What Is the Expansion Limit of Our Universe?

[Garth]

Actually I am not saying we have to 'make' DM or the Higgs Boson/Inflaton in the laboratory, just that we have to observe them under controlled conditions before we can be certain of their existence and nature.

The whole point of empirical science is that it is based on what is observed rather on that which is not.

Garth

 

[Wallace]

The whole point of empirical science is that it is based on what is observed rather on that which is not.

Garth

Precisely, and DM and DE have (or the effects that we currently think could be ascribed to these entities) been observed. Many times, in great detail. Observing the gravitational lensing of DM is a galaxy cluster is no different from measuring the scattering potential of a sub-atomic particle in an accelerator. We observe the effects of something we cannot see on something that we can.

Lab experiments are not fundamentally different from cosmology observations. If it is not possible to observe DM and DE 'under controlled conditions' (which may or may not be the case) do you really think we should therefore cease to study the cosmos, since 'we could never be sure' of what we discover, in which case, what is the point? That is the logical extension of your position. Cosmology says that we need DM and DE (or something that looks like them), further observations will only refine the parameters. Given this then, we should stop spending any money on cosmology until we find these in a lab, since anything else that cosmology discovers about their properties is only 'provisional'.

 

[Garth]

There is a fundamental difference between an experiment in which the conditions can be controlled, and varied, and the remote sensing of a distant object.

Of course I do not think we should cease to study the cosmos, studying and understanding the cosmos has been a life long passion for me, however we need to be mindful of our limitations.

"Cosmologists are often in error but never in doubt" - - Lev Landau

Garth

 

[Wallace]

Right, but promoting the Milne model is not being 'mindful of our limitations', it goes much further than that.

The best we can do it be as honest as possible about what the data that we can get says. If we have strong evidence for a certain physical theory then let's be happy to say so, rather than trying to squirm out of what the data says because of what we think it should say.

We can never make stars, galaxies, heck until relatively recently we couldn't test gravity other than by observing the sky. Yet we've learned a lot about these things (even if we have a long way to go).

 

[Garth]

But I am not necessarily promoting the Milne model, but as an alternative and simpler model to the standard I think it deserves consideration, alongside MOND or other possibilities.

The motivation for the strictly linear expansion model is that it does not require Inflation, the motivation for MOND is that it does not require DM. While the Higgs Boson and DM particle remain undetected 'in the lab' these alternatives deserve to be kept in mind.

If in the Milne model case, R(t) \propto t, k = -1, high \Omega_b ~ 0.69, low h ~ 0.65, does not produce the correct large scale structure formation over its longer developmental time, as you report, (BTW is there a published reference to this work?) then one question would be: "Is there anything that can be added to make it work? - such as non-interacting DM?"

If the MOND model requires a mechanism to make it work then what combination of additional scalar, vector and tensor fields will provide this?

If this seems 'ad hoc' then that would be a valid criticism, but no more so than with the standard model that required Inflation/DM/DE to make it work.

As a matter of personal 'preference' the model my own work throws up is R(t) \propto t, k = +1, and that has its problems, I know! I am not promoting that here, as a published theory I have been allowed to discuss it in another thread.

But when others report that a simple model, such as the Milne, passes basic cosmological constraints I take interest. At least it provides an alternative to test the standard model against.

Garth

 

[Wallace]

If in the Milne model case, R(t) \propto t, k = -1, high \Omega_b ~ 0.69, low h ~ 0.65, does not produce the correct large scale structure formation over its longer developmental time, as you report, (BTW is there a published reference to this work?) then one question would be: "Is there anything that can be added to make it work? - such as non-interacting DM?"

I haven't published this stuff, I just ran some models once to have a squiz. I don't think it's worth publishing since it's a pretty obvious result. I'm sure there are some older (~ 2dF heyday) papers that would have some figures, I'll see what I can find. As for adding DM, my simulation were with DM, as far as most N-body simulations go matter is matter, whether dark or baryonic (people do add 'gas physics' and 'star formation' in some studies but the way to do this rigorously is not yet established). So no, DM doesn't throw Milne a life line.

If this seems 'ad hoc' then that would be a valid criticism, but no more so than with the standard model that required Inflation/DM/DE to make it work.

The problem with MOND is that you need to have a different theory of gravity to fit each galaxy or cluster, since the same parameters do not fit each galaxy. Now that is what I call ad hoc! DM and DE are not ad hoc in this way, since the whole reason it is sometimes known as the 'concordant' model is that the same parameter values work for every data set. That's an incredibly powerful and significant result and far from 'ad hoc'.

But when others report that a simple model, such as the Milne, passes basic cosmological constraints I take interest. At least it provides an alternative to test the standard model against.

Garth

You can always find papers to support pretty much any position. You need to go a little deeper than the abstract to see if the claims are valid though and unfortunately several papers you have pointed myself and others to on this topic have been fairly shoddy to say the least. Science is not a democracy, I'm not discounting these paper because they are in the minority numerically, but they do not demonstrate their claims with any rigour (at least not that I have seen).

 

[Garth]

Nevertheless, even if present alternatives fall short, I continue to maintain that the mainstream interpretation of cosmological observations must be taken as 'provisional' until the existence of these dark particles/energy is confirmed under controlled conditions.

Garth

 

[Chronos]

Cosmology is always provisional. We can't create planets in the laboratory, but most agree they do exist. Right now, the preponderance of evidence is in the concordant court. And it looks pretty firm. While a single observation could unsaddle that horseman, none have yet suceeded. I think it fair to concede the concordance model is a reasonable approximation. But that does not diminish your efforts, Garth. Asking a few hard questions is always worthwhile. The concordance model does not always yield answers, but is not demonstrably false. I don't mind walking along the fence row, but am reluctant to climb it.

 

[Garth]

Of course, I agree with that statement.

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.

Garth

 

[Voltage]

...The problem with MOND is that you need to have a different theory of gravity to fit each galaxy or cluster, since the same parameters do not fit each galaxy...

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

 

[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 . .