Adding dark matter to the model is adding mass. Dark matter has mass. It's just not visible mass. In other words, the dark matter hypothesis is that the actual mass of galaxies is larger than the mass we infer from what we can see.Structure seeker said:I'm curious to know how they explain galaxy rotation curves without adding mass?
So what? This is an "A" level thread in the cosmology forum. That means you are expected to have background knowledge in the subject matter equivalent to a graduate student. If you don't have that background knowledge, you need to go get it if you want to be able to make useful posts in this thread. We are not going to clutter up an "A" level thread with explanations of things that anyone with the requisite background knowledge should already know. That includes the basic tenets of the ##\Lambda C D M## model and the dark matter hypothesis and the reasons for it. Anyone posting in this thread needs to already know all those things.Structure seeker said:cosmology is not my area of expertise, but mathematics with an inclination towards physics
Your terminology here might be somewhat confusing. Dark matter is a "GR" solution--one which contains matter (stress-energy) that is not visible and whose only effects are gravitational. It still uses the Einstein Field Equation of GR, with no modifications; the only "modification" from a model that only includes the matter visible to us is to also include matter that is not visible to us. The matter content (stress-energy tensor) is a free parameter in GR.astronomer said:it has been established beyond discussion that a DM halo can stabilize the disk (it should be recalled that the stability of disk galaxies was the first hint of the existence of DM halos in disk galaxies, before the observation of flat rotation curves). What about GR?
This is the point I was making, maybe you put it better!PeterDonis said:The ##\Lambda CDM## model is based on the Friedmann equations, which in turn are solutions to the Einstein Field Equation of GR.
None of this has anything to do with the ##\Lambda CDM## model. You are talking about models of individual galaxies that are developed to try and explain their observed rotation curves. ##\Lambda CDM## is a model of the universe as a whole.
ohwilleke said:The most obvious candidate for the discrepancy is that the self-interaction term of the GR Lagrangian includes a physical constant that should in principle be determinable from first principles in GR using only Newton's constant G and the geometry of the mass distribution. Deur doesn't actually calculate this constant from first principles in GR, however. Instead, he uses the same observations used to establish the MOND critical acceleration a0 to calibrate this constant in the case of mass distributions with a geometry in line with an idealized spiral galaxy. He basically has faith that the physical constant in question could be calculated and would reproduce the observed value, without actually doing that involved calculation and no later paper from Deur has attempted to do that calculation.
As best I can discern, however, the paper reviewing Deur's work, basically does appear to use first principles to establish the value of this physical constant and in doing so finds that the effect is to small.
My analysis above of what I surmise is going on involves some guesswork and reading between the lines because the paper critical of Deur's approach basically reconstructs its own scalar approximation from scratch and compares its end results with Deur's, rather than going step by step through the analysis that Deur did in order to pinpoint where they believe he veered off course. It could be that this is not actually the issue, but it seems like the point of Deur's analysis most prone to a magnitude of conventional GR self-interaction effect outcome.
If I am right about what is going on, then the basic issue is that Deur is implicitly modifying GR by assuming a stronger coupling between gravitons of a given mass-energy than the coupling between gravitons and other fundamental particles from the Standard Model. (I'd be particularly curious if the strength of the attraction between gravitons was implicitly the square of the naive expectation, but I don't know how the observationally estimated parameter and the first principles parameter compare to each other.)
I can see where the problem lies. Possibly, I should have made my entries clearer, but I was using the 'ΛCDM' term in this thread to mean the use of dark matter in galaxy dynamics, since that was what this thread was discussing. I should have specified dark matter and not ΛCDM (c.f. #254). Apologies for the confusion.PAllen said:This is just absurd. Lambda-DCM is described by a GR metric and nothing else - the FLRW metric family. All its physical content is contained in this full GR model. This is a theory of cosmology. There is no Lambd-CDM model of galaxy. That is a separate problem, in which dark matter is typically used, but it is not using the Lambda-CDM cosmological model.
It is interesting that both the quotes above (from PAllen: 'There is no lamba-ΛCDM model of galaxies', and PeterDonis: 'ΛCDM; that is a model of the universe, not of galaxies', appear to want to separate galaxy dynamics from full ΛCDM. I thought one of the strengths of ΛCDM was that it was a full description of the cosmos. Have I got this wrong? MOND and even Deur's position is of a unifying description of the cosmos.PeterDonis said:ΛCDM; that is a model of the universe, not of galaxies.
Yes. Nobody has "a full description of the cosmos" in the sense you mean--a single unified detailed model that accounts for everything at all scales.Adrian59 said:I thought one of the strengths of ΛCDM was that it was a full description of the cosmos. Have I got this wrong?
No, they aren't. As above, nobody has a single unified detailed model that accounts for everything at all scales. MOND's and Deur's models of galaxies do not also include the evolution of the universe as a whole. They might claim that they could, using similar general principles, construct separate models of the universe as a whole, but those would still be separate models.Adrian59 said:MOND and even Deur's position is of a unifying description of the cosmos.
Deur claims that field self-interaction "increases" gravity within and as a consequence "decreases" gravity outside mass distributions and hence mimics Dark Matter and ##\Lambda## as well. In his model of the universe the SN Ia data aren't interpreted as accelerated expansion but as to be due to the universe' anisotropy, in other words, the cosmological principle doesn't hold according to Deur. Consequently Deur's modified Friedmann equations don't contain ##\Lambda## but an anisotropy term instead.PeterDonis said:No, they aren't. As above, nobody has a single unified detailed model that accounts for everything at all scales. MOND's and Deur's models of galaxies do not also include the evolution of the universe as a whole. They might claim that they could, using similar general principles, construct separate models of the universe as a whole, but those would still be separate models.
Yes, but in my description, this corresponds to the theory (GR), not the specific model. He still does not have a single unified model that captures both the evolution of the universe as a whole and the dynamics of individual galaxies. Nobody does.timmdeeg said:Deur claims that field self-interaction "increases" gravity within and as a consequence "decreases" gravity outside mass distributions
I agree that we haven't got a single equation to describe all scales, but I am less sure that ΛCDM is not a theory of galaxies as well. Probably I need to be more specific. I suppose what I am really referring to is what is known as the 'Standard Model of Cosmology' (SMC) which as its name implies is a complete model, and as such should have all scales covered. I often somewhat lazily use the term ΛCDM as synonymous with SMC. To me the SMC contains the big bang, inflation, baryogenesis, nucleosynthesis, dark matter and dark energy. Strictly speaking ΛCDM is dark energy (Λ) and cold dark matter (CDM). Of course behind all this is GR and the Friedmann equations on the large scale.PeterDonis said:Yes. Nobody has "a full description of the cosmos" in the sense you mean--a single unified detailed model that accounts for everything at all scales.
##\Lambda C D M## uses a unified theory, General Relativity, that covers phenomena at all scales. But ##\Lambda C D M## itself is a particular model built using that theory to cover a particular domain, the evolution of the universe as a whole. It certainly does not also include detailed models of individual galaxies; like all models of the universe as a whole, it treats the matter in the universe as a continuous fluid and ignores the clumping at smaller scales.
Similarly, models of galaxies that attempt to explain their rotation curves are models of galaxies. They are not models of either the universe as a whole or systems on smaller scales, such as individual stars or solar systems. This applies to MOND and Deur's models just as much as to dark matter models.
Ok, this makes it clearer what you meant, but you should understand that it is not standard usage. In fact the term "Standard Model of Cosmology" is not standard usage either as far as I know. But it is true that the field of cosmology contains models at all scales and that those models are supposed to be consistent with each other. It is also true that dark matter is believed by most cosmologists to be necessary in models at multiple scales (at the very least, the universe as a whole, and at the scale of galaxies).Adrian59 said:I often somewhat lazily use the term ΛCDM as synonymous with SMC.
Having been active in this thread, and as such I have read the references offered by other contributors, including this one from Lasenby et al. I thought that there was an issue with this paper that I mentioned in #211. I asked for clarification on this but none has been forthcoming. Has anyone re-read this reference and able to comment. To aid any comment I will copy my original question below:PAllen said:This paper is claimed to be thoroughly refuted by https://arxiv.org/abs/2303.06115, so the debate goes on.
Adrian59 said:However, maybe someone can clear up an issue that I've found in the Lasenby et al paper, already referenced. The authors examine NGC 1560 saying, 'we will restrict our attention here to the model having the MN density profile (16), which can be treated almost entirely analytically and suffices to demonstrate the shortcomings of the overall approach'. Using this Miyamoto–Nagai (MN) density profile, they get equation (21) which is soluble by a numerical method. They comment that one can get a simpler expression by approximating and solving this expression (22). Both solutions are plotted in figure 2.
What they do not plot in this figure are the experimentally observed values though the authors do say 'Although the rotation curves obtained using either (21) or the analytic approximation (22) appear to fit the rotation curve data for NGC 1560 shown ... in a pleasing way'. Although, had they plotted the experimentally observed velocity values, one would see that these derived curves from the gravito-magnetic approach are compatible with these experimentally derived values. So it is difficult to square this with an alleged failure of this novel approach.
The authors, then, plot the curve without a gravito-magnetic correction and get a standard rising and decreasing curve, the one usually shown as evidence of the need for dark matter, on a separate graph (figure 3), and plot the same combined line from figure 2. They comment that this standard 'curve peaks at velocities around 420 km s−1 (readopting SI units for the moment); this is much higher than one would expect for what is meant to be a dwarf galaxy'.
But, where do you get this plot from since the experimentally observed values are no where near this, but quite accurately match the values obtained with the gravito-magnetic approach?
why DM is required by newtonian gravity also in NON rotating system. Unless GEM answer in a clear way to this point, the whole discussion looks quite strangeastronomer said:I think GEM solutions are quite clearly ruled out. The burden of proof is on the proponent, not on people producing counter arguments, and the counter arguments seem quite strong. In any case the single argument that I think should close the debate about GEM (and we can notice that no answer is provided) is why DM is required by newtonian gravity also in NON rotating system. Unless GEM answer in a clear way to this point, the whole discussion looks quite strange