Dark matter ....

[Mordred]

 

You raise some good points @Markus Hanke there so I'm going to detail the above interms of how N=Body codes such as Gadget use in Millenium and the Mare-Nordstrum simulation apply the above factors described by your sand dune analogy. There is several stages to consider Jeans instability which provide in-fall rates due to gravitational collapse

https://en.wikipedia.org/wiki/Jeans_instability

the expansion rates must also be considered as well as the momentum terms of the particles involved. At first the density perturbations are linear however as they deviate from linear to non linear there is an intermediate stage where one doesn't require a full non linear treatment. This is the Zel'Dovich approximation

\[\vec{r}(t)=a(t)\vec{x}+b(t)\vec{f}(\vec{x})\]

the first term is the expansion rates and the second term is the peculiar velocities of the vector field \(\vec{f}(t)\)

The formula shows a production of voids separated by walls of dark matter

https://en.wikipedia.org/wiki/Zeldovich_pancake

now the problem with this is that it will break down when the density perturbations start crossing each other  so we then have to employ a full non-linear treatment

\[\frac{d^2 \vec{r}_i(t)}{dt^2}=\sum_{k\neq i}\frac{GM_k m_i}{|\vec{r}_k-\vec{r}_i|^2}(\vec{r}_k-\vec{r}_i)\]

the above is incredibly difficult to compute for large N body simulations so one has to employ Fourier transformations to solve the Poisson equations this leads to PM (particle_Mesh)

https://en.wikipedia.org/wiki/Particle_mesh

and the improvement P3M (particle-particle-particle-mesh)

https://en.wikipedia.org/wiki/P3M

for DM halos itself one can employ the spherical symmetric approximation

Press-Schechter mass function for halos

https://en.wikipedia.org/wiki/Press–Schechter_formalism

all the above naturally involve

https://en.wikipedia.org/wiki/Virial_theorem

so as one can see the situation is extremely complex for large N-Body simulations which is the fundamental point you raised in your post

 

 

11 hours ago, Markus Hanke said:

 

 

 

These are all excellent points. Unfortunately I’m up to my eyeballs in the real life at the moment, so I’ll need to come back to this at a later point.

Consider the following though. Suppose you have an alien scientist whose species lives down in the ocean of a water-world (no solid land). One day he notices some sand on the bottom of the ocean, and begins to wonder: what would happen if you had a very large amount of grains of sand, without water, just under the influence of wind and gravity? He knows Newtonian physics, and he knows the Navier-Stokes equations. Based on these, he figures that each grain is blown about by the wind, pulled down by gravity, bounces about a bit in pretty much chaotic patterns, and might come to rest somewhere. Over large areas and long times, each point on the sand plain is equally likely to become the resting spot of a sand grain - so it’s reasonable to expect that all inhomogeneities smooth out over time, and you end up with a more or less flat expanse of sand eventually.

So now he jumps into his (water-filled) UFO and visits Earth. He lands in a desert, and imagine his surprise when he sees this:

A naive application of Newtonian gravity and Navier-Stokes fluid dynamics would give no indication that a large number of essentially isolated sand grains undergoing essentially chaotic dynamics would give rise to large scale ordered structures such as these. So our alien scientist would be forgiven in concluding that there must be some other influence that leads to the formation of dunes.

The situation in GR is similar. Each star or galaxy taken in isolation is locally near-Newtonian, and would thus be expected to behave that way on all scales. However, an n-body system with very large n undergoing chaotic dynamics under the laws of GR might form global spacetime geometries that are not immediately predictable, just like sand grains and the formation of dunes (which is meant just an analogy, btw). This holds for stars in a galaxy, or for the interaction between galaxies, or for galaxies in the universe.

The point is we don’t know if that’s the case or not, because we don’t have the computing power necessary to model a GR n-body problem with very large n. So this is just a hypothesis, based on the fact that metrics don’t add linearly; the overall metric of an n-body system is not the sum of n metrics for the n constituent bodies. So it’s possible at least in principle that the actual global spacetime might look like it contains more mass than we can observe, even though in actual fact it doesn’t.

That’s not really what I’m saying. We can use GR quite accurately so long as it is permissible to make enough simplifying assumptions to render the maths manageable. For example, a single body that can be considered isolated (asymptotic flatness) and is symmetric enough can be easily modelled, and the result matches observation very closely.

I think the problems arise only if we are dealing with n-body systems, because the non-linearities inherent in GR may not smooth out and become negligible; they might in fact compound in large enough systems. And the trouble is we don’t have enough computing power to actually run such simulations, for large n. 

 

1 hour ago, DanMP said:

Maybe not only sterile neutrinos but also relic neutrinos. Or something else. 

@joigus answered this  

1 hour ago, DanMP said:

 

How is MOND explaining gravitational time dilation? 

 

 

In the same manner as GR though with the new applicable mass terms MOND is compatible with GR 

1 hour ago, DanMP said:

 

The gas we know is made by atoms and molecules. Molecules can transform part of kinetic energy in vibration and rotation, slowing down.  Also they may react/transform and ultimately clump. In extreme concentrations/pressure, hydrogen and helium can undergo nuclear fusion. On the other hand, DM particles may be incapable to bond in any way, and just collide with each other, without losing speed. Would such a "model" lead to the halo we know/inferred? 

yes the above is correct and how you described DM does allow for halo formation but also as the filament to void separations of LSS filament structure. See above for the related formulas 

edit forgot to add an important detail Zel'dovich pancake development actually leads to the NFW profile use for galaxy rotation curves.

https://en.wikipedia.org/wiki/Navarro–Frenk–White_profile

Edited August 17 by Mordred

[Eise]

1 hour ago, DanMP said:

On 8/14/2024 at 5:23 PM, Eise said:

So if there are different explanations, then MOND, as explanation for rotation curves, is still a candidate.

How is MOND explaining gravitational time dilation? 

What did I say MOND is supposed to explain? To be sure, I also made the 'if' bold. Hope you now understand my sentence. 

I sense some aggressiveness in your questions and reactions, which make you blind for what people are really saying. 

1 hour ago, DanMP said:

On the other hand, DM particles may be incapable to bond in any way, and just collide with each other, without losing speed. Would such a "model" lead to the halo we know/inferred? 

If I remember my physics correctly, if the collisions are absolutely elastic, then yes. One particle may lose some kinetic energy to another one, but the other one gains it. 

Grrr... Partially cross posted with Mordred again... But yes, starting a posting, then eat an evening meal, and then continue the posting, might be not so a good idea.

[Mordred]

45 minutes ago, Eise said:

 

Grrr... Partially cross posted with Mordred again... But yes, starting a posting, then eat an evening meal, and then continue the posting, might be not so a good idea.

also doesn't help that I often have to edit latex etc so my posts tend to take a considerable time to complete and tend to get cross posted by numerous others as a result 

So I will readily take the blame or part of the blame in this case lmao

This is some studies I've been working on in my spare time in my Nucleosynthesis thread in Speculations. For right hand neutrinos using Majaronna mass terms the related mass terms via CKMS and PMNS mass mixing matrices are as follows as nearly as I've been able to gather from research. I recognize that most won't understand below but its here to indicate that my earlier statement has validity.

 

\[m\overline{\Psi}\Psi=(m\overline{\Psi_l}\Psi_r+\overline{\Psi_r}\Psi)\]

\[\mathcal{L}=(D_\mu\Phi^\dagger)(D_\mu\Phi)-V(\Phi^\dagger\Phi)\]

4 effective degrees of freedom doublet complex scalar field.

with 

\[D_\mu\Phi=(\partial_\mu+igW_\mu-\frac{i}{2}\acute{g}B_\mu)\Phi\]\

\[V(\Phi^\dagger\Phi)=-\mu^2\Phi^\dagger\Phi+\frac{1}{2}\lambda(\Phi^\dagger\Phi)^2,\mu^2>0\]

in Unitary gauge

\[\mathcal{L}=\frac{\lambda}{4}v^4\]

\[+\frac{1}{2}\partial_\mu H \partial^\mu H-\lambda v^2H^2+\frac{\lambda}{\sqrt{2}}vH^3+\frac{\lambda}{8}H^4\]

\[+\frac{1}{4}(v+(\frac{1}{2}H)^2(W_mu^1W_\mu^2W_\mu^3B_\mu)\begin{pmatrix}g^2&0&0&0\\0&g^2&0&0\\0&0&g^2&g\acute{g}\\0&0&\acute{g}g&\acute{g}^2 \end{pmatrix}\begin{pmatrix}W^{1\mu}\\W^{2\mu}\\W^{3\mu}\\B^\mu\end{pmatrix}\]

Right hand neutrino singlet needs charge conjugate for Majorana mass term (singlet requirement)

\[\Psi^c=C\overline{\Psi}^T\]

charge conjugate spinor

\[C=i\gamma^2\gamma^0\] 

Chirality

\[P_L\Psi_R^C=\Psi_R\]

mass term requires

\[\overline\Psi^C\Psi\] grants gauge invariance for singlets only.

\[\mathcal{L}_{v.mass}=hv_{ij}\overline{I}_{Li}V_{Rj}\Phi+\frac{1}{2}M_{ij}\overline{V_{ri}}V_{rj}+h.c\]

Higgs expectation value turns the Higgs coupling matrix into the Dirac mass matrix. Majorana mass matrix eugenvalues can be much higher than the Dirac mass.

diagonal of

\[\Psi^L,\Psi_R\] leads to three light modes v_i with mass matrix

\[m_v=-MD^{-1}M_D^T\]

MajorN mass in typical GUT 

\[M\propto10^{15},,GeV\]

further details on Majorana mass matrix

https://arxiv.org/pdf/1307.0988.pdf

https://arxiv.org/pdf/hep-ph/9702253.pdf

The other detail is if the above has accuracy then the cross sections for anti neutrinos would be similar to below

A possible antineutrino cross section calculation massless case

\[\vec{v}_e+p\longrightarrow n+e^+\]

Fermi constant=\(1.1663787(6)*10^{-4} GeV^{-2}\)

\[\frac{d\sigma}{d\Omega}=\frac{S|M|^2\acute{p}^2}{M_2|\vec{p_1}|2|\vec{p_1}|(E_1+m_2c^2)-|\vec{p_1}|\prime{E_1}cos\theta}\]

Fermi theory

\[|M|^2=E\acute{E}|M_0^2|=E\acute{E}(M_Pc^2)^2G^2_F\]

\[\frac{d\sigma}{d\Omega}=(\frac{h}{8\pi}^2)\frac{M_pc^4(\acute{E})^2G^3_F}{[(E+M_p^2)-Ecos\theta]}\]

\[\frac{d\sigma}{d\Omega}=(\frac{h}{8\pi}^2)\frac{M_pc^4(\acute{E})^2G^3_F}{M_pc^2}(1+\mathcal{O}(\frac{E}{M_oc^2})\]

\[\sigma=(\frac{\hbar cG_F\acute{E}^2}{8\pi})^2\simeq 10^{-45} cm^2\] prior to electroweak symmetry breaking 

A possible antineutrino cross section calculation massless case

\[\vec{v}_e+p\longrightarrow n+e^+\]

Fermi constant=\(1.1663787(6)*10^{-4} GeV^{-2}\)

\[\frac{d\sigma}{d\Omega}=\frac{S|M|^2\acute{p}^2}{M_2|\vec{p_1}|2|\vec{p_1}|(E_1+m_2c^2)-|\vec{p_1}|\prime{E_1}cos\theta}\]

Fermi theory

\[|M|^2=E\acute{E}|M_0^2|=E\acute{E}(M_Pc^2)^2G^2_F\]

\[\frac{d\sigma}{d\Omega}=(\frac{h}{8\pi}^2)\frac{M_pc^4(\acute{E})^2G^3_F}{[(E+M_p^2)-Ecos\theta]}\]

\[\frac{d\sigma}{d\Omega}=(\frac{h}{8\pi}^2)\frac{M_pc^4(\acute{E})^2G^3_F}{M_pc^2}(1+\mathcal{O}(\frac{E}{M_oc^2})\]

\[\sigma=(\frac{\hbar cG_F\acute{E}^2}{8\pi})^2\simeq 10^{-45} cm^2\] 

 

as stated this is simply to be informative that there is standard model methods to help make accurate predictions for something such as anti neutrinos prior to discovery and with this be able to look for signatures and evidence.

 

 

This is an overview of the types of signatures were looking for this particular set of slides gives an example of the DM cross section under DM decay.

https://www.hip.fi/cosmoseminars/wp-content/uploads/sites/15/2020/10/Drewes-2020.pdf

related papers

DARK MATTER AS STERILE NEUTRINOS

http://arxiv.org/abs/1402.4119
http://arxiv.org/abs/1402.2301
http://arxiv.org/abs/1306.4954

in direct answer to the excellent question by 

2 hours ago, DanMP said:

Maybe not only sterile neutrinos but also relic neutrinos. Or something else.  

the sterile neutrinos must have a mean lifetime longer than the age of the universe to match the cross section provided by

https://www.hip.fi/cosmoseminars/wp-content/uploads/sites/15/2020/10/Drewes-2020.pdf

further details on the reason for the mean lifetime provided by the article

further articles

Next decade of sterile neutrino studies

by Alexey Boyarsky, Dmytro Iakubovskyi, Oleg Ruchayskiy

https://arxiv.org/pdf/1306.4954.pdf

Detection of An Unidentified Emission Line in the Stacked X-ray spectrum of Galaxy Clusters

Esra Bulbul, Maxim Markevitch, Adam Foster, Randall K. Smith, Michael Loewenstein, Scott W. Randall

https://arxiv.org/abs/1402.2301

Neutrino Masses, Mixing, and Oscillations

Revised October 2021 by M.C. Gonzalez-Garcia (YITP, Stony Brook; ICREA, Barcelona; ICC, U. of Barcelona) and M. Yokoyama (UTokyo; Kavli IPMU (WPI), UTokyo).

https://pdg.lbl.gov/2022/reviews/rpp2022-rev-neutrino-mixing.pdf

this portion will help relate Fermi's Golden rules in terms of those cross sections provided including mean lifetime from the Breit Wigner distrbution cross sections.

Fermi's Golden Rule

\[\Gamma=\frac{2\pi}{\hbar}|V_{fi}|^2\frac{dN}{DE_f}\]

density of states

\[\langle x|\psi\rangle\propto exp(ik\cdot x)\]

with periodic boundary condition as "a"\[k_x=2\pi n/a\]

number of momentum states

\[dN=\frac{d^3p}{(2\pi)^2}V\]

decay rate

\[\Gamma\]

Hamilton coupling matrix element between initial and final state

\[V_{fi}\]

density of final state

\[\frac{dN}{dE_f}\]

number of particles remaining at time t (decay law)

\[\frac{dN}{dt}=-\Gamma N\]

average proper lifetime probability

\[p(t)\delta t=-\frac{1}{N}\frac{dN}{dt}\delta t=\Gamma\exp-(\Gamma t)\delta t\]

mean lifetime \[\tau=<t>=\frac{\int_0^\infty tp (t) dt}{\int_0^\infty p (t) dt}=\frac{1}{\Gamma}\]

relativistic decay rate set 

\[L_o=\beta\gamma c\tau\] average number after some distance x

\[N=N_0\exp(-x/l_0)\]

Edited August 17 by Mordred

[Markus Hanke]

13 hours ago, DanMP said:

If this is a "mathematical artefact" how can we trust the validity of GR on large scales?!?

This is just the point - if the hypothesis is correct, then, if we were able to actually solve the Einstein equations for a very large n along with the correct boundary conditions, the result would exactly match observation, without having to postulate the presence of anything else. 

In other words, Dark Matter would just be the error introduced by using idealised computations, rather than an actual n-body problem with large n.

13 hours ago, DanMP said:

How/why we can say that GR is in agreement with all observations?

The domain of applicability of GR is naturally limited - as a classical model, it can’t eg account for quantum effects, so it won’t ever be able to explain all observations. You just use it within its proper domain.

The question is just how far this domain extends up and down, and we won’t be able to definitively answer that question until we have clarity as to the precise nature of DM. It is definitely possible that an alternative model of gravity is required on larger scales, which would restrict the validity of GR to the intermediate scale (~tens/hundreds of Ly?). The problem with that is of course that all currently known alternatives to GR themselves suffer from a variety of problems, and don’t match all observations.

14 hours ago, DanMP said:

Or you are wrong.

The idea with the n-body dynamics is just a hypothesis, to add to the “particles” and “alternative model” options. Of course it could be wrong, though we have no way to check right now.

But then again, we know already that ordered structures can emerge from otherwise chaotic dynamics in non-linear n-body systems; so the concept isn’t that far-fetched at all.

13 hours ago, DanMP said:

If this is a "mathematical artefact" how can we trust the validity of GR on large scales?!?

This is just the point - if the hypothesis is correct, then, if we were able to actually solve the Einstein equations for a very large n along with the correct boundary conditions, the result would exactly match observation, without having to postulate the presence of anything else. 

In other words, Dark Matter would just be the error introduced by using idealised computations, rather than an actual n-body problem with large n.

13 hours ago, DanMP said:

How/why we can say that GR is in agreement with all observations?

The domain of applicability of GR is naturally limited - as a classical model, it can’t eg account for quantum effects, so it won’t ever be able to explain all observations. You just use it within its proper domain.

The question is just how far this domain extends up and down, and we won’t be able to definitively answer that question until we have clarity as to the precise nature of DM. It is definitely possible that an alternative model of gravity is required on larger scales, which would restrict the validity of GR to the intermediate scale (~tens/hundreds of Ly?). The problem with that is of course that all currently known alternatives to GR themselves suffer from a variety of problems, and don’t match all observations.

14 hours ago, DanMP said:

Or you are wrong.

The idea with the n-body dynamics is just a hypothesis, to add to the “particles” and “alternative model” options. Of course it could be wrong, though we have no way to check right now.

But then again, we know already that ordered structures can emerge from otherwise chaotic dynamics in non-linear n-body systems; so the concept isn’t that far-fetched at all.

13 hours ago, DanMP said:

If this is a "mathematical artefact" how can we trust the validity of GR on large scales?!?

This is just the point - if the hypothesis is correct, then, if we were able to actually solve the Einstein equations for a very large n along with the correct boundary conditions, the result would exactly match observation, without having to postulate the presence of anything else. 

In other words, Dark Matter would just be the error introduced by using idealised computations, rather than an actual n-body problem with large n.

13 hours ago, DanMP said:

How/why we can say that GR is in agreement with all observations?

The domain of applicability of GR is naturally limited - as a classical model, it can’t eg account for quantum effects, so it won’t ever be able to explain all observations. You just use it within its proper domain.

The question is just how far this domain extends up and down, and we won’t be able to definitively answer that question until we have clarity as to the precise nature of DM. It is definitely possible that an alternative model of gravity is required on larger scales, which would restrict the validity of GR to the intermediate scale (~tens/hundreds of Ly?). The problem with that is of course that all currently known alternatives to GR themselves suffer from a variety of problems, and don’t match all observations.

14 hours ago, DanMP said:

Or you are wrong.

The idea with the n-body dynamics is just a hypothesis, to add to the “particles” and “alternative model” options. Of course it could be wrong, though we have no way to check right now.

But then again, we know already that ordered structures can emerge from otherwise chaotic dynamics in non-linear n-body systems; so the concept isn’t that far-fetched at all.

13 hours ago, DanMP said:

If this is a "mathematical artefact" how can we trust the validity of GR on large scales?!?

This is just the point - if the hypothesis is correct, then, if we were able to actually solve the Einstein equations for a very large n along with the correct boundary conditions, the result would exactly match observation, without having to postulate the presence of anything else. 

In other words, Dark Matter would just be the error introduced by using idealised computations, rather than an actual n-body problem with large n.

13 hours ago, DanMP said:

How/why we can say that GR is in agreement with all observations?

The domain of applicability of GR is naturally limited - as a classical model, it can’t eg account for quantum effects, so it won’t ever be able to explain all observations. You just use it within its proper domain.

The question is just how far this domain extends up and down, and we won’t be able to definitively answer that question until we have clarity as to the precise nature of DM. It is definitely possible that an alternative model of gravity is required on larger scales, which would restrict the validity of GR to the intermediate scale (~tens/hundreds of Ly?). The problem with that is of course that all currently known alternatives to GR themselves suffer from a variety of problems, and don’t match all observations.

14 hours ago, DanMP said:

Or you are wrong.

The idea with the n-body dynamics is just a hypothesis, to add to the “particles” and “alternative model” options. Of course it could be wrong, though we have no way to check right now.

But then again, we know already that ordered structures can emerge from otherwise chaotic dynamics in non-linear n-body systems; so the concept isn’t that far-fetched at all.

PS. I have no idea why the above post appears several times in a row?? This has never happened before, and I defo didn’t type it that way…

[DanMP]

20 hours ago, joigus said:

You need a mechanism to explain why those excess neutrinos are there, and why they are decoupled from the rest of the matter.

Neutrinos, of any/all kind, are my second choice for DM particles. I wrote "or something else" ...

 

20 hours ago, joigus said:

The attribute "relic" only says that they are remnant from the big bang. Nothing more.

There is something more: they are slowed down due to expansion.

 

 

[Mordred]

44 minutes ago, DanMP said:

 

There is something more: they are slowed down due to expansion.

 

 

No expansion doesn't affect a particles momentum terms. The relic neutrinos need to be slow (cold/non relativistic) to begin with.

Edit rather slow once they acquire mass from electroweak symmetry breaking.

Edited August 18 by Mordred

[DanMP]

20 hours ago, Mordred said:

21 hours ago, DanMP said:

 

How is MOND explaining gravitational time dilation? 

Expand  

 

In the same manner as GR though ...

I didn't understand your explanation and didn't find any. Sorry for the clipped quote, I'm having trouble to select the text from my tablet. 

 

20 hours ago, Mordred said:

yes the above is correct and how you described DM does allow for halo formation 

Thank you. This is the first time I receive a positive reply in relation with (a part of)  my "theory" on DM (posted 6 years ago).

 

19 hours ago, Eise said:

What did I say MOND is supposed to explain? To be sure, I also made the 'if' bold. Hope you now understand my sentence. 

I sense some aggressiveness in your questions and reactions, which make you blind for what people are really saying. 

I understood what you said. Sorry if I sound aggressive, but an alternative gravity theory, like MOND, should cover all the aspects, not only the rotation curves, in order to be "the correct one". And I really don't understand how a Newtonian theory can explain gravitational time dilation. 

 

19 hours ago, Eise said:

If I remember my physics correctly, if the collisions are absolutely elastic, then yes. One particle may lose some kinetic energy to another one, but the other one gains it. 

Thank you.

By the way, your observation (One particle may lose some kinetic energy to another one, but the other one gains it), reminds me 2 things. First, neutrino collisions with electrons (the way we detect them) may slow neutrinos down. Second, if the collision is with another, static, neutrino, the emerging neutrino would be the static one, possibly different, explaining the mystery of neutrino in-flight transformation ... 

[Mordred]

That's fine I recall the reply GR can be applied to MOND even though MOND modifies f=ma. The change  simply needs to carry over when you run calculations using GR. In essence it's own class of GR solution.

For the record I would not be surprised if there isn't a paper out on how to use GR with MOND. Rather in this case I would expect there would be.

 

[DanMP]

17 minutes ago, Mordred said:

No expansion doesn't affect a particles momentum terms. The relic neutrinos need to be slow (cold/non relativistic) to begin with.

I learned about relic neutrinos from Swansont in another discussion/thread and, if I remember correctly, he said that they are slowed down due to expansion. We should ask him. (I don't know how).

 

14 minutes ago, Mordred said:

GR can be applied to MOND even though MOND modifies f=ma. The change  simply needs to carry over when you run calculations using GR. In essence it's own class of GR solution.

Then it (MOND) sounds like a modified GR rather than a modified Newtonian theory ...

[Mordred]

As expansion allows electroweak symmetry breaking to occur in that manner it's accurate that includes cosmological redshift however one must also recognize that cosmological redshift affects how we measure the kinetic energy in the same manner as it effects how we measure its mass term.

However the particle itself has an invariant mass and kinetic energy term (momentum) which defines the particle. (If either changes its no longer the same particle type)  One must take into account observer effects as per GR when measuring either quantity. So it's likely Swansont was describing observer effects due to expansion

Edited August 18 by Mordred

[DanMP]

8 hours ago, Markus Hanke said:

This is just the point - if the hypothesis is correct, then, if we were able to actually solve the Einstein equations for a very large n along with the correct boundary conditions, the result would exactly match observation, without having to postulate the presence of anything else. 

You don't get it, to me is cheating if you postulate the presence of DM just to "exactly match" the observations. The same is with the "peculiar velocities", used in order to justify/cover a difference in redshift.

[Eise]

43 minutes ago, DanMP said:

I understood what you said.

I don't think so. You are just too pre-occupied with DM to see what I mean. 

45 minutes ago, DanMP said:

an alternative gravity theory, like MOND, should cover all the aspects, not only the rotation curves

No, not necessarily. I agree that 'one solution for all' phenomena that hint at there being more mass than we can see is the most 'beautiful' solution. But that being said, science is always in for a surprise. As long as we have not observed DM directly, it still could be that there are different solutions for these different phenomena. Maybe MOND is the correct solution for the rotation curves, maybe we do not apply GR correctly on the universe as a whole, and so we interpret the CMB incorrectly, and the Bullet Cluster phenomenon has again another explanation. I don't know, and you neither.

So it is a 'two-stage rocket': 

• do all these phenomena have the same cause, namely DM?
• If so, what is DM?

I am open to the idea that the answer of the first question is 'no'. You already take it for granted that the answer is 'yes', and therefore speculate (already) about what DM is.

And looking back at this 2018 thread of yours, I assume that you are blind for my, in my opinion, methodologically correct approach, that even the first question is still not answered. Just be sure that when you speculate, you are correct about the established science. And I am not so sure if this meets that criterion:

1 hour ago, DanMP said:

First, neutrino collisions with electrons (the way we detect them) may slow neutrinos down. Second, if the collision is with another, static, neutrino, the emerging neutrino would be the static one, possibly different, explaining the mystery of neutrino in-flight transformation ... 

[joigus]

3 hours ago, DanMP said:

Neutrinos, of any/all kind, are my second choice for DM particles. I wrote "or something else" ...

 

That wasn't the point. The point was "relic", as in "relic neutrinos" says nothing about how strongly coupled they are to themselves or to other types of matter. "Relic" just means they were produce at some point during the big bang, and are still there.

Then there's "something else". Oh, well... Of course DM could be something else. That leaves the question relic vs sterile intact. In fact, if I'm not mistaken, all sterile neutrinos would have to be relic, as later in the life of the universe it's just too late for them to be produced in any amounts.

3 hours ago, DanMP said:

There is something more: they are slowed down due to expansion.

 

My "nothing more" wasn't about the kinematics. It was about the nature of the neutrinos.

They are slowed down with respect to what? Do you mean their peculiar velocity? (their velocity with respect to the co-moving frame?) That doesn't sound right, but I would have to think about it. They would certainly be red-shifted. Velocity in cosmology is a bit tricky. 

Are you sure swansont didn't say "red-shifted"?

[Mordred]

31 minutes ago, joigus said:

Velocity in cosmology is a bit tricky. 

Are you sure swansont didn't say "red-shifted"?

Velocity in cosmology via the FLRW metric is to a commoving observer which in essence is tying that observer to the scale factor  a(t) for proper velocity as opposed to peculiar velocity such as from an observer on Earth measuring the cosmological event horizon which is based on Hubbles law v=HD.  Hope that helps.

Edited August 18 by Mordred

[Mordred]

Forgot to add the  velocity equation 

\[v(t)=\frac{\dot{a}(t)}{a(t)}d(t)=H(t)d(t)=H_0d(t)\]

[joigus]

4 hours ago, Mordred said:

Velocity in cosmology via the FLRW metric is to a commoving observer which in essence is tying that observer to the scale factor  a(t) for proper velocity as opposed to peculiar velocity such as from an observer on Earth measuring the cosmological event horizon which is based on Hubbles law v=HD.  Hope that helps.

Thank you, Mordred. That v is the cosmological Hubble flow. What I was referring to was the peculiar velocity. dr/dt-H₀d

https://en.wikipedia.org/wiki/Peculiar_velocity

4 hours ago, joigus said:

They are slowed down with respect to what? Do you mean their peculiar velocity? (their velocity with respect to the co-moving frame?)

I just wanted to engage @DanMP. I wanted them to explain in more detail what they mean exactly by "neutrinos slowing down". Are they slowing down wrt the galactic rest frame? Ie, is their peculiar velocity slowing down?

I don't think it is.

I said something incorrect, btw, I said "red-shift" which is applied to light, not to neutrinos, although they do have a De Broglie wavelenth. I suppose your comments on momentum previously referred to something like that?:

9 hours ago, Mordred said:

No expansion doesn't affect a particles momentum terms.

 

 

[Mordred]

Actually redshift applies to all particles but I understand your referring to photons being the meansof how we measure an objects redshift.  Yes neutrinos does have a high momentum term however due to its weak interactions any scattering  collisions is greatly reduced. 

 Were both trying to get DanP to clarify to which class of observer. 

Ie applying the four momentum but recognizing observer effects to the particles four momentum term.

Edited August 19 by Mordred

[Markus Hanke]

13 hours ago, DanMP said:

You don't get it, to me is cheating if you postulate the presence of DM just to "exactly match" the observations.

Yes I get you, but to me this simply is extrapolating a model which we know works very well on solar system scales, to larger scales. After all, there is no immediate reason to assume that gravity works differently on galactic scales than on solar scales - while that could be so, we have no evidence that that’s actually the case. Therefore do I think it’s important to try and find ways to figure out what GR actually predicts for large n-body systems, rather than just simplified models with an unknown error factor. 

One might also say that possible alternatives such as MOND etc are “cheating”, because all of those models postulate things (extra fields, new universal constants etc) for which we have otherwise no evidence. At the very least, GR is the simplest possible metric model of gravity that is fully relativistic.

[DanMP]

10 hours ago, joigus said:

I just wanted to engage @DanMP. I wanted them to explain in more detail what they mean exactly by "neutrinos slowing down".

I searched and found what I learned from Swansont. It was: the cosmic neutrino background. And the relic neutrino energy became smaller, not their speed, as I wrote/implied. I assumed that lower energy means lower kinetic energy, but maybe I was wrong. Anyway, as I said, neutrinos are my second option, and I wrote about relic neutrinos mainly because Mordred option for DM was sterile neutrinos and he said that they are not enough (if I remember correctly). 

 

18 hours ago, Eise said:

 

20 hours ago, DanMP said:

an alternative gravity theory, like MOND, should cover all the aspects, not only the rotation curves

No, not necessarily.

Really? You said:

18 hours ago, Eise said:

 be sure that when you speculate, you are correct about the established science

Gravitational lensing and gravitational time dilation are not "established science" (more accurate, real facts/observations, because a new theory must be in agreement with the facts, all of them, not necessarily with the previous/existing theories)? You really think that MOND may ignore gravitational lensing and time dilation? 

 

 

[Eise]

42 minutes ago, DanMP said:

Really? You said:

19 hours ago, Eise said:

 be sure that when you speculate, you are correct about the established science

Gravitational lensing and gravitational time dilation are not "established science" (more accurate, real facts/observations, because a new theory must be in agreement with the facts, all of them, not necessarily with the previous/existing theories)? You really think that MOND may ignore gravitational lensing and time dilation? 

Yep. And you still do not understand me. I think I have to yell:

It is not established that dark matter is the common cause of all phenomena that we attribute to it. 

Did you hear me now?

I find @Markus Hanke's Ansatz very interesting. However, I am not sure if it could explain e.g. the separation of DM from normal matter. And also, do not forget that such observations are heavily theory-loaded, so I also cannot totally exclude the idea that these observations are wrong. Maybe look up a few youtube videos of Sabine Hossenfelder about dark matter.

[Mordred]

1 hour ago, DanMP said:

I searched and found what I learned from Swansont. It was: the cosmic neutrino background. And the relic neutrino energy became smaller, not their speed, as I wrote/implied. I assumed that lower energy means lower kinetic energy, but maybe I was wrong. Anyway, as I said, neutrinos are my second option, and I wrote about relic neutrinos mainly because Mordred option for DM was sterile neutrinos and he said that they are not enough 

 

Sounds like what's being described is the CNN blackbody temperature decrease due to expansion which means the mean average kinetic energy is reduced due to reduction in number density of neutrinos.

That makes sense now the current CNB temperature from calculation is 1.97 Kelvin however in the past much higher.

Yes even with 3 generations of sterile neutrinos to replace DM you would need more than the baryonic particles. As you recall there is more DM than baryonic matter.

 That's the essential clincher against a sterile neutrino solution. One can show sterile neutrinos has a good match in mean lifetime and weakly interactive with the non relativistic (cold) characteristics. It's the sheer number required that's the main issue.

Edited August 19 by Mordred

[DanMP]

7 hours ago, Markus Hanke said:

Yes I get you, but to me this simply is extrapolating a model which we know works very well on solar system scales, to larger scales. After all, there is no immediate reason to assume that gravity works differently on galactic scales than on solar scales - while that could be so, we have no evidence that that’s actually the case. Therefore do I think it’s important to try and find ways to figure out what GR actually predicts for large n-body systems, rather than just simplified models with an unknown error factor. 

One might also say that possible alternatives such as MOND etc are “cheating”, because all of those models postulate things (extra fields, new universal constants etc) for which we have otherwise no evidence. At the very least, GR is the simplest possible metric model of gravity that is fully relativistic.

I agree, thank you. 

 

54 minutes ago, Eise said:

you still do not understand me. I think I have to yell:

It is not established that dark matter is the common cause of all phenomena that we attribute to it. 

Did you hear me now?

Now you are being aggressive.

 

I simply didn't address the idea that dark matter may not be the common cause of all phenomena that we attribute to it. Now, if you insist, I do answer: yes, you may be right.

 

On the other hand, what is your point? All scientists who are trying to find and explain DM should stop, because "it is not established that dark matter is the common cause of all phenomena that we attribute to it"? You are serious? 

 

1 hour ago, Eise said:

2 hours ago, DanMP said:

You really think that MOND may ignore gravitational lensing and time dilation? 

Yep.

Why?

[Eise]

1 hour ago, DanMP said:

Now you are being aggressive.

Yep. Your continuous misunderstanding is tiresome. And the misunderstanding has its followup here:

1 hour ago, DanMP said:

All scientists who are trying to find and explain DM should stop, because "it is not established that dark matter is the common cause of all phenomena that we attribute to it"? You are serious? 

Where did I say scientists should stop? It is a critique on your position, that you already take DM for granted, and therefore close your mind for other explanations, or combinations of thereof.

And this:

1 hour ago, DanMP said:

2 hours ago, Eise said:

3 hours ago, DanMP said:

You really think that MOND may ignore gravitational lensing and time dilation? 

Yep.

Why?

Because MOND is meant to explain galaxies' rotation curves, and possibly galaxy's movements in clusters. Just to say it again, I like a solution with DM as one solution for all, I wrote that already several times. DM in your ears would also explain why you after at least 2 repetitions of the same point, still do not get what I am saying: just don't fixate on DM as a solution for all phenomena that it should explain. 

Oops, missed this:

1 hour ago, DanMP said:

I simply didn't address the idea that dark matter may not be the common cause of all phenomena that we attribute to it. Now, if you insist, I do answer: yes, you may be right.

Well, to be pedantic, I am right. Not in the idea that there are other (combinations of) solution(s), but simply the fact that the science is still open. 

Edited August 19 by Eise
Added 'Oops'...

[joigus]

15 hours ago, Mordred said:

Actually redshift applies to all particles but I understand your referring to photons being the meansof how we measure an objects redshift.

Yes, exactly. But it's beside the point anyway. Thank you again.

I would never say, ie, a proton running away from me is being redshifted!  

[Mordred]

10 minutes ago, joigus said:

Yes, exactly. But it's beside the point anyway. Thank you again.

I would never say, ie, a proton running away from me is being redshifted!  

Yeah one of the hassles of how to describe something outside of the math lol. Leave that one for the metaphysics arguments give them something useful lol

Edited August 19 by Mordred