The basic point I described is not proposing an alternative to dark matter, and I'm not asking for a refutation of Kroupa's well-known preference for alternative gravity theories. I'm just pointing out that Kroupa has described a prediction of the dark matter hypothesis that does not appear to match observations. I'm wondering if any proponent of the dark matter hypothesis has a counter argument to this: either an explanation of why the dark matter hypothesis doesn't make the prediction Kroupa is describing, or an explanation of why that prediction does match observations after all. This seems to me to be a perfectly reasonable question that does not require any discussion of alternative theories at all.
I'm not infallible but I do have a PhD in the study of CPV in particle physics so I understand this at a level enough to pass a doctoral viva.
Alternate gravity theories struggle with gravitational lensing. This is a proven observable optical phenomena in the study of the universe.
Kroupa is still trying to explain the rotational differences between observation and "normal" Newtonian/Einstein gravity. This is colloquially referred to as Dark Matter. A gravitational (matter like) effect, which doesn't interact optically via standard EM as we can see the galaxies and stars. Hence the name Dark Matter.
Are not what I'm asking about here. I know Kroupa pushes them, but that's not the part of Kroupa's work that I'm asking about.
> Kroupa is still trying to explain the rotational differences between observation and "normal" Newtonian/Einstein gravity.
Not in the specific item I'm asking about. In that specific item, he's not talking about any alternative theories of gravity at all. He's pointing out what he claims is a conflict between the predictions of the dark matter hypothesis and actual observations. I've already explained what a counter argument to that would look like. Do you have one?
Unfortunately you are getting a little crossed from your perspective. You're asking a question that to experts is structured oddly because you can't disentangle things you're trying to separate.
I'll try to explain.
Again dark matter is an attempt to explain a collection of observations that don't match current accepted theoretical models.
There "classical dark matter hypothesises" is just that. That these observations can be explained by a quantizable matter candidate of some form with some yet to be determined properties.
The only non particle descriptions of "dark matter phenomena" is a modified gravity framework. These attempt to address the "classical dark matter phenomena" whilst disentangling it from the "CPV phenomena". This is fine but breaks the big bang model at the stage of baryogenesis (matter formation from pure energy).
There is the possibility that you can explain "classical dark matter phenomena" without demanding it also be responsible for CPV. However, if you do that the CPV step of the big bang model demands additional particle candidates which then raises the problem of what do they look like and where are they and do they interact with the Higgs field and therefore have mass.
There are no counter arguments to "classical dark matter phenomena" because they are just that. Observed natural phenomena which do not fit into our current understanding of the universe.
A "complete dark matter theory" tends to explain CPV+DM. These again don't make many testable predictions other than a (potentially TeV scale) weekly interacting particle candidate. So the testable prediction is that a dark matter candidate exists and we can observe it.
> I've already explained what a counter argument to that would look like. Do you have one?
I'll choose to read that in a relaxed manner, albeit it's a bit direct.
There is no counter argument to natural phenomena. The main 2 theories are that dark matter particles exist and we can find them or we can't.
MOND et al make few predictions and struggle to observe defined "gravitational phenomena", hence why there is little interest in them.
There are differences between DM particle models but almost all agree that the major premise is that we might/should be able to see certain particle types given how they interact.
> You're asking a question that to experts is structured oddly because you can't disentangle things you're trying to separate.
To me this reads as the answer "no" to the question I asked. Nobody actually has a counter argument. "Experts" have simply chosen to dismiss the matter without any argument at all.
To expand on this somewhat: you appear to believe that I am asking for some kind of separation of observations, so that we consider only the particular observations Kroupa refers to (orbits of small satellite galaxies around large galaxies) and ignore all the other observations. That is not what I am asking. I am asking for a simple response from "experts" who support the dark matter hypothesis in general to these two questions:
(1) Is Kroupa correct in his claims about what dark matter models of "halos" around galaxies predict regarding the orbits of small satellite galaxies around larger galaxies? If not, why not?
(2) Is Kroupa correct that actual observations of the orbits of small satellite galaxies around larger galaxies do not match the predictions of dark matter models? If not, why not?
These seem to me like straightforward questions of the kind that scientists pose to other scientists all the time, and it does not seem credible to me that "experts" would not have responses to them along any of several obvious lines:
(A) Kroupa's computations of the predictions of dark matter "halo" models are wrong. Here are the correct predictions, which match observations.
(B) Kroupa's claims about the predictions are correct, but his comparison with the data is wrong. Here is the correct comparison with the data, which shows that the models match observations.
(C) Kroupa's claims that the predictions don't match observations in this particular domain are correct taken in isolation. However, when the entirety of all observations are considered, including ones that have nothing to do with individual galaxies (such as the evolution of the early universe), the dark matter models do much better at minimizing the overall error between model and observation than any of the alternative models, including the ones Kroupa favors.
You seem to be leaning towards something like (C), but if so, you could just say (C) and be done with it. Instead, you appear to be trying to avoid any possible admission that there is any discrepancy between dark matter models and observations at all, which IMO is not a good strategy. No model ever matches 100% of observations.
> dark matter is an attempt to explain a collection of observations that don't match current accepted theoretical models
I would not put it that way. I would put it that dark matter is an attempt to explain a collection of observations using currently accepted theoretical models (General Relativity and the relevant solutions to the Einstein Field Equation), by making use of a free parameter in the models, namely the assumed stress-energy tensor (or distribution of matter, if you like). By adding a "dark matter" component to the distribution of matter, which does not interact electromagnetically and therefore is not visible to us by any means other than its gravitational effects, many observations can be matched much better.
MOND, by contrast, proposes modifying the theory of gravity in general. That, to me, is a much bigger change than adding a component to the distribution of matter.
> There is no counter argument to natural phenomena.
So what? I wasn't asking for a counter argument to "natural phenomena". I was asking for a counter argument to a claim that a particular model makes predictions that don't match observations. Claims like this are made all the time in science, and the appropriate response is to give a counter argument along the lines I described. I have not seen one, and as I noted above, all your remarks have given me the strong impression that there isn't one, and that "experts" have no interest in providing one. That answers my question, although no doubt the inferences I draw from that answer will be very different than yours.
In my opinion, the big bang model, along with all of GR - LCDM is broken and can not be fixed.
Please take a look at the modified gravity proposal that I have put forward. This idea incorporates a mechanism that would explain why gravity might slightly deviate from Newtonian and/or GR in some circumstances.
This paper also adapts GR in such a way that it is consistent with galactic rotation rates, the anisotropies of the CMB, and cosmological expansion -- while showing that the simple operation of gravity is the cause of each of these phenomena.
This idea, Cyclic Gravity and Cosmology (CGC) predicts that there are discrete specific sizes allowable for macro-objects. The instability of Bennu and the fact that it behaves more like loosely held scree rather than a compact mass -- is an example of a mass that is not exactly at one of the discrete allowable sizes. Please also refer to the link I included wherein I uploaded a video simulation of the formation of a solar system using this type of force law. (This is in section 18 of the paper)
I would greatly appreciate any comments on this idea. A copy may be downloaded here:
Tl;Dr c is closest to what you're asking but it's more like.
(D) Kroupka is making bold claims that have to be verified first in the scientific arena very loudly and publically. That is odd.
He is claiming that DM doesn't exist which is effectively saying the particle solution to the problem can't exist. This breaks the big bang model as we understand it as I've stated. And is offering a gravity based solution which explains some small phenomena which he claims that not only that this isn't modelled accurately currently but that it can't be using the DM models. That claim requires huge effort to correctly simulate the matter field as it _might_ exist and that requires more assumptions. This could mean certain particle solutions are ruled out or that we need to revise the particle model (fantastic we learned something), but it's much less likely that it's completely dead just because one test says so.
His model may be valid. His data might be good. His conclusion has little to do with either.
Just because a theory doesn't match all observations doesn't mean people stop working on it. If that happened the foundations leading to the Higgs scalar boson wouldn't have been researched as at the time everyone was interested in understanding other field theories.
Ok some background.
DM research was introduced as a solution to the problem of the distribution of mass within galaxies. I.e. orbital velocities of stars within a single galaxy.
In order to try and understand this phenomena models were used to simulate the distribution of "matter fields" within galaxies.
Now modern modelling has evolved to simulate complex systems of multiple galaxies or even larger.
We now suspect and have strong evidence for huge mega structures (MPa mega parsecs in size) made of some "DM goop" in the universe.
Modeling of this is notoriously difficult and relies on numerous assumptions because we can't model everything exactly so we take sensible shortcuts which shouldn't impact the physics of interest for the simulation.
Large simulations have problems the further they evolve. This is a stochastic not model dependent statement. Modelling a thing we're yet to observe is even more difficult.
Most claims that DM can't exist are people modifying gravity in some way to remove DM as a physical particle that you can build a DM hammer out of in principle. These claims are not entirely baseless but really struggle when you try to match large gravitational structures we have seen so far in the universe (cosmic filaments).
Kroupa claiming his simulation has to be the only simulation to explain a problem (if he makes or infers this) would be false. Everything in science has requirement of reproducibility which goes beyond share the source and run the tool.
Kroupa claiming that his model perfectly describes this set of observations might be interesting, but is yet to be demonstrated at different scales. Again this has to satisfy orbital momenta of stars within a galaxy as well as demonstrating larger scale effects such as gravitational lensing.
Dark matter simulations _all of them_ will have some amount of disagreement or uncertainty. That is the nature of scientific computer modelling and comparison to scientificly collected data. Uncertainty and error estimation/understanding is a huge part of this field. Anyone claiming exact results better have a huge amount of data to back it up.
This is either a conversation that would have been easier by Skype. I'm guessing you don't have a strong background in a formal science. I'm not saying this from some pedastle, it's a joint question/observation of you focusing on the socialogical implications and exactness of a thing.
There is no lack of interest from experts to talk about their field. There is almost never a lack of wanting to move away from the "status quo" in the scientific community. Especially in blue sky particle and astro research at a professional level. The public perception of this existing comes from 2 issues. 1) "loud" individuals who shout loudly that they're not being heard will be ignored. That isn't discourse, that is someone acting like a small child within this area. Unfortunately they often get awarded by funding and we're stuck with them. As they get pushed aside for shouting rather than discussing they tend to get louder. 2) the media loves to paint the image of scientists sitting there as gatekeepers to some theoretical framework. This is fun in movies etc but mostly we talk about this in science education as a case of remember the mistakes from the last 100 years. As a result you will be looked at strangely for dismissing an idea out of hand without proof or testability.
> (D) Kroupka is making bold claims that have to be verified first in the scientific arena very loudly and publically.
Meaning basically that other experts haven't had time yet to check his work? How long is that expected to take? The paper I referenced was published in 2015.
> He is claiming that DM doesn't exist
Yes, on the basis of his claim that dark matter models make predictions about the orbits of small satellite galaxies around larger galaxies that don't match observations. As I've said, these kinds of claims get made in science all the time. Scientific models are supposed to be tested against observations. When someone sees a mismatch, they're supposed to say so. That's part of how science is supposed to work.
> This breaks the big bang model as we understand it as I've stated.
More precisely, it "breaks" the belief that we can construct a valid model of our universe using standard General Relativity, on the basis of the claim that we cannot find any assumption for the distribution of matter that gives a satisfactory fit against all observations. That's the larger claim he's making and the basis of his advocacy for alternative theories of gravity, yes. But, as I've already stated, that larger claim is not what I have been asking about.
> it's much less likely that it's completely dead just because one test says so.
That doesn't change the fact that there should be some answer to how that one test comes out: either it does come out the way he claims, or it doesn't. Which is all I was asking about. If the answer is "nobody else has had the time or resources to check as yet", then that's the answer.
> Ok some background.
While this is all valid information it's not information I really needed. I'm well aware of the difficulties involved in constructing these models and checking them against observations.
> I'm guessing you don't have a strong background in a formal science.
Your guess would be wrong. I don't currently do scientific research for a living, but that doesn't mean I don't have a lot of background in how science works.
> you focusing on the socialogical implications and exactness of a thing.
I have no idea where you are getting this from. I haven't asked about "sociological implications" at all. You're the one who keeps bringing that up. And I have already explicitly recognized that no scientific model matches observations with 100% exactness.
> The public perception
Has nothing to do with what I've been asking about. I haven't been asking general questions about how the scientific field of cosmology works or whether or not cosmologists are supporting a "status quo" or whether or not "dissenters" in science get a fair hearing. I've been asking a very specific question which I've already described several times.
> Meaning basically that other experts haven't had time yet to check his work? How long is that expected to take? The paper I referenced was published in 2015.
Likely never, as I have explained. Someone loud will be pushed aside from the community and simulations of this type/scale take too long to 'jsut test' or 'just repeat'.
> Yes, on the basis of his claim that dark matter models make predictions about the orbits of small satellite galaxies around larger galaxies that don't match observations. As I've said, these kinds of claims get made in science all the time. Scientific models are supposed to be tested against observations. When someone sees a mismatch, they're supposed to say so. That's part of how science is supposed to work.
This is not a reasinable assertion based on his work so, in simple terms, no.
> More precisely, it "breaks" the belief that we can construct a valid model of our universe using standard General Relativity, on the basis of the claim that we cannot find any assumption for the distribution of matter that gives a satisfactory fit against all observations. That's the larger claim he's making and the basis of his advocacy for alternative theories of gravity, yes. But, as I've already stated, that larger claim is not what I have been asking about.
No you cannot make claims in isolation that break the rest of science that's not how it works. Ever.
> While this is all valid information it's not information I really needed. I'm well aware of the difficulties involved in constructing these models and checking them against observations.
This is _essential_ to the discussion to underatand what is being discussed. To throw the topic aside is showing a lack of focussing on the article, but I think
> That doesn't change the fact that there should be some answer to how that one test comes out: either it does come out the way he claims, or it doesn't. Which is all I was asking about. If the answer is "nobody else has had the time or resources to check as yet", then that's the answer.
Again, a loud person jumping up and down with a simple test over a single overvation will not get the community to divert funding, time and effort. Reasonable arguments for diverting of limited resources are to approach the community with fact based evidence and reason.
> Your guess would be wrong. I don't currently do scientific research for a living, but that doesn't mean I don't have a lot of background in how science works.
Please stop being blunt and defensive, I'm trying to be polite.
> I have no idea where you are getting this from. I haven't asked about "sociological implications" at all. You're the one who keeps bringing that up. And I have already explicitly recognized that no scientific model matches observations with 100% exactness.
You brought it up in a paragraph with the theme of "how the community should change" which is the main shouting point from this article.
> Has nothing to do with what I've been asking about.
as above.
OK, to avoid this going further I think I've in good faith discussed everything in reasonable detail. You may feel different, you're entitled to and you're entitled to reply. But to move on I feel I've given you the large benefit of an experienced insight that is for you to make of what you chose. Have a great day/week/year, I won't be replying further.
Ok, that answers my question. I won't comment on the rest except to say that I don't think I have the same views about how science works (or should work) that you do. But scientists all have the freedom to choose what to work on, so ultimately that will determine how all this plays out.
> You brought it up in a paragraph with the theme of "how the community should change"
To be clear, nothing I have posted in this thread was intended to make any such claim. As I have said all along, I was only interested in a specific answer to a specific question, which you have now made clear. As I noted just above, scientists are all free to choose what they will work on, and I have no issue with that at all. Kroupa is free to express his opinions about more general questions like "how the community should change", and other scientists are free to ignore him, which is what seems to be happening. All of that is beyond the scope of what I was asking about.
Why? Who made you an infallible authority?
> none of these alternatives
The basic point I described is not proposing an alternative to dark matter, and I'm not asking for a refutation of Kroupa's well-known preference for alternative gravity theories. I'm just pointing out that Kroupa has described a prediction of the dark matter hypothesis that does not appear to match observations. I'm wondering if any proponent of the dark matter hypothesis has a counter argument to this: either an explanation of why the dark matter hypothesis doesn't make the prediction Kroupa is describing, or an explanation of why that prediction does match observations after all. This seems to me to be a perfectly reasonable question that does not require any discussion of alternative theories at all.