Dark matter ....

[exchemist]

2 hours ago, Mordred said:

A good way to understand the difference is that DM doesn't interact with the EM field nor the strong force. So lets use a simplified analogy.

Two DM particles approach one another. Without EM interactions or strong force interactions they will simply drift by each other.

Whereas two baryonic particles will have a higher likelyhood ie form into atoms etc.

 

 

It is the lack of strong force and EM interaction that keep DM from clumping though they can form halos due to gravity.

Thanks, though actually I find @Eise's explanation is the clearer, as he specifically refers to the absence of collisions, which  seems to be the key point at issue. 

[joigus]

4 hours ago, julius2 said:

We all watched Star Wars as kids. Why can't we have a future like this?

7 hours ago, MigL said:

Don't use Star Wars as your source for science, please. You might end up thinking you can hear explosions in outer space, or travel at superluminal speed.

There's no reason I can think of that we cannot have Figrin D'an and the Modal Nodes some day. But don't hold your breath.  

[Mordred]

3 hours ago, exchemist said:

Thanks, though actually I find @Eise's explanation is the clearer, as he specifically refers to the absence of collisions, which  seems to be the key point at issue. 

No problem whichever explanation works best with you as we're both stating the same answer

[julius2]

3 hours ago, joigus said:

Don't use Star Wars as your source for science, please. You might end up thinking you can hear explosions in outer space, or travel at superluminal speed.

There's no reason I can think of that we cannot have Figrin D'an and the Modal Nodes some day. But don't hold your breath.  

Lol

17 hours ago, exchemist said:

It is explained here: https://en.wikipedia.org/wiki/Galaxy_rotation_curve

There is an animation which helps. The left hand galaxy rotates as would be expected on the basis of the mass calculated from the visible stars, while the right is closer to what is actually found. The difference is that in the right hand case extra, invisible, mass is distributed throughout the galaxy from the centre out to the edge. 

So that's the problem, or one of them. What else could account for the anomalous rotation pattern? There is a hypothesis that perhaps the law of gravitation works somehow differently at very long range from what we see in our own solar system. That is what the article refers to as "Modified Newtonian Dynamics", or MOND. But not many people seem to like that idea very much, as it makes some predictions which are not borne out by observation. The particle physicists on the other hand, struggle to imagine what kind of hitherto undetected particle, or particles, could make up the extra matter with gravitational mass needed to account for the astronomical observations.  

So we are in a quandary. The hunt is on for mystery particles, while people also struggle to refine MOND in an attempt to overcome some of its shortcomings, i.e. things it predicts we should see but which we don't.   

Left hand galaxy?

 

2 minutes ago, julius2 said:

Lol

Left hand galaxy?

 

Please put in to layman terms. I can understand EVERYTHING when it is in layman terms.

14 hours ago, Mordred said:

Piece of advise it would be impossible to stitch together some model for DM without having a sufficient mainstream understanding in the related mainstream physics theories and models.

For example how does one determine the measurable effects via indirect evidence unless one knows how calculate galaxy rotation curves or what influence DM would have on expansion.

If you cannot perform those calculations then you have zero ability to test any theory with regards to DM.

 There is also mainstream formulas that describe and define the most likely distributions of DM in terms of their early universe large scale structure formation contributions.

(Trust me it's not an easy formula to use).

The formula will return a Gaussian distribution (it's also one I've never mentioned before on this forum)

In a nutshell, what are galaxy rotation curves?

[Eise]

14 minutes ago, julius2 said:

In a nutshell, what are galaxy rotation curves?

Discussing DM, and not knowing what rotation curves are? One of the strongest empirical hints that DM exists (except the alternative, MOND, turns out to correct).

Edited July 31 by Eise

[Phi for All]

16 minutes ago, julius2 said:

Left hand galaxy?

 

Please put in to layman terms. I can understand EVERYTHING when it is in layman terms.

The galaxy on your left, or left hand, in the simulated picture. Terms don't get much more layman than that.

[exchemist]

23 minutes ago, julius2 said:

Lol

Left hand galaxy?

 

Please put in to layman terms. I can understand EVERYTHING when it is in layman terms.

In a nutshell, what are galaxy rotation curves?

Read the effing link, you lazy idiot. It is in layman's terms, with next to no mathematics. If you can't even be bothered to do that, when I've gone to the trouble of not only providing it but directing you to the animation and explaining what it means,  you don't deserve any further help. So read the link, look at the animation and then, if there are aspects you still don't understand, ask about them specifically. 

As for the notion that you can start dissing the science and offering your own theory, when you have shown no capacity to try to learn anything about the topic, what a joke that is. 

Christ Almighty! 

 

Edited July 31 by exchemist

[joigus]

58 minutes ago, julius2 said:

In a nutshell, what are galaxy rotation curves?

Rotation speed of galaxies stars plotted against distance to galactic centre. It's explained in link provided by exchemist.

Left-hand means "on the left side".

Edited July 31 by joigus
correction

[DanMP]

On 7/31/2024 at 9:10 AM, Eise said:

Because, if DM really is composed of WIMPS, it does not interact with itself either. That means it does not contract to a galactic disk, because this is due to collisions of normal matter particles. For collisions you need some interaction. In the case of normal matter that would be EM mostly.

Weak interaction is an interaction. So, if DM is made of WIMPS, then we should be able to detect them (if their mass/energy is big/massive enough) and we didn't. Also they may collide with each other. If the collision is mostly between each other and elastic there would be no reasons for slowing down and clumping.

 

DM halo may be a sphere because DM particles are interacting/colliding like the molecules in a gas?

 

 

On 7/31/2024 at 4:53 PM, Eise said:

Discussing DM, and not knowing what rotation curves are? One of the strongest empirical hints that DM exists (except the alternative, MOND, turns out to correct).

Reaĺy, MOND turns out to be correct? How MOND explains gravitational lensing and Bullet cluster?

[Eise]

On 8/5/2024 at 6:42 PM, DanMP said:

Weak interaction is an interaction.

Yes, but that is not what is meant by 'WIMP':

Quote

There exists no formal definition of a WIMP, but broadly, it is an elementary particle which interacts via gravity and any other force (or forces), potentially not part of the Standard Model, which is as weak as or weaker than the weak nuclear force, but also non-vanishing in strength.

Bold by me.

On 8/5/2024 at 6:42 PM, DanMP said:

DM halo may be a sphere because DM particles are interacting/colliding like the molecules in a gas?

Wouldn't that mean that most galaxies also have a gas halo around them? Maybe they have, but I assume that such a gas halo would be extremely thin. In the end, most mass is concentrated in the galactic plane. Because the gas collided, got concentrated in the galactic plain so much that it could form stars. As explained by Mordred and me above.

On 8/5/2024 at 6:42 PM, DanMP said:

Reaĺy, MOND turns out to be correct? How MOND explains gravitational lensing and Bullet cluster?

It doesn't. But my reaction was about rotation curves of galaxies. Ockham's Razor (slightly misused as Ockham originally meant it) suggests that having one single explanation for all phenomena that we connect to DM would be better. But maybe we need different explanations. Until we detect DM directly, and got enough knowledge about their properties, we simply have no proof. So if there are different explanations, then MOND, as explanation for rotation curves, is still a candidate.

My opinion is that DM in some form, maybe WIMPs, maybe something else exists. But that is my opinion, the science is still open.

Edited August 14 by Eise

[Mordred]

As far as opinions on DM I've always leaned toward sterile neutrinos even though I have examined treatments using Majaronna mass coupling RH neutrinos and 3 species I still haven't seen how to account for the total mass.

The research is still ongoing in that regard.

[Markus Hanke]

13 hours ago, Mordred said:

As far as opinions on DM I've always leaned toward sterile neutrinos

The current overall state of affairs seems to be that:

1. All efforts in directly detecting the more plausible types of DM particles have come up negative, and particle physicists are forced to consider more and more exotic extensions to the Standard Model to come up with workable alternatives

2. There appears to be little to no statistically significant evidence to support any one of the various alternative gravity models, since they all suffer from more or less significant problems

While I think the current evidence isn’t strong enough to definitively rule out either the particle or the alternative gravity model option, I personally tend towards the third option, which avoids both of those - namely that DM is actually an artefact of our inability to produce solutions to the ordinary GR field equations that aren’t idealised.

For example, even the best numerical approaches to modelling a spiral galaxy in the context of the GR equations need to be idealised - it’s going to be some sort of continuous dust distribution, with appropriate density curves and initial and boundary conditions. But a real galaxy is not that - it’s a discreet set of a large number of individual sources of gravity, all of which interact gravitationally and often also mechanically. Due to the non-linear nature of the GR equations, it is really not possible (with our current tools) to tell what kind of an error is introduced by idealising this situation to make it model-lable. We don’t have nearly enough computing power (by many orders of magnitude) to numerically solve a GR n-body problem with n on the order of ~100 billion, plus realistic boundary conditions.

At least in principle the discrepancy between model and observation which we call Dark Matter could just be the error introduced by idealising a real-world scenario in a non-linear model. DM could be nothing more than a mathematical artefact.

[Mordred]

It's one of the possibilities though one that I find rather tricky particularly when you further consider a few details.

Those details include the need for DM for early universe large scale structure formation. Gravitational lensing effects not fully accountable by nearby baryonic matter.

Another detail is often missed is that when one goes to measure galaxy rotation curves it's necessary to use mass to luminosity relations. The Mass to luminosity relations show that only 10 to 20 percent the total luminosity can be accounted by baryonic matter content. Even though DM doesn't interact with the EM field it does affect gravity. It is this effect that further shows up in the mass luminosity relations.

Part of my courses was using spectography to examine M31 and other local galaxies and examine the mass-luminosity to rotation curves. 

 This is one detail Zwicky noted when he first examined rotation curves and pushed the examinations beyond mathematical error.

One you rarely ever see discussed is the integrated early and late time Sache Wolfe effects due to overdensity and underdensity regions (this effect also includes localized expansion rates ).

Other possibilities not mentioned yet being Machos and axioms. Though those possibilities I don't follow but they are still current approaches.

 

Edited August 15 by Mordred

[MigL]

8 hours ago, Markus Hanke said:

At least in principle the discrepancy between model and observation which we call Dark Matter could just be the error introduced by idealising a real-world scenario in a non-linear model.

Further to the points Mordred brought up, is anyone even using the EFE to analyze galaxy rotation curves ?
The discrepancy is readily evident when using the Newtonian model for gravity ( which I realize is also an approximation so possibly subject to similar errors ).
As a matter of fact, one proposed solution is MOdified Newtonian Dynamics ( MOND ), but as you state, that has significant issues.

And it's not because Non-Idealized General Relativistic Dynamics (NIGRD ) doesn't roll off the tongue as easily 😄 .

[Mordred]

yes There are treatments using the EFE here is one example

https://arxiv.org/pdf/2405.04933

another example using Gravitomagnetism

https://arxiv.org/pdf/2303.06115

one article I particularly like that isn't model specific other than GR.

TOWARDS A FULL GENERAL RELATIVISTIC APPROACH TO GALAXIES

https://arxiv.org/pdf/2106.12818

Edited August 16 by Mordred

[Eise]

On 8/15/2024 at 7:41 AM, Markus Hanke said:

But a real galaxy is not that - it’s a discreet set of a large number of individual sources of gravity, all of which interact gravitationally and often also mechanically. Due to the non-linear nature of the GR equations, it is really not possible (with our current tools) to tell what kind of an error is introduced by idealising this situation to make it model-lable.

Interesting Ansatz. 2 Points:

• Aren't the distances between stars and so that big, that Newtonian mechanics can be used? (Therefore MOND).
• Would it possibly explain all observations that hint at DM? For galactic rotation curves and movements of galaxies in cluster I can imagine it does. But CMB and separation between DM and baryonic matter in colliding galaxy clusters?

20 hours ago, Mordred said:

This is one detail Zwicky noted when he first examined rotation curves and pushed the examinations beyond mathematical error.

Zwicky studied the movements of galaxies in galaxy clusters, and concluded that there was not enough visible matter to explain their movements. Just a tiny, historical detail.

It was Jan Hendrik Oort:

Quote

In 1932, Jan Hendrik Oort became the first to report that measurements of the stars in the solar neighborhood indicated that they moved faster than expected when a mass distribution based upon visible matter was assumed, but these measurements were later determined to be essentially erroneous.

Zwicky was Swiss, Oort was Dutch, so as a Dutchman living in Switzerland, I take this very personal . 

Edited August 16 by Eise

[KJW]

I think one needs to remember that Newtonian gravitation only contributes half of any gravitational lensing effects.

 

 

[Mordred]

Zwicky did use the mass luminosity relations to make his velocity determination. Though Oort also did as well. 

Invariably the mass-luminosity relations is required though that often gets missed as most ppl typically focus on the redshift relations.

 Both are involved, in point of detail in this instance in order to determine redshift you need the mass luminosity relations to begin with however for some reason readers don't find the luminosity relations itself as relevant as the redshift...

I will leave it in the hands of the historians as to who is considered the father of DM.

[DanMP]

On 8/15/2024 at 8:41 AM, Markus Hanke said:

While I think the current evidence isn’t strong enough to definitively rule out either the particle or the alternative gravity model option, I personally tend towards the third option, which avoids both of those - namely that DM is actually an artefact of our inability to produce solutions to the ordinary GR field equations that aren’t idealised.

For example, even the best numerical approaches to modelling a spiral galaxy in the context of the GR equations need to be idealised - it’s going to be some sort of continuous dust distribution, with appropriate density curves and initial and boundary conditions. But a real galaxy is not that - it’s a discreet set of a large number of individual sources of gravity, all of which interact gravitationally and often also mechanically. Due to the non-linear nature of the GR equations, it is really not possible (with our current tools) to tell what kind of an error is introduced by idealising this situation to make it model-lable. We don’t have nearly enough computing power (by many orders of magnitude) to numerically solve a GR n-body problem with n on the order of ~100 billion, plus realistic boundary conditions.

At least in principle the discrepancy between model and observation which we call Dark Matter could just be the error introduced by idealising a real-world scenario in a non-linear model. DM could be nothing more than a mathematical artefact.

I can't believe that you wrote/admit such a thing, namely that we are incapable to accurately use GR. In this case, how/why we can say that GR is correct (in agreement with all observations)?

[Mordred]

58 minutes ago, DanMP said:

I can't believe that you wrote/admit such a thing, namely that we are incapable to accurately use GR. In this case, how/why we can say that GR is correct (in agreement with all observations)?

It's not that GR is inaccurate. The difficulty is that with a galaxy you have an axisymmetric spacetime with a disk that also has rotation.

That only covers certain galaxy types. Each galaxy type would require its own set of EFE solutions.

If you look at the links I included in response to Migl you will see a proposed set of solutions for spiral galaxies. 

[Markus Hanke]

On 8/15/2024 at 2:07 PM, Mordred said:

the need for DM for early universe large scale structure formation

 

On 8/15/2024 at 2:07 PM, Mordred said:

mass to luminosity relations

 

21 hours ago, Eise said:

Aren't the distances between stars and so that big, that Newtonian mechanics can be used?

 

21 hours ago, Eise said:

But CMB and separation between DM and baryonic matter in colliding galaxy clusters?

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.

14 hours ago, DanMP said:

I can't believe that you wrote/admit such a thing, namely that we are incapable to accurately use GR.

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. 

[DanMP]

23 hours ago, Mordred said:

On 8/16/2024 at 6:09 PM, DanMP said:

... we are incapable to accurately use GR. In this case, how/why we can say that GR is correct (in agreement with all observations)?

Expand  

It's not that GR is inaccurate

I wrote: "we are incapable to accurately use GR " ...

 

9 hours ago, Markus Hanke said:

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. 

In the real world n is extremely large. And you said :

On 8/15/2024 at 8:41 AM, Markus Hanke said:

At least in principle the discrepancy between model and observation which we call Dark Matter could just be the error introduced by idealising a real-world scenario in a non-linear model. DM could be nothing more than a mathematical artefact.

DM is huge. If this is a "mathematical artefact" how can we trust the validity of GR on large scales?!? How/why we can say that GR is in agreement with all observations? It sounds like cheating. Or you are wrong. 

[Mordred]

14 minutes ago, DanMP said:

I wrote: "we are incapable to accurately use GR " ...

 

In the real world n is extremely large. And you said :

DM is huge. If this is a "mathematical artefact" how can we trust the validity of GR on large scales?!? How/why we can say that GR is in agreement with all observations? It sounds like cheating. Or you are wrong. 

We aren't incapable but no single set of equations solve every problem. GR by itself isn't suitable to deal with infall rates v outfall rates as applicable to LSS and galaxy formation, nor formation of DM halos. Other hydrostatic formulas are required. I will be detailing those involved as I had planned on doing so in regards to Markus last post using his sand dune analogy. (though I will drop the analogy itself.)

 

[DanMP]

On 8/14/2024 at 7:16 PM, Mordred said:

As far as opinions on DM I've always leaned toward sterile neutrinos even though I have examined treatments using Majaronna mass coupling RH neutrinos and 3 species I still haven't seen how to account for the total mass.

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

 

On 8/14/2024 at 6:23 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? 

 

On 8/14/2024 at 6:23 PM, Eise said:

Wouldn't that mean that most galaxies also have a gas halo around them? Maybe they have, but I assume that such a gas halo would be extremely thin. In the end, most mass is concentrated in the galactic plane. Because the gas collided, got concentrated in the galactic plain so much that it could form stars

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? 

[joigus]

32 minutes ago, DanMP said:

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

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

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

So what you said is a bit like saying "maybe the murderer is any old person, instead of that particular suspect" in a murder case. You see...