non-GR theories of gravity
Phillip Helbig---remove CLOTHES to reply
In the cosmological community, the idea of dark matter is well
established, though a significant minority think that the idea of a
modified law of gravity might be a better explanation to explain things
which dark matter explains. In particular, such theories have made
testable predictions which a) differ from those of the dark-matter
hypothesis and b) have been confirmed while c) having fewer parameters
than the dark-matter hypotheses. Sounds like a good scientific theory
to me.
Of course, there are justified objections to the idea of MOND (Modified
Newtonian Dynamics). However, the following papers go beyond the idea
of the simple form of MOND to counter many of these objections:
astro-ph/0403694
astro-ph/0412652
What do people who work in gravitational theory think of these papers?
(They are by Jacob Bekenstein, who is certainly not a crackpot.) The
astronomical community might be more open to modifications of the law of
gravity since it a) is confronted by problems which require it OR a
competing hypothesis like dark matter which is 1) not DIRECTLY verified
and 2) has problems of its own (see the papers mentioned above) and b)
most people in cosmology and astrophysics don't use full-blown GR in
their day-to-day work so perhaps aren't as emotionally attached to it as
a theory. The papers above, however, should be accessible and
interesting to people on the other side of the fence. How big an issue
is the idea of alternative theories of gravity in the field of
gravitational research as opposed to the field of
astrophysics/cosmology?
greywolf42
wrote in message news:csjhje$4vj$1...@online.de...
>
> In the cosmological community, the idea of dark matter is well
> established, though a significant minority think that the idea of a
> modified law of gravity might be a better explanation to explain things
> which dark matter explains.
One must be careful to separate the two different reasons for the
postulation of "dark matter". The first is the discrepancy between the
observed mass distribution of stars and the motion of (the gas in) spiral
galaxies. The second is the needs of big bang cosmology to "cook" the
isotopic ratios properly (the observed mass is not sufficient).
Even if the first reason is explained by a MOND, the second will remain.
> In particular, such theories have made
> testable predictions which a) differ from those of the dark-matter
> hypothesis and b) have been confirmed while c) having fewer parameters
> than the dark-matter hypotheses. Sounds like a good scientific theory
> to me.
An even simpler one is the observation that gas does not move by gravity
alone. And that the measurements of galactic rotation curves are all based
on gas motions, or O and B stars recently condensed from same.
> Of course, there are justified objections to the idea of MOND (Modified
> Newtonian Dynamics). However, the following papers go beyond the idea
> of the simple form of MOND to counter many of these objections:
>
> astro-ph/0403694
> astro-ph/0412652
>
> What do people who work in gravitational theory think of these papers?
> (They are by Jacob Bekenstein, who is certainly not a crackpot.)
What evidence can you cite that Bekenstein is not a 'crackpot'?
> The
> astronomical community might be more open to modifications of the law of
> gravity since it a) is confronted by problems which require it OR a
> competing hypothesis like dark matter which is 1) not DIRECTLY verified
> and 2) has problems of its own (see the papers mentioned above) and b)
> most people in cosmology and astrophysics don't use full-blown GR in
> their day-to-day work so perhaps aren't as emotionally attached to it as
> a theory. The papers above, however, should be accessible and
> interesting to people on the other side of the fence. How big an issue
> is the idea of alternative theories of gravity in the field of
> gravitational research as opposed to the field of
> astrophysics/cosmology?
I suspect you will find out. ;)
--
greywolf42
ubi dubium ibi libertas
{remove planet for return e-mail}
Phillip Helbig---remove CLOTHES to reply
<min...@marssim-ss.com> writes:
> One must be careful to separate the two different reasons for the
> postulation of "dark matter". The first is the discrepancy between the
> observed mass distribution of stars and the motion of (the gas in) spiral
> galaxies. The second is the needs of big bang cosmology to "cook" the
> isotopic ratios properly (the observed mass is not sufficient).
>
> Even if the first reason is explained by a MOND, the second will remain.
Are you saying that nucleosynthesis demands non-baryonic dark matter?
> An even simpler one is the observation that gas does not move by gravity
> alone. And that the measurements of galactic rotation curves are all based
> on gas motions, or O and B stars recently condensed from same.
As originally noticed by Zwicky a very long time ago, there is also a
"missing mass" problem in clusters of galaxies. What is observed here
is the redshift of the entire spectrum of the entire galaxy, which
doesn't necessarily contain a lot of gas or young stars.
> What evidence can you cite that Bekenstein is not a 'crackpot'?
I've read some of his papers. He also gets a low score on John Baez's
crackpot index. :-)
carlip...@physics.ucdavis.edu
> <hel...@astro.multiclothesvax.de> wrote:
> In the cosmological community, the idea of dark matter is well
> established, though a significant minority think that the idea of a
> modified law of gravity might be a better explanation to explain things
> which dark matter explains. In particular, such theories have made
> testable predictions which a) differ from those of the dark-matter
> hypothesis and b) have been confirmed while c) having fewer parameters
> than the dark-matter hypotheses. Sounds like a good scientific theory
> to me.
> Of course, there are justified objections to the idea of MOND (Modified
> Newtonian Dynamics). However, the following papers go beyond the idea
> of the simple form of MOND to counter many of these objections:
> astro-ph/0403694
> astro-ph/0412652
> What do people who work in gravitational theory think of these papers?
> (They are by Jacob Bekenstein, who is certainly not a crackpot.)
MOND is an interesting enough idea that a number of people have tried
to develop it into a sensible relativistic theory that doesn't obviously
fail in one way or another. There was a nice argument a year and a half
ago by Soussa and Woodard showing that you couldn't do this in a purely
metric formalism without getting the wrong results for gravitational
lensing (astro-ph/0307358). By adding an extra vector and an extra scalar,
Bekenstein may have managed to get around this constraint, though at the
expense of an extraordinarily complicated model.
An immediate worry is that by introducing a unit vector field, Bekenstein
is going to get spontaneous Lorentz invariance violation. There has been
some work on similar, although not identical, theories in a very different
context -- searching for ways to further test Lorentz invariance -- by
Jacobson and Mattingly. There, the presence of a unit vector field causes
a number of potentially undesirable results. For example, it can easily
screw up binary pulsar orbital decay (you get new radiative modes), and
can lead to a variety of Solar System problems. Bekenstein says that the
post-Newtonian parameters related to preferred frame effects haven't yet
been computed in his model, and I think it's likely that when they are,
they will at least require some very fine tuning of coupling constants to
get consistency with observation.
Beyond that, though, I would read Bekenstein's papers as a demonstration
of how hard it is to get a phenomenologically viable version of MOND. Note,
for example, that his action contains an arbitrary function F that has to
be carefully chosen, and looks very peculiar (look at eqn. (5.12) of the
preprint astro-ph/0412652), as well as very peculiar kinetic terms for the
scalar field in the action. All in all, it's a nice demonstration of why
one might prefer dark matter.
Steve Carlip
melroy...@hotmail.com
Well its a great question. I too would love to know the answer :-)
see this paper by Aguiree which was however before Bekenstein's papers.
astro-ph/0310572
I have a related question. What do experts think about Mannheim's
conformal
gravity theory (which is different from MOND but not discussed much in
literature)
which also can explain galactic rotation curves (see eg.
astro-ph/9807122 and other
papers which are referenced.) as well as acceleration of the universe
without
dark matter and dark energy respectively. Maybe experts on this forum
such as Steve Carlip, John Baez, Ted Bunn and others could comment.
jgr...@mailaps.org
I see no one more expert than I has replied to your question, so I will
reply.
Nevertheless, I have read every abstract on gr-qc for at least five
years,
so I think my opinion is at least partially informed.
Furthermore, alternate theories of gravity are one of my main
interests.
I think the answer depends on how you classify string theory and loop
quantum gravity.
If you consider these quantum gravity theories as "alternate theories
of gravity" rather than as
"quantizations of general relativity" , then alternate theories of
gravity are a very big issue in the field of gravitational research.
If you make the opposite choice, then alternate theories are studied
by few and ignored by everyone else.
Theories not connected with quantum gravity are discussed very
infrequently, and only a handful of theorists (if that many) ever
publish anything on MOND.
Actually, I just searched the ArXiv for MOND in abstract, all years .
Results: astro-ph 95 hits; gr-qc 19 hits.
Besides Milgrom, Sanders, McGaugh, and Bekenstein, less than ten other
papers, (some with multiple authors).
You could read them all in a morning, if you wanted to,
But that's more than nothing.
Best,
Jim Graber
I.Vecchi
>
> ... I would read Bekenstein's papers as a demonstration
> of how hard it is to get a phenomenologically viable version of MOND.
Note,
> for example, that his action contains an arbitrary function F that
has to
> be carefully chosen, and looks very peculiar (look at eqn. (5.12) of
the
> preprint astro-ph/0412652), as well as very peculiar kinetic terms
for the
> scalar field in the action. All in all, it's a nice demonstration of
why
> one might prefer dark matter.
>
Bekenstein states that "dark matter is less falsifiable [than TeVeS]
because of the essentially unlimited choice of halo models and choices
of their free parameters". MOND is an obviously flawed theory, but it
is simple and has some remarkable predictions to its credit. On the
other hand an unlimited choice of parameters can indeed rescue (almost)
any theory and may be a good reason to prefer dark matter. I guess it
depends on what you want to do.
As far as I am concerned, dark matter is on a par with direct
intervention by the Virgin Mary.
Still, I see little merit in TeVes that isn't "cut and pasted" or not
already there in MOND, excepts perhaps the Einstein vs. "physical"
metrics idea, which might be an unwitting phenomenological
approximation of a quantum duality .
There is a remark by Hawking at the closure of his debate with Penrose
in [1] which keeps me mulling.
[Don't fall into the] "trap of assuming that there is a single metric
for space-time, as there is in classical theory. In quantum theory, on
the other hand, one has to do a path integral over all possible
metrics. There will be different saddle points in the metric for
different questions. In particular the saddle point metrics for the
questions that outside observers ask will be
different from the the saddle point metric of an infalling observer."
I don't think the relevance of Hawking's remark is limited to black
holes. There is a breed of "questions" that apparently trigger the need
for "dark matter" or MOND-like massaging of the space-time metric. The
a0 acceleration threshold in MOND may have something to do with
gravitational quantisation inducing "a breakdown of Newtonian dynamics
in the limit of small accelerations" ([4]).
There are some pleasant MOND surveys online (see [2], [3], [6] as well
as Milgrom's [4]). MOND is mentioned also by people dealing with the
Pioneer 10 and 11 acceleration anomaly ([5]), who might be interested
in Bekenstein's "failure to achieve a perfect Newtonian limit in the
outer solar system".
IV
[1] http://www.hawking.org.uk/text/public/public.html
[2] http:/www.astro.psu.edu/users/caryl/a480/M.O.N.D..ppt
[3] http://www-thphys.physics.ox.ac.uk/users/JamesBinney/MOND.ppt
[4] M. Milgrom "MOND -- a pedagogical review" at
http://http://arxiv.org/abs/astro-ph/0112069
[5] http://arxiv.org/abs/gr-qc/0104064
[6] http://math.ucr.edu/home/baez/week206.html
---------------------------
T: "Define crackpot!"
S: "Someone with a mind of his own"
greywolf42
Phillip Helbig---remove CLOTHES to reply <hel...@astro.multiCLOTHESvax.de>
wrote in message news:cste2o$kom$2...@online.de...
> In article <uVxHd.957$rc4...@fe07.usenetserver.com>, "greywolf42"
> <min...@marssim-ss.com> writes:
>
> > One must be careful to separate the two different reasons for the
> > postulation of "dark matter". The first is the discrepancy between the
> > observed mass distribution of stars and the motion of (the gas in)
> > spiral galaxies. The second is the needs of big bang cosmology to
> > "cook" the isotopic ratios properly (the observed mass is not
> > sufficient).
> >
> > Even if the first reason is explained by a MOND, the second will remain.
>
> Are you saying that nucleosynthesis demands non-baryonic dark matter?
No.
> > An even simpler one is the observation that gas does not move by gravity
> > alone. And that the measurements of galactic rotation curves are all
> > based on gas motions, or O and B stars recently condensed from same.
>
> As originally noticed by Zwicky a very long time ago, there is also a
> "missing mass" problem in clusters of galaxies.
That would be a different problem. Because there is no "missing mass"
problem within elliptical galaxies.
> What is observed here
> is the redshift of the entire spectrum of the entire galaxy, which
> doesn't necessarily contain a lot of gas or young stars.
You don't need "a lot" of gas or "a lot" of young stars.
> > What evidence can you cite that Bekenstein is not a 'crackpot'?
>
> I've read some of his papers. He also gets a low score on John Baez's
> crackpot index. :-)
I meant, did you have any specific reasons. I already knew that you didn't
think he was a crackpot. What factors did you consider?
Nicolaas Vroom
<hel...@astro.multiCLOTHESvax.de> schreef in bericht
news:csjhje$4vj$1...@online.de...
>
> In the cosmological community, the idea of dark matter is well
> established, though a significant minority think that the idea of a
> modified law of gravity might be a better explanation to explain things
> which dark matter explains. In particular, such theories have made
> testable predictions which a) differ from those of the dark-matter
> hypothesis and b) have been confirmed while c) having fewer parameters
> than the dark-matter hypotheses. Sounds like a good scientific theory
> to me.
>
> Of course, there are justified objections to the idea of MOND (Modified
> Newtonian Dynamics). However, the following papers go beyond the idea
> of the simple form of MOND to counter many of these objections:
SNIP
It is not my primary intention to chalenge MOND.
What is much more important is to chalenge the issue
how much dark matter there is in an average spiral galaxy
(Like our Milky Way)
In order to answer that question it is important to make
a distinction between two types of ordinairy matter:
visible and invisible.
(Ordinairy matter can be described by Newton's Law)
Visible ordinairy matter are the star sized objects
in our Galaxy. They shine light.
Invisible ordinairy matter are the planet sized objects
in our Galaxy. They do not shine (or very little)
The question is how much visible ordinary matter
versus invisible ordinary matter is there in an average galaxy.
The issue is that it is easy possible that there can be a lot
of invisible ordinary matter outside the disc of a spiral galaxy
which is invisible from our place in the universe i.e. Earth.
In a simulation, using only Newton's Law and only the
visible ordinary matter (3D) the rotation curve will not be flat.
However, strange as its sounds, only relative small amounts
of invisble ordinary matter (not in the halo) in the disc
and outside the disc have to be added to make the
rotation curve flat (using Newton's Law)
The question to answer is how much of this invisible
ordinary matter can be added before some one declares:
you have so much added and now it should become
visible.
The question how to tackle a simulation using Newton's
Law is (partly) answered in my lastest posting in the thread:
"How important is GR in order to calc the precession of Mercury"
in the newsgroups sci.astro.research and sci.physics.research.
In fact in that thread I remove the visible component that we see
the stars in the past.
Instead I treat all visible objects and invisible objects identical
like ordinairy objects and I position them
all within one frame with the same clock reading.
(But before you do that the above mentioned issue
has to be solved)
For more detail about such a simulation see my homepage.
Hopes this helps.
Nicolaas Vroom
http://users.pandora.be/nicvroom/
Phillip Helbig---remove CLOTHES to reply
In article <y2SId.1749$rc4...@fe07.usenetserver.com>, "greywolf42"
<min...@marssim-ss.com> writes:
> > In article <uVxHd.957$rc4...@fe07.usenetserver.com>, "greywolf42"
> > <min...@marssim-ss.com> writes:
> >
> > > One must be careful to separate the two different reasons for the
> > > postulation of "dark matter". The first is the discrepancy between the
> > > observed mass distribution of stars and the motion of (the gas in)
> > > spiral galaxies. The second is the needs of big bang cosmology to
> > > "cook" the isotopic ratios properly (the observed mass is not
> > > sufficient).
> > >
> > > Even if the first reason is explained by a MOND, the second will remain.
> >
> > Are you saying that nucleosynthesis demands non-baryonic dark matter?
>
> No.
Then what ARE you saying? It is unclear to me.
> > > What evidence can you cite that Bekenstein is not a 'crackpot'?
> >
> > I've read some of his papers. He also gets a low score on John Baez's
> > crackpot index. :-)
>
> I meant, did you have any specific reasons. I already knew that you didn't
> think he was a crackpot. What factors did you consider?
I suppose that arguing that someone is NOT a crackpot is like arguing
for the innocence of someone in court, i.e. it is very difficult. It
makes more sense to argue for the guilt of a suspect, and similarly it
is easier to say why someone is a crackpot than to say why someone else
is not. I meant that I have seen no evidence to indicate that he is a
crackpot.
jcgon...@yahoo.com
melroy...@hotmail.com wrote:
> Well its a great question. I too would love to know the answer :-)
> see this paper by Aguiree which was however before Bekenstein's
papers.
> astro-ph/0310572
>
> I have a related question. What do experts think about Mannheim's
> conformal
> gravity theory (which is different from MOND but not discussed much
in
> literature)
> which also can explain galactic rotation curves (see eg.
> astro-ph/9807122 and other
> papers which are referenced.) as well as acceleration of the universe
> without
> dark matter and dark energy respectively. Maybe experts on this
forum
> such as Steve Carlip, John Baez, Ted Bunn and others could comment.
>
Conformal gravity may be to MOND what general relativity is to
Newtonian gravity. I don't know about Mannheim's conformal gravity but
I know a little anout Segal's conformal gravity especially as it is
used by Tony Smith. John Baez would know Segal's conformal gravity
since Baez worked with Segal as a grad student.
Nicolaas Vroom
"Phillip Helbig"
<hel...@astro.multiCLOTHESvax.de> schreef in bericht
news:csjhje$4vj$1...@online.de...
>
> established, though a significant minority think that the idea of a
> modified law of gravity might be a better explanation to explain things
> which dark matter explains. In particular, such theories have made
> testable predictions which a) differ from those of the dark-matter
> hypothesis and b) have been confirmed while c) having fewer parameters
> than the dark-matter hypotheses. Sounds like a good scientific theory
> to me.
>
> Of course, there are justified objections to the idea of MOND (Modified
> Newtonian Dynamics). However, the following papers go beyond the idea
> of the simple form of MOND to counter many of these objections:
SNIP
Martin Hardcastle
greywolf42 <min...@marssim-ss.com> wrote:
>That would be a different problem. Because there is no "missing mass"
>problem within elliptical galaxies.
This isn't actually true: in massive elliptical galaxies, dark matter
is required by the X-ray observations, just as it is in clusters. See
e.g. the review of X-ray properties of ellipticals by Matthews &
Brighenti (2003 ARA&A 41 191), or Bill Keel's page here:
http://www.astr.ua.edu/keel/galaxies/darkmatter.html
You're doubtless thinking of recent results on dwarf ellipticals, but
they haven't made dark matter go away.
Martin
--
Martin Hardcastle
School of Physics, Astronomy and Mathematics, University of Hertfordshire, UK
Please replace the xxx.xxx.xxx in the header with star.herts.ac.uk to mail me
greywolf42
news:KNp*Bp...@news.chiark.greenend.org.uk...
> >That would be a different problem. Because there is no "missing mass"
> >problem within elliptical galaxies.
>
> This isn't actually true: in massive elliptical galaxies, dark matter
> is required by the X-ray observations, just as it is in clusters. See
> e.g. the review of X-ray properties of ellipticals by Matthews &
> Brighenti (2003 ARA&A 41 191), or Bill Keel's page here:
> http://www.astr.ua.edu/keel/galaxies/darkmatter.html
> You're doubtless thinking of recent results on dwarf ellipticals, but
> they haven't made dark matter go away.
No, I'm not thinking only of dwarf ellipticals. The "problems" with
elliptical galaxies that you describe are cosmological ones ... not orbital
ones.
As explicitly noted on the page that you reference -- and as noted by
myself -- orbital / rotational "problems" are found only in disk galaxies.
A completely different argument (the "hydrostatic argument") is used to come
up with claims about missing mass in ellipticals. However, these arguments
provide no repeatable prediction of what level of "missing mass" is needed.
Hence, they are likely to simply result from a bad assumption.
--
greywolf42
ubi dubium ibi libertas
{remove planet for e-mail}
Joseph Lazio
g> "Martin Hardcastle" <m.hard...@xxx.xxx.xxx> wrote in message
g> news:KNp*Bp...@news.chiark.greenend.org.uk...
>>> That would be a different problem. Because there is no "missing
>>> mass" problem within elliptical galaxies.
>>
>> This isn't actually true: in massive elliptical galaxies, dark
>> matter is required by the X-ray observations, just as it is in
>> clusters. See e.g. the review of X-ray properties of ellipticals by
>> Matthews & Brighenti (2003 ARA&A 41 191), or Bill Keel's page here:
>> http://www.astr.ua.edu/keel/galaxies/darkmatter.html
>> You're doubtless thinking of recent results on dwarf ellipticals,
>> but they haven't made dark matter go away.
g> No, I'm not thinking only of dwarf ellipticals. The "problems"
g> with elliptical galaxies that you describe are cosmological ones
g> ... not orbital ones.
g> As explicitly noted on the page that you reference [...] orbital /
g> rotational "problems" are found only in disk galaxies. A
g> completely different argument (the "hydrostatic argument") is used
g> to come up with claims about missing mass in ellipticals. However,
g> these arguments provide no repeatable prediction of what level of
g> "missing mass" is needed. Hence, they are likely to simply result
g> from a bad assumption.
Perhaps, but you haven't shown that some other, equally plausible
assumption makes the need for dark matter go away.
I also think you're confused about the process. There is no
"elliptical galaxy theory" that would allow one to predict from first
principles what the stellar velocity dispersion (or other observable)
is. Rather, like many problems, one has a set of measurements (e.g.,
stellar velocity dispersions as a function of position), and the task
is to find a parsimonious model for the mass distribution within the
galaxy (the so-called "inverse problem").
As for "repeatable prediction," the only way I can think that this
would enter would be if one were to obtain a second set of data,
comparable in quality to the first set, and test whether the model
determined from the first set continued to match the second set. I
suspect that's been done, but I'm not going to pore through the
literature to determine it. Of course, in the real world, often the
second (or third or ...) set of data is of higher quality, so it is
not surprising that the first attempt to make a model isn't successful
with later data.
Finally, relevant to this discussion seems to be two recent papers on
astro-ph:
1. astro-ph/0502037
Title: Modelling Kinematics and Dark Matter: The Halos of Elliptical Galaxies
Authors: Ortwin Gerhard
2. astro-ph/0501622
Title: Dark-Matter Haloes in Elliptical Galaxies: Lost and Found
Authors: A. Dekel, F. Stoehr, G.A. Mamon, T.J. Cox, J.R. Primack
--
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No means no, stop rape. | http://patriot.net/%7Ejlazio/
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Bjoern Feuerbacher
> "Martin Hardcastle" <m.hard...@xxx.xxx.xxx> wrote in message
> news:KNp*Bp...@news.chiark.greenend.org.uk...
>
>>In article <y2SId.1749$rc4...@fe07.usenetserver.com>,
>>greywolf42 <min...@marssim-ss.com> wrote:
>
>
>>>That would be a different problem. Because there is no "missing mass"
>>>problem within elliptical galaxies.
>>
>>This isn't actually true: in massive elliptical galaxies, dark matter
>>is required by the X-ray observations, just as it is in clusters. See
>>e.g. the review of X-ray properties of ellipticals by Matthews &
>>Brighenti (2003 ARA&A 41 191), or Bill Keel's page here:
>>http://www.astr.ua.edu/keel/galaxies/darkmatter.html
>>You're doubtless thinking of recent results on dwarf ellipticals, but
>>they haven't made dark matter go away.
>
>
> No, I'm not thinking only of dwarf ellipticals. The "problems" with
> elliptical galaxies that you describe are cosmological ones ... not orbital
> ones.
Err, they are neither "orbital" nor "cosmological" problems (as you
yourself admit below - the "hydrostatic argument"). We have evidence for
the presence of dark matter by looking at the amount of hot gas bound in
and around the ellipticals. And we get that amount from X-ray observations.
> As explicitly noted on the page that you reference -- and as noted by
> myself -- orbital / rotational "problems" are found only in disk galaxies.
> A completely different argument (the "hydrostatic argument") is used to come
> up with claims about missing mass in ellipticals.
1) Note that the page says that the evidence comes *mostly* from the
hydrostatic argument. Not exclusively.
2) This argument is *not* cosmological. So your assertion above was
simply false.
> However, these arguments
> provide no repeatable prediction of what level of "missing mass" is needed.
What exactly do you mean with "repeatable prediction" and "level" here?
> Hence, they are likely to simply result from a bad assumption.
Please explain how the described observations could be explained without
dark matter.
Bye,
Bjoern
Kent Paul Dolan
> In the cosmological community, the idea of dark matter is well
> established, though a significant minority think that the idea of a
> modified law of gravity might be a better explanation to explain things
> which dark matter explains. In particular, such theories have made
> testable predictions which a) differ from those of the dark-matter
> hypothesis and b) have been confirmed while c) having fewer parameters
> than the dark-matter hypotheses. Sounds like a good scientific theory
> to me.
Alternatives to dark matter probably just got a lot harder
to support, with the discovery of a galaxy 99.8% dark matter,
checked for over four years for possible alternative explanations
before it was announced:
http://www.space.com/scienceastronomy/050223_dark_galaxy.html
FYI
xanthian.
--
Posted via Mailgate.ORG Server - http://www.Mailgate.ORG
Phillip Helbig---remove CLOTHES to reply
<xant...@well.com> writes:
> Alternatives to dark matter probably just got a lot harder
> to support, with the discovery of a galaxy 99.8% dark matter,
> checked for over four years for possible alternative explanations
> before it was announced:
>
> http://www.space.com/scienceastronomy/050223_dark_galaxy.html
Not really. The existence of one dark galaxy doesn't make MOND or some
alternative theory impossible. Some die-hards might say that if ANY
dark matter is discovered, then we might as well give up MOND since the
"no dark matter at all" scenario can't be saved, but that's more an
emotional than a scientific response. It reminds me of the (rather
arrogant) claims by some so-called and self-styled "experts" a few years
ago that EITHER Omega_matter must be 1 OR the universe must be flat with
no non-baryonic matter, because anything more complicated would be too
ugly. The universe, however, is not under pressure to live up to
anyone's expectations.
Personally, I think there is nothing mysterious about dark matter.
After all, why should everything be so convenient so as to glow so that
an astronomer can see it? However, both sides need to avoid setting up
straw men which can be easily criticised.
Matthew Parry
>
> Alternatives to dark matter probably just got a lot harder
> to support, with the discovery of a galaxy 99.8% dark matter,
>
That's not conclusive yet. Others think it's just a huge
cloud of hydrogen ejected from a nearby one-armed spiral
galaxy - The single arm being evidence of a collision.
--
Matthew Parry, <me...@tpg.com.au> <URL:http://users.tpg.com.au/mettw/>
garth
Is the observation of Dark Matter (DM) just an artefact of
non-Newtonian gravity (the MOND approach)?
If not then is it baryonic or some non-baryonic as yet undiscovered,
exotic particle?
I agree that we haven't seen all that is there, the question is how
much of the unseen mass can be explained by baryonic matter.
The standard constraint on baryonic DM is BB nucleosynthesis, which
puts it at a maximum of about 4% closure density of which only 10% of
that is visible.
However the 'freely coasting' universe model {R(t) ~ t},
(http://arxiv.org/abs/astro-ph/0502370), not only does not require
Inflation, because it does not suffer from the horizon, density and
smoothness problems of GR in the first place, but also predicts a
baryonic density of about 20% closure density. So DM exists but is
baryonic. The problem is explaining why it has not been observed, for
example a majority of it reside in a number of intermediate mass black
holes.
The problem with the freely coasting model, in which density and
pressure are decoupled from the dynamics of cosmological expansion, is
the absence of a mechanism to deliver such a dynamic. Self Creation
Cosmology predicts such an expansion.
(http://www.kluweronline.com/oasis.htm/5092775 and
http://arxiv.org/abs/gr-qc/0405094.)
Furthermore as a conformally flat model it does not require Dark Energy
either.
Garth
DHOLLINGSWORTH2
> Personally, I think there is nothing mysterious about dark matter.
> After all, why should everything be so convenient so as to glow so that
> an astronomer can see it? However, both sides need to avoid setting up
> straw men which can be easily criticised.
What happens when they find element X with several thousand Protons/neutrons
in the nucleus?
Hontas F. Farmer III
> "Phillip Helbig" <hel...@astro.multivax.de> wrote:
>
>> In the cosmological community, the idea of dark matter is well
>> established, though a significant minority think that the idea of a
>> modified law of gravity might be a better explanation to explain things
>> which dark matter explains. In particular, such theories have made
>> testable predictions which a) differ from those of the dark-matter
>> hypothesis and b) have been confirmed while c) having fewer parameters
>> than the dark-matter hypotheses. Sounds like a good scientific theory
>> to me.
>
> Alternatives to dark matter probably just got a lot harder
> to support, with the discovery of a galaxy 99.8% dark matter,
> checked for over four years for possible alternative explanations
> before it was announced:
We cannot discount the possibility that both MOND and dark matter may be
partially correct and the synthesis of both will be the full explaination.
--
Let me get this straight we "advanced" from telgraphs to email?-George
Carlin
Phillip Helbig---remove CLOTHES to reply
<DHOLLIN...@cox.net> writes:
Assuming that it is even moderately stable (which seems unlikely), no
problem as long as there aren't many of them around.
ohwilleke
see this article:
http://www.arxiv.org/abs/gr-qc/0409089
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ohwilleke
First, there are two ways to think about MOND (and by extension TeVeS).
One is as a modification of gravity. Another (with a somewhat
different formulation which Milgrom and some other have explored in
papers) is as a formula describing the distribution of dark matter in
galaxies. One of the problems with dark matter theory is that while
dark matter theory permits a great many degrees of freedom, MOND's
phenomenological success shows that outside galactic clusters, the
distribution of dark matter in a galactic system can be entirely
predicted from the distribution of visibile light in the system. Thus,
the hard part of dark matter theory is not so much explaining how dark
matter could produce the results we see, but explaining why it does so
in only one way, when the theory allows for so many other possibilities
and there is no obvious mechanism for the fine tuned relationship
between visible matter and dark matter. Also, in a related matter, one
has to explain why galaxies that are known to be a product of a
collision of two other galaxies have different amounts of dark matter
than are typical of the galaxies that produce them.
Second, the requirement that dark matter by non-bayronic is a real
significant theoretical leap. One has to posit the existence of all
sorts of particles that there is simply nothing more than the musing of
supersymmetry theorists to support the existence of. Put another way,
you either have to significantly revise the standard model of quantum
mechanics, or you have to significantly revise general relativity with
something like TeVeS. The fact that dark matter must be non-bayronic
makes the choice a pretty stark one.
Third, there is a logical mechanism by which MOND can arise from
quantum gravity. If your quantum gravity is non-abelian, then quantum
gravity starts to look to the flip side of QCD in the same way that
weak force and QED unificiation comes about. This gives rise to
self-interaction of the gravitational field and hence to MOND like
effects when this self-interaction become significant compared to other
parts of the field. Notably, at least one recent paper (not at hand, I
apologize) has argued that while QM theories of a graviton do produce a
Rank 2 tensor formalism, that the Rank 2 tensor that results is not and
cannot be the same Rank 2 tensor used in the GR equations.
Fourth, a hydrogen cloud would be a place you would expect strong MOND
effects because in the absence of any stellar objects, you are looking
at weak gravitational fields which is where you would expect strong
MOND effects.
Fifth, with regard to Bekenstein's credentials . . . he is also an
established black hole theorist (really his main area of research),
with papers that are not radical, has a PhD in the field, and is a long
time member of an established physics faculty.
Sixth, there is little reason, a priori to think that WMAP or
baryogenesis would be signficantly different in a lamda CDM universe
and in a TeVeS universe. More commonly, lamda CDM and TeVeS produce
similar predictions with very diffierent mechanism.
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Joseph Lazio
PH> Personally, I think there is nothing mysterious about dark matter.
PH> After all, why should everything be so convenient so as to glow so
PH> that an astronomer can see it?
Particularly given that we know of one kind of dark matter already,
neutrinos. They appear to have some mass and they do not interact via
the electromagnetic force (i.e., they are dark). If there's one kind
of dark matter, why not another?
I.Vecchi
> ... we know of one kind of dark matter already,
> neutrinos. They appear to have some mass and they do not interact
via
> the electromagnetic force (i.e., they are dark). If there's one kind
> of dark matter, why not another?
>
Yeah, why not?
Because physics is about making testable predictions, not about waving
problems away. MOND has an impressive series of accurate predictions
to its credit (and it has apparently failed some tests too ), while
dark matter is virtually unfalsifiable ([1]).
Well, since I am at it, I'll make a prediction too. It's a follow up
to my other post in this thread.
Consider MOND's basic equation
mu(a/a0)*a=-NABLA(GPN) (1)
GPN being the Newtonian gravitational potential and mu being a function
such that m(x)=1 for x>>1 and m(x)=x for x<<1. Milgrom regards a0~10
exp(-8) sec cm exp(2) as a fundamental constant.
Having noticed that missing mass is mostly needed in dim objects, one
may speculate that a0 and its fellow free parameter M/L in MOND are
actually phenomenological parameters adjusting for the capacity of the
information channel (CoIC) relaying the observer and the observed
object (or more accurately, the corresponding quantum fluctuation) , in
the galaxies' case the hydrogen atoms from which information is
extracted.
For a standard candle CoIC decreases as 1/Dist exp(2), where Dist
denotes the distance between the observer and the candle. In general
CoIC is something like InfC/Dist exp(2), InfC being the the amount of
information the object churns out, which scales with (and can be
replaced by) its luminosity L.
One can then replace (1) with
mu(a/k(CoiC))*a=-NABLA(GPN) (2)
where k is a decreasing function, rapidly approaching 0 as CoIC
increases and such that k(CoIC)~a0 when CoIC has a value corresponding
to systems where MOND has been verified. Equation (2) encodes the
conjecture that the observed dynamics deviates from the Newtonian
regime as the capacity of the information channel sinks.
In the surveys I have inspected, distant galaxies are not considered,
since they yield data of inferior quality (precisely because they have
a small CoIC). On the basis of (2) I conjecture that, once properly
measured, they will exhibit large discrepancies between visible and
classical Newtonian dynamical mass at accelerations larger than a0.
I take bets on the outcome.
IV
melroy...@hotmail.com
reply <hel...@astro.multiCLOTHESvax.de> writes: > > PH> Personally, I
think there is nothing mysterious about dark matter. > PH> After all,
why should everything be so convenient so as to glow so > PH> that an
of dark matter already, > neutrinos. They appear to have some mass and
they do not interact via > the electromagnetic force (i.e., they are
that neutrino is a hot-dark matter candidate and we knew in the 80's
itself that hot dark matter dominated universe is more or less ruled
out. note that besides dark matter (which implied that on galactic
scales gravity is more attractive as compared to Newtonian gravity)
you also have to account for dark energy (which implies that on
cosmologica scales gravity is repulsive) IMO in my opinion a theory
which accounts for both ought to be taken seriously. I don't know if
Bekenstein's TeVS theory can account for the acceleration of the
universe.
However Mannheim's conformal gravity theory does account for both.
see
http://www.mit.edu/people/cabi/blog/2005/02/mannheims-conformal-gravity.html
for some discussion on this.
Melroy
Joseph Lazio
m> Joseph Lazio wrote:
>>>>>> "PH" == Phillip Helbig---remove CLOTHES to reply
>>>>>> <hel...@astro.multiCLOTHESvax.de> writes:
PH> Personally, I think there is nothing mysterious about dark
PH> matter. After all, why should everything be so convenient so
PH> as to glow so that an astronomer can see it?
>> Particularly given that we know of one kind of dark matter already,
>> neutrinos. They appear to have some mass and they do not interact
>> via the electromagnetic force (i.e., they are dark). If there's
>> one kind of dark matter, why not another?
m> except that neutrino is a hot-dark matter candidate
People seem to be misinterpreting my comment. With respect to the
current state of cosmology, there are at least three options:
a) Our census of the Universe is incomplete;
b) There's new physics; or
c) All of the above.
I think that the most simple explanation is that GR is the correct
form of gravity on macroscopic scales and that we have not detected
all of the particles in the Universe.
Even for luminous matter, depending upon how you split things, there
are at least two different ways to describe it: baryonic (i.e.,
protons, neutrons, etc.) and leptons (electrons, muons, etc.). Of
course, some leptons are also dark matter, notably neutrinos.
m> and we knew in the 80's itself that hot dark matter dominated
m> universe is more or less ruled out.
Yes, but that doesn't rule out the possibility that there are other
forms of dark matter, e.g., cold dark matter.
m> note that besides dark matter (which implied that on galactic
m> scales gravity is more attractive as compared to Newtonian gravity)
m> you also have to account for dark energy (which implies that on
m> cosmologica scales gravity is repulsive) IMO in my opinion a theory
m> which accounts for both ought to be taken seriously.
Of course, GR does both quite perfectly. There's no requirement
within GR that mass be luminous, and the addition of a cosmological
constant is one way of explaining dark energy.
melroy...@hotmail.com
astro-ph/0504130 which shows that Bekenstein's TeVes theory can
also explain cosmic acceleration.
Enjoy.
Melroy