non-GR theories of gravity

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?

Phillip Helbig---remove CLOTHES to reply
wrote in message ...

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}

In article , "greywolf42"
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. :-)

In sci.astro.research Phillip Helbig---remove CLOTHES to reply
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

Phillip Helbig---remove CLOTHES to reply 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.) 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 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

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.



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?

wrote:


... 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/u...inney/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"

Phillip Helbig---remove CLOTHES to reply
wrote in message ...
In article , "greywolf42"
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?

--
greywolf42
ubi dubium ibi libertas
{remove planet for return e-mail}

"Phillip Helbig"
schreef in bericht
...

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/

In article , "greywolf42"
writes:

In article , "greywolf42"
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.