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Concerning Dark Matter


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I have been thinking about dark matter lately (it's been very cloudy here and my job is boring). 

When I first heard that it was a thing, and was necessary for current theories to work, I thought 'Why does there need to be dark matter, can it not just be ordinary matter that we can't see, and black holes and stuff?'. Then I learned that It didn't interact with anything, and there had to be more of it than actual matter so I thought 'OK'. I have been watching some things on Youtube and now I see that black holes are being mooted again.

Can anyone explain the current thinking on this?

 

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Astro Noodles 

As I understand it, when we observe things such as stars going around galaxies, the amount of gravity acting on the stars is more than it should be. There must be something else "pulling the stars" that we cannot see...dark matter.

Mark

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There are various theories as to the composition of dark matter. Several propose "new" particles like axions. The were first proposed in 1977 to solve problems in quantum chromodynamics, and given their small energies could be responsible for the dark matter. There are several other options. I am currently involved in designing detection algorithms for the Cherenkov Telescope Array to look for potential candidates by observing gamma rays from potentially dark-matter rich dwarf galaxies. Hopefully, careful observations might solve the mystery

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If Dark Energy is exponentially expanding the universe and Dark Matter adds hugely to the total mass of the universe something seems to be wrong with my simple understanding - surely mass should slow expansion???

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It won’t answer your question but if you haven’t seen it have a look at “Black Holes - The Edge of All We Know” on Netflix. My wife went to bed but I found it interesting…….

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24 minutes ago, azrabella said:

If Dark Energy is exponentially expanding the universe and Dark Matter adds hugely to the total mass of the universe something seems to be wrong with my simple understanding - surely mass should slow expansion???

Dark energy works on the expansion of the bulk of the universe, dark matter locally contracts matter into stars, galaxies, clusters, walls etc, whereas dark energy expands the voids between these structures. We need dark energy to explain the apparent accelleration of the expansion of the universe, but dark matter because the visible matter cannot explain all the structures we see.

 

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So the thinking is that dark matter is going to turn out to be a combination of thing? Axions, other exotic particles and primordial black holes, and maybe other things as well?

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1 hour ago, Astro Noodles said:

So the thinking is that dark matter is going to turn out to be a combination of thing? Axions, other exotic particles and primordial black holes, and maybe other things as well?

Possible, we simply don't know 

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2 hours ago, Astro Noodles said:

So the thinking is that dark matter is going to turn out to be a combination of thing? Axions, other exotic particles and primordial black holes, and maybe other things as well?

As I understand it, there is some suggestion that although Dark Matter doesn't interact with 'normal' matter (except gravitationally), it may well interact with itself. 

The reason that this is suspected seems to relate to the way that the distribution of Dark Matter in the very central parts of a galaxy is slightly different to what would be expected if there were no interactions at all. 

It thus seems possible (to me at least) that this may represent a new force altogether to go along with the Electromagnetic, Strong and Weak forces, with a new boson as well to mediate the force. 

It's exciting of course, but tricky to see how it would be possible to run experiments that would let us find out more. 

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A report on a Zoom talk about dark matter research that I "attended" last fall:

 

On 09/12/2020 at 10:19, George Jones said:

I meant to get to this earlier, but I have been doing a COVID-related overload, and things have been somewhat hectic.

The talk included theoretical results of the paper to which I linked in my previous post. The authors of the paper take the view that gravity is okay, and that new unseen and transparent dark matter is needed to account for the motions of stars in galaxies, and galaxies in clusters of galaxies. Observations of the relative abundances of primordial elements, and of the Cosmic Microwave Background (CMB), indicate that dark matter is not made of the particles that account for normal matter's mass, protons and neutrons.

To date, we only have evidence that dark matter interacts via gravity, but many physicist think/hope that dark matter interacts via other (quantum) forces. They think that dark matter has quantum interactions, because this would give scenarios for dark matter production in the early universe similar to the production of normal matter, e.g., matter antimatter annihilation/creation that continues until the universe expands enough to stop these processes. Physicists hope that dark matter interacts with normal matter, as this gives ways for experimentally seeing signatures of dark matter.

If there are other interactions (besides gravity) involving dark matter interactions, they could be between: 1) dark matter and dark matter (just as there are interactions between normal matter and normal matter), and/or 2) dark matter and normal matter.

The very interesting talk was about theoretical models that have both 1) and 2), where 1) is used to generate the masses of dark matter particles, and 2) is used to predict observable effects of the models.

Two scenarios were considered for generation dark matter mass. One was a possible a dark electromagnetic-like interaction between dark matter and dark matter. The analogy is not exact, as, unlike normal photons, the dark photons have mass that they acquire from a proposed dark Higgs-like particle. Another possibility is that there is a dark colour-like force. In the normal colour nucleon force, gluons carry the colour force, and thus gluons can interact with gluons. In the proposed dark colour-like interactions, dark gluons interact to form massive dark glueballls.

In both types of models, it is proposed that there is a "feeble" interaction between dark matter and normal matter. This feeble interaction can inject energy into the normal universe. If his injection is early enough, it can affect the relative primordial abundances of elements. If this energy injection is later, it can affect the the thermal spectrum of the CMB. If this injection is later still, it can affect the anisotropy spectrum of the CMB. High precision cosmological measurements could reveal these effects.

Possible results for the LHC and for high precision cosmological measurements probe complementary interaction strengths. The cosmological results are for weaker interactions.

 

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On 28/06/2021 at 21:23, michael.h.f.wilkinson said:

Dark energy works on the expansion of the bulk of the universe, dark matter locally contracts matter into stars, galaxies, clusters, walls etc, whereas dark energy expands the voids between these structures. We need dark energy to explain the apparent accelleration of the expansion of the universe, but dark matter because the visible matter cannot explain all the structures we see.

 

In most cases this is true but apparently there is always something that comes along and opens up a tin of worms to spoil or make theories hard to prove.

Mystery of Galaxy's Missing Dark Matter Deepens | NASA

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11 hours ago, michael.h.f.wilkinson said:

MOND opens an entirely different can of worms. Hasn't been ruled out yet, however

I think it's fair to say that there isn't a single formulation of MOND that hasn't been ruled out for one reason or another.

There may be new formulations that haven't been tested yet.

As I recall, even Milgrom accepts that MOND requires some form of DM to make things work.

 

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31 minutes ago, Gfamily said:

I think it's fair to say that there isn't a single formulation of MOND that hasn't been ruled out for one reason or another.

There may be new formulations that haven't been tested yet.

As I recall, even Milgrom accepts that MOND requires some form of DM to make things work.

 

I agree, all formulations I can recall had problems (hence the can of worms), but it is always difficult to rule out MOND completely (especially with some sprinkling of DM)

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19 hours ago, bomberbaz said:

In most cases this is true but apparently there is always something that comes along and opens up a tin of worms to spoil or make theories hard to prove.

Mystery of Galaxy's Missing Dark Matter Deepens | NASA

 

Pieter van Dokkum, the lead author of the research explained at this link, takes this as evidence for dark matter and against MOND (MOdified Newtonian Dynamics). From the link

Quote

Nevertheless, van Dokkum thinks finding a galaxy lacking dark matter tells astronomers something about the invisible substance. "In our 2018 paper, we suggested that if you have a galaxy without dark matter, and other similar galaxies seem to have it, that means that dark matter is actually real and it exists," van Dokkum said. "It's not a mirage."

 

From the Introduction section of the actual 2021 research paper,

Quote

The EFE, unique to MOND, causes a low mass galaxy in orbit around a massive galaxy to have a lower velocity dispersion than the same object in isolation. Thus, the low velocity dispersions of NGC 1052–DF2 and NGC 1052–DF4 may be consistent with the expectations from MOND if both galaxies are in close proximity to NGC 1052 (Kroupa et al. 2018).

 

and, from the paper's Conclusion,

Quote

The relative distance also places a new constraint on the interpretation of NGC 1052–DF2 and NGC 1052–DF4 in the context of MOND (see§1). ... the observed velocity dispersions of both NGC 1052–DF2 and NGC 1052–DF4 are consistent with MOND if both galaxies are physically close (<300 kpc; Kroupaet al. 2018) to a massive galaxy. The most obvious candidate for this massive galaxy is NGC 1052 itself, but our relative distance measurement of ∆D= 2.1±0.5 Mpc rules out NGC 1052–DF2 and NGC 1052–DF4 both being within 300 kpc of NGC 1052.

 

 

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  • 1 month later...

IMVHO (In My Very Humble Opinion) Dark matter is a flawed theory. I cannot understand why most of the scientific community embrace it. The terrain does not follow the map, therefore the terrain is wrong - I just don't get it. But then I don't have the mathematical skills to follow the argument's. Good luck finding any real observation of dark matter - I postulate you won't. And I don't think I will stand corrected on that in my lifetime.  I hope I will humbly have to admit I am wrong though.

I think just because something can be written mathematically it does not necessarily make it true.

Examples:

In the center of a black hole is a singularity.

I have an apple. George Jones have an apple. Therefore me and George have two apples together  (1+1=2). The math is undoubtedly true, so it must be a fact! Let me roam the house and look for that apple...

I did not find any apples, but I do have some tomatoes. Math is just a language, you can say anything you want, but that does not necessarily make it so.

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On 28/06/2021 at 20:34, mdstuart said:

That trumps my answer!

I thought your answer was spot on  - basically what we teach at school ( A level/Higher)  :) 

Jim 

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On 28/06/2021 at 20:57, azrabella said:

If Dark Energy is exponentially expanding the universe and Dark Matter adds hugely to the total mass of the universe something seems to be wrong with my simple understanding - surely mass should slow expansion???

Ah that is the question - the answer will decide the fate of the universe - dark energy dominates and we have the" big rip ",  gravity dominates and the " big crush " .  I don't fancy either , the alternative, the, " slow heat death ",  isn't much better (all the lights go out and eventually nothing happens , literally nothing can happen) - good thing we won't be around to find out :) 

Jim 

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1 hour ago, Viktiste said:

IMVHO (In My Very Humble Opinion) Dark matter is a flawed theory. I cannot understand why most of the scientific community embrace it. The terrain does not follow the map, therefore the terrain is wrong - I just don't get it. But then I don't have the mathematical skills to follow the argument's. Good luck finding any real observation of dark matter - I postulate you won't. And I don't think I will stand corrected on that in my lifetime.  I hope I will humbly have to admit I am wrong though.

I think just because something can be written mathematically it does not necessarily make it true.

Examples:

In the center of a black hole is a singularity.

I have an apple. George Jones have an apple. Therefore me and George have two apples together  (1+1=2). The math is undoubtedly true, so it must be a fact! Let me roam the house and look for that apple...

I did not find any apples, but I do have some tomatoes. Math is just a language, you can say anything you want, but that does not necessarily make it so.

Re your apple problem  - all you have is an incorrect condition , your founding statement is false - you did not have an apple as you claimed in your opening statement .    And no you cannot say anything you want in maths , that is the point , that is why it is so powerful and why it is remarkably successful in describing the universe . 

 

Why do you think dark matter is a flawed theory?   Forget the name , perhaps it is too loaded - I wish they hadn't used the term "matter"   and even "dark" , that's to poetic and allows misconception.   Call it weakly interacting Bla Bla and that removes some of the mystique.  Whatever it is at the moment it is a  phenomena/property that appears to responsible for  galaxy's having a higher rotational velocity than we can account for with our understanding of "normal matter".  So whether it turns out to be a form of matter of not - something is causing the galaxies to be birling around the universe faster than they should - so what is it?  Dark matter or weakly interacting blah blah :) 

Jim 

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What is it?  We don't know, except we don't know - so why pretend to know? 

Why can I not say anything I want in math? Why can somebody else (ok I realize that this may sound stupid...) say anything in math, for example everything existed in a point of 0 diameter at some point? Take the CBR and back calculate - does that make it a fact?

I get the misconception induced by the term "Dark...".  I think because so much of what we observe fits Einstein's Law of General Relativity we are taking it as a fact. So when we see something that does not fit to it, we invent a convenient way out? I'm not claiming to have  the answers, but it makes me wonder. I'm pretty damn sure I will never see any evidence of Dark Matter in my lifetime.

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