What Is the Expansion Limit of Our Universe?

[Garth]

Can you tell me where I could find out more about this, Wallace?

Perhaps?

Angus, G. W.; Famaey, B.; Zhao, H. S.: Can MOND take a bullet? Analytical comparisons of three versions of MOND beyond spherical symmetry

Abstract
A proper test of modified Newtonian dynamics (MOND) in systems of non-trivial geometries depends on modelling subtle differences in several versions of its postulated theories. This is especially true for lensing and dynamics of barely virialized galaxy clusters with typical gravity of scale a0. The original MOND formula, the classical single-field modification of the Poisson equation, and the multifield general relativistic theory of Bekenstein (tensor-vector-scalar, TeVeS) all lead to different predictions as we stray from spherical symmetry. In this paper, we study a class of analytical MONDian models for a system with a semi-Hernquist baryonic profile. After presenting the analytical distribution function of the baryons in spherical limits, we develop orbits and gravitational lensing of the models in non-spherical geometries. In particular, we can generate a multicentred baryonic system with a weak lensing signal resembling that of the merging galaxy cluster 1E 0657-56 with a bullet-like light distribution. We finally present analytical scale-free highly non-spherical models to show the subtle differences between the single-field classical MOND theory and the multifield TeVeS theory

Milgrom, Mordehai:http://adsabs.harvard.edu/abs/2007a...he mass discrepancies in tidal dwarf galaxies

Abstract
I consider in light of MOND the three debris galaxies discussed recently by Bournaud et al.. These exhibit mass discrepancies of a factor of a few within several scale lengths of the visible galaxy, which, arguably, flies in the face of the cold dark matter paradigm. I show here that the rotational velocities predicted by MOND agree well with the observed velocities for each of the three galaxies, with only the observed baryonic matter as the source of gravity. There is thus no need to invoke a new form of baryonic, yet-undetected matter that dominates the disc of spiral galaxies, as advocated by Bournaud et al.. I argue that the presence of such ubiquitous disc dark matter is, in any event, rather unlikely on other grounds.

Garth

 

[Chronos]

I think GPB was a bad idea. But my opinion is irrelevant. I don't get a vote on these projects. The results were mostly known before that mission. I'd have rather seen the money spent on the Hubble telescope.

 

[Garth]

I think GPB was a bad idea. But my opinion is irrelevant. I don't get a vote on these projects. The results were mostly known before that mission. I'd have rather seen the money spent on the Hubble telescope.

Which is a good example of what I was saying:

However an over-confidence placed in the mainstream model may discourage effort that may put in developing other alternatives.

The reason it is good to have other viable alternative models is that they provide something against which the main model may be tested. The nature of both the main model and an alternative determines what questions should be asked and what experiments are worthwhile.

Witness the question about whether the $800 million spent on the Gravity Probe B experiment was well spent or not.

If you think the mainstream model and the theory it is based on - GR - is so well established, then there is no need to test it, and the money so spent is wasted.

IMHO if the final results from that experiment are exactly those predicted by GR then at least that would falsify a whole sheath of alternative theories.

However the hint of a non-GR result, (see http://einstein.stanford.edu/content/aps_posters/APS_talk_Everitt.pdf page 21), at present seen through the noise of unexpected experimental error makes the next few months exciting!

Garth

 

[Wallace]

Can you tell me where I could find out more about this, Wallace?

If you're still interested, check out the papers Garth suggests in post #61. As I've said the issue with MOND, and these two papers are a reasonable example, is that you need to tweak the paramters of the gravity theory for each system studied. So the parameters used in each paper, that find that MOND is a good fit, are different. Use the theory of gravity of one paper with the other and the fit would not be as good.

There are many many papers that detail the best fit parameters for the LCDM model across a wide range of data sets. The MOND crowd are still operating as they did years ago, taking individual systems and constructing ad hoc parametrisations to fit them. What really needs to be done is the let the MOND parameters vary and confront the predictions with the full data set (at once) to arrive at the best fit MOND model for the Universe. If this fit could be shown to be concordant with all available data (as has been done with LCDM) then things will become very interesting. The fact that MONDians have not attempted (to my knowledge) this kind of open and thorough analysis makes me very suspicious. This is the first thing I would do if I was to look at MOND.

Before anyone suggests that this would be too much work for a few mavericks to attempt, I really don't think it would be that hard. Take somthing like http://cosmologist.info/cosmomc/" , adjust the SN likelyhood for the MOND m vs z prediction, modify the CMB code for MOND and your pretty much done. Even ignoring the structure data, which may be a bit tricker to handle with MOND, you would get a good idea of how well MOND can independantly handle these two data sets, and have an idea of how well constrained the gravity parameters are, so if the marginal distributions of the gravity parameters are wide that's pretty clear evidence that either physial laws are not homogenous or that MOND isn't a good theory.

 

[Pakbabydoll]

ok yea I am totally lost... I take my first physics class this fall and have read some basic physics books but these calculations went right over my head.. can you email me and explain please?

 

[Chronos]

It's not too much work, Wallace, as you probably know. The efforts to date simply have not been . . . convincing. And I'm not insisting they must be at this time, just pointing out they have a long row to hoe before MOND can be considered a serious contender.

Footnote for Garth: I agree the research was useful and may produce useful new science. I only object to the price tag. But hindsight is 20-20. The money was mostly spent before the new results were derived. Damned if you do, damned if you don't. I'm thinking of putting that on my resume to NASA . .