs/gr-qc/0508052
I think this was also published in American J. Physics, but I don't
have the reference.

Finally, I should also point out

Tamara M. Davis, Charles H. Lineweaver
"Expanding Confusion: common misconceptions of cosmological horizons
and the superluminal expansion of the Universe"
http://arxiv.org/abs/astro-ph/0310808

--
-- From: "Jonathan Thornburg [remove -animal to reply]" <jthorn@astro.indiana-zebra.edu>
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