Dark matter, dark energy

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marcus

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kurious said:
So maybe MOND comes into play at large distances.
Mond is ad hoc (underlying theory, if any, unknown) so
everyone can have his own thumbrule for it----his own version of mond. Yes?

Smolin version MOND out of that lecture I gave the link for
says not what you say "mond comes into play at large distances"

It says mond comes into play at low accelerations

there is a lower threshhold of acceleration----whenever the centripetal acceleration falls below this threshhold then there is a geometric mean modification which brings the acceleration up a little (not up all the way to the theshhold but a little raised up towards the threshhold, from the bare Newtonian value)

this version of mond would explain the Pioneer anomaly. the vehicle was far enough from the sun that the centripetal accel. had fallen below this threshhold---and then it was larger than expected

this version of mond achieves the nice fit to the galaxy rotation curves.
when the stars are far enough from the center that their centripetal accel is down below threshhold, then their accel is larger than expected.

this threshold accel is connected to the CC.

the connection is by way of a length L = Lambda-1/2

I personally do not advocate this length L.
The big question is what is it doing in our universe----if it is related to galaxy rotation curve and to pioneer anomaly, and also to acceleration of expansion.

the length L is 9.5 billion LY. that is what, if you square it and take the reciprocal, you get the cosmological constant current estimate.
 

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turbo-1 said:
I wondered about that how the MOND folks approach gravitational lensing and found this paper.

http://citebase.eprints.org/cgi-bin/citations?id=oai:arXiv.org:astro-ph/9406051 [Broken]
Anything more recent (that paper is >10 years old now)? For example, this 2002 page seems to suggest that for both "Galaxy Clusters" and "Gravitational Lensing", MOND is 'uncertain but not promising'
 
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marcus

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Nereid said:
Anything more recent (that paper is >10 years old now)? For example, this 2002 page seems to suggest that for both "Galaxy Clusters" and "Gravitational Lensing", MOND is 'uncertain but not promising'
thanks Nereid! this link you gave
http://www.astro.umd.edu/~ssm/mond/mondvsDM.html

is also very helpful. I had not realized how successful MOND was in predicting other things. It is even more impressive than I thought. So I agree all the more with your "certainly interesting" assessment.

However am still leery because cant imagine any underlying theory
 

marcus

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Perhaps MOND can be explained by a flow of neutral particles entering our galaxy and pushing stars and the pioneer spacecraft towards gravitational sources, making
the force of gravity seem stronger than it should be.
 
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marcus said:
thanks Nereid! this link you gave
http://www.astro.umd.edu/~ssm/mond/mondvsDM.html

is also very helpful. I had not realized how successful MOND was in predicting other things. It is even more impressive than I thought. So I agree all the more with your "certainly interesting" assessment.

However am still leery because cant imagine any underlying theory
I have a sneaking suspicion that MOND is due to an as yet unknown affects of GR. This could be gravitational redshifting of rest mass, or perhaps gravitational redshifting of gravitons, or perhaps the affect of the gravitational effects of the universe as a whole as the universe becomes less dense.
 

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MOND is pretty interesting, and it points out that we may not understand gravity as well as we had hoped. One consequence is that we may once again have to regard gravitation as a force acting over a distance through a medium. GR says that mass curves space-time, and orbits are merely paths along momentum-conserving geodesics, which may be true, but we may at some point have to reconsider that concept, if dark matter remains as elusive as ever and MOND continues to make accurate predictions regarding the behavior of galaxies. If MOND survives the next few years, we may also have to reconsider whether gravitational mass and inertial mass are truly equivalent. Exciting times.
 

marcus

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turbo-1 said:
MOND is pretty interesting, and it points out that we may not understand gravity as well as we had hoped. One consequence is that we may once again have to regard gravitation as a force acting over a distance through a medium. GR says that mass curves space-time, and orbits are merely paths along momentum-conserving geodesics, which may be true, but we may at some point have to reconsider that concept, if dark matter remains as elusive as ever and MOND continues to make accurate predictions regarding the behavior of galaxies. If MOND survives the next few years, we may also have to reconsider whether gravitational mass and inertial mass are truly equivalent. Exciting times.
yes turbo exciting times
let's be cautious about giving up on GR (and falling into the arms of action at a distance) because it could be that a small modification of the geometric approach will achieve the results of mond.

you might look at the second half of page 15 of
http://arxiv.org/gr-qc/0406100 [Broken]

the game is, can you get mond-like effects by bending the tangent space (where energy and momentum and acceleration are born) a little.
well that is putting it too vaguely, but you can see what Florian Girelli says in the paper.
 
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Nereid

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The August 2002 issue of Scientific American has an article by Milgrom ("Does Dark Matter Really Exist?"). Here's what he had to say about GR and cosmology in that article (extracts only):

"Successful as it may be, MOND is, at the moment, a limited phenomenological theory. [...] And MOND is limited because it cannot yet be applied to all the relevant phenomena at hand. The main reason is that MOND has not been incorporated into a theory that obeys the principles of relativity, either special or general. [...] The phenomena that fall outside the present purview of MOND are those that involve, on the one hand, accelerations smaller than a0 (so that MOND plays a role) and, on the other, extreme speeds or extremely strong gravity (so that relativity is called for). [...] Light propogating in the gravitational fields of galactic systems [satisfies] both criteria. MOND cannot properly treat this motion, which pertains to gravitational lensing. [...] A second system which requires MOND and relativity is the universe at large. It follows that cosmology cannot be treated in MOND."
 

marcus

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Nereid said:
The August 2002 issue of Scientific American has an article by Milgrom ("Does Dark Matter Really Exist?"). Here's what he had to say about GR and cosmology in that article (extracts only):

"Successful as it may be, MOND is, at the moment, a limited phenomenological theory. [...] And MOND is limited because it cannot yet be applied to all the relevant phenomena at hand. The main reason is that MOND has not been incorporated into a theory that obeys the principles of relativity, either special or general. [...] The phenomena that fall outside the present purview of MOND are those that involve, on the one hand, accelerations smaller than a0 (so that MOND plays a role) and, on the other, extreme speeds or extremely strong gravity (so that relativity is called for). [...] Light propogating in the gravitational fields of galactic systems [satisfies] both criteria. MOND cannot properly treat this motion, which pertains to gravitational lensing. [...] A second system which requires MOND and relativity is the universe at large. It follows that cosmology cannot be treated in MOND."
The main reason is that MOND has not been incorporated into a theory that obeys the principles of relativity

this is a useful perspective (from the standpoint of August 2002) because it highlights what is new in the following 2004 papers:

http://arxiv.org/gr-qc/0406100 [Broken]

The prospect of building mond-like effects into a GR-type theory
is what they are talking about on the second half of page 15 and in the last 3 or 4 lines of the conclusions paragraph at the end.

See also another 2004 link I just gave a few posts back.
http://ws2004.ift.uni.wroc.pl/html.html [Broken]

or pages 3 and 4 of
http://arxiv.org/hep-th/0406276 [Broken]
where mond is again approached from a cosmological constant direction.

At this point we are, i believe, merely talking about some coherent lines of investigation being taken by a few people---the 3 authors of one paper and the 2 authors of the other and possibly others I dont know of. Oh yes a couple of physicists at Monpellier too. Just a handful. And this line of investigation could not pan out! If-you-want-to-be-sure-of-results-go-somewhere-else-sort of thing.

The significance is what your August 2002 quote points out, the need to build mond into a GR-type theory, which is precisely what these people see as a promising line of research.
 
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Nereid

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Thanks marcus, I should have acknowledged your posts (and the papers); perhaps I was too focussed on passing on what the founder of MOND had to say about these two aspects. Good to see that much has happened in only two years!
 

marcus

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Nereid said:
Thanks marcus, I should have acknowledged your posts (and the papers); perhaps I was too focussed on passing on what the founder of MOND had to say about these two aspects. Good to see that much has happened in only two years!
Dear Nereid, you are generous. I wish that much had happened in this department, and we would both be very pleased. But IMO the most that can be said is that this is a line of investigation which appears to a handful of people to show promise. As far as I know there is little else to report. Perhaps if you check the links out yourself you may see something substantial which I missed.

The quote from Milgrom, the urMONDer, is just what we need to establish a kind of baseline. They must incorporate mondy effects in a (nice) theory, which they have not done yet, and we have Milgrom's word for it. :smile:
 

Chronos

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For the record, I despise dark matter and dark energy. I am very resistant to the idea the universe is mostly composed of 'invisible stuff'. And no, I don't have a better theory. MOND is ad hoc and I have a visceral distrust of ad hoc theories. Err, did I mention I think string theory is the most contrived waste of time in the history of science? Beat me with a cricket bat till I come to my senses.
 
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Kurious:

MOND cannot explain gravitational lensing according to Milgrom.
So whatever causes MOND interacts weakly with photons?
Also Greywolf has said on these forums that if electromagnetism is taken into
account, the anaomalously high velocities in galaxies can be accounted for.
 

marcus

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Kurious:

1.MOND cannot explain gravitational lensing according to Milgrom.

2.So whatever causes MOND interacts weakly with photons?

3.Also Greywolf has said on these forums that if electromagnetism is taken into account, the anaomalously high velocities in galaxies can be accounted for.
-----end quote----

1. Do you have something recent from Milgrom? All I have seen is something dated August 2002. Should we write Mordehai Milgrom email and ask him his current opinion? Also I dont know whether to consider him the authority on MOND (no matter what he says in 2004) since he is primarily known for having been the first to postulate it, quite some time ago. Is he still at the forefront? Personally I have no idea.

2. Whatever causes MOND interacts weakly with photons? "Whatever causes" would be some underlying physics incorporating the observed MOND effect, still to be discovered. Whatever underlying physics, if it and the effect are real, would probably fit the observed gravitational lensing as well.

3. :confused:

------------------

Dear Kurious, I urge you to have a look at Smolin's lecture slides on this.
I've given the link several times.
http://ws2004.ift.uni.wroc.pl/html.html [Broken]
Go to "Lectures" and click on Smolin's third talk.

either mond is real and has some nice underlying physics, or not. we should be finding that out and hopefully soon.
there is a strange coincidence that points to the possibility of some nice physics, and it might be purely coincidence or it might not.

you ought to be aware of that coincidence, perhaps you are.

the coincidence is between the cosmological constant Lambda that governs the accelerating expansion
and the acceleration threshhold of the MOND effect

if someone can either explain that coincidence, or show that it is purely coincidence and therefore insignificant, it would be a bit of progress
 
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marcus said:
The significance is what your August 2002 quote points out, the need to build mond into a GR-type theory, which is precisely what these people see as a promising line of research.
Do these "MOND"ifications you speak of mean altering the equations of GR a little with some more fudge factors added to Einstein's field equations? Or do they mean to apply the present GR equations in a different way?

It occurs to me that if the frequency changes of string vibrations alters mass due to gravitational effects, then you would see a breaking in symmetry between inertial mass and gravitational mass, as is done in MOND. The gravitational affects on mass would change, but it would seem that in the local frame of reference the inertial mass would not be affected by the change due to the distance to other massive objects.
 

turbo

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Googling on gravity and pendulum, I found this paper regarding an interesting pendulum experiment. Does anyone here know if these results have been replicated anywhere?

From the paper below: "The results of Fourier analysis show, that the transferred first harmonic gravitational energy is much lower compared to the theoretical calculation that was based on Newtonian theory. The reason of this discrepancy is due to the significantly decreasing interaction force between the equal masses, which was proven by a proper phase analysis of the excitation. From an evaluation of the measured data, we have concluded that the energy transfer between equal masses (both being 24 kg) was less then seven percent of the theoretical value based on the Newtonian gravity model."

http://www.journaloftheoretics.com/Articles/3-6/Grav-pub.htm
 
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Marcus:
the coincidence is between the cosmological constant Lambda that governs the accelerating expansion
and the acceleration threshhold of the MOND effect

Kurious:
Milgrom pointed out originally that if
you divide the speed of light by the age of the universe
in seconds you get 10^-10 - the MOND threshold.
I think it is a good idea to contact Milgrom and see what he
has to say about MOND.


"proponents of MOND point out that some of the candidates for dark
matter such as WIMPs are as astonishing as asserting that gravity behaves differently over long distances than we normally think"

from an interesting overview of MOND at:
http://encyclopedia.thefreedictionary.com/Modified Newtonian dynamics
 
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marcus

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kurious said:
Kurious:
Milgrom pointed out originally that if
you divide the speed of light by the age of the universe
in seconds you get 10^-10 - the MOND threshold.
I think it is a good idea to contact Milgrom and see what he
has to say about MOND.
Marcus: :smile: yes but dont you think that is rather lame?
why would the age of the universe have anything to do with it?

the galaxies we see (and measure rotation curves for) come from
all different times in the history of the universe

they all have different "ages of the universe" so should they all
have different MOND acceleration threshholds? But no.

Please consider other possibilities. By now one would expect that in the ordinary course of events other people would have done more constructive things with Milgrom's suggestion than Milgrom did himself originally. If a suggestion is any good, people take it and go with it.
 
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marcus

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Dear Kurious,

the age of the universe is not a fundamental physical constant.

there are some fundamental physical constants, like the mass of the electron, and fine structure alpha, and Newton G, and the speed of light.

they are nice.

if there is a nice physical theory underlying the observed MOND effect then the approximate threshhold will probably not be sharp, it will most likely be an acceleration marking the rough location where a smooth curve has a smooth change in behavior

that place, that threshhold, will be calculable from fundamental physical constants-----or it will itself be a fundamental constant----this is the way things have turned out in the past. It is the way physics is organized and how it has grown up.

When Milgrom saw this acceleration threshhold he didnt have Lambda, so he punted.
Lambda the CC contains the length 9.5 billion lightyears----call it L.
If Lambda is a fundamental constant (to be one day listed with the others at the NIST website) then so is L.

If Milgrom had known about L in the 1980s, he would not have had to punt and make a wild seemingly rather shallow guess. he would have recognized, or someone would have told him,
that the acceleration is c2/L

then the age of the universe would not have been mentioned.

unfortunately nobody had a figure for the CC until 1998
that was the year cosmology totally changed.

anyway that's my personal take on it. I'd be interested to know what Milgrom says now, and i hope you write him email, but i would not
automatically take it as authoritative about MOND (who is an authority?
MOND is a fascinating flukey business)
 
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This distance L is about 10^25 - 10^26 metres.
The Newtonian calculation I did earlier in this thread
gives a decceleration due to gravity of 10^-11 m/s^2
at this distance range.
This is of a similar order of magnitude to the acceleration
of supernovae due to dark energy at the same distance.
It is as though gravity has changed signs.This may be a trivial point or it may not.
 

Nereid

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kurious said:
This distance L is about 10^25 - 10^26 metres.
The Newtonian calculation I did earlier in this thread
gives a decceleration due to gravity of 10^-11 m/s^2
at this distance range.
This is of a similar order of magnitude to the acceleration
of supernovae due to dark energy at the same distance.
It is as though gravity has changed signs.This may be a trivial point or it may not.
How would you go about determining whether it's trivial or not? What experiments or observations would you suggest that might help (in principle ones are perfectly OK)?
 

marcus

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kurious said:
This distance L is about 10^25 - 10^26 metres.
The Newtonian calculation I did earlier in this thread
gives a decceleration due to gravity of 10^-11 m/s^2
at this distance range.
.
Hi Kurious, it is great to be doing calculations with you! Let me check
your figures. The relevant thing one wants to do, assuming both L and c are fundamental constants, is calculate the only acceleration which it is possible to calculate directly from a speed and a length, namely
c2/L

I assume this is what you calculated, so I will see if I get the same thing!

A lightyear is 9.46E15 meters and L is 9.5E9 lightyears
so L is 8.99E25 meters, but let us be relaxed and call it 9E25 meters.

Now squaring the speed of light gives a number like 9E16 and we have to divide that by the number 9E25 we got earlier for L. So we get

E-9 meter per second per second.

10-9 meter per second per second

One nanometer per second per second.

But wait Kurious! this does not seem to be the same as what you got.

Have I made a mistake?

Or did you use some other method for calculating an acceleration?

did you by any chance use the Hubble parameter? Remember that this is not a constant and has changed radically in the course of a few billion years---one cannot use it as a constant for this type of thing.

Anyway please clue me in how you got something two orders of magnitude different.
 

marcus

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Nereid said:
How would you go about determining whether it's trivial or not? What experiments or observations would you suggest that might help (in principle ones are perfectly OK)?
Would it be OK, Nereid, to focus first of all on seeing how Kurious got the number 10-11 before we talk about designing experiments?
the number itself strikes me as two orders of magnitude too small and may indicate some misunderstanding which we might clear up
 

Nereid

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marcus said:
Would it be OK, Nereid, to focus first of all on seeing how Kurious got the number 10-11 before we talk about designing experiments?
the number itself strikes me as two orders of magnitude too small and may indicate some misunderstanding which we might clear up
Well, kurious is free to answer either your post or mine, or Chronos' ... or free to ignore all of us :cry:

Personally, I like to see if folk who have questions and ideas can think through the implications of these for themselves (you're all totally shocked, right? Not in a million years did you think that Nereid had such thoughts! the shock! the horror!!) ... and some may think I have a particular interest in doing experiments and making observations :cool: (I can't for the life of me think how anyone could form such an opinion, but, it's a free world).
 

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