Understanding Dark Matter: A Simple Explanation

[tolove]

My Google searches are result in either something too complicated, or confusing non scientific journal articles.

I always thought black holes were what people were referring to by dark matter. And I also was under the impression that small black holes should have all evaporated by now if they ever existed.

Dark matter is some new kind of particle? What's the justification for adding a new particle? I read something that said we could be certain dark matter doesn't exist in supervoids because an overall lower temperature was measured there. But if dark matter isn't emitting light, how can it emit heat (I can't imagine things colliding much in regions that large)?

 

[phinds]

Dark matter is an unknown form of matter whose presence has been shown beyond dispute in at least a couple of different ways. It is called "dark" matter because as you apparently already know, it does not absorb or emit electromagnetic radiation. It only interacts with normal matter and itself gravitationally. This has serious ramifications. If does not clump and form stars or planets because dark matter particles (if indeed it is even made of particles, which does seem likely) do not collide with each other. It exists primarily in halos at the outer areas of galaxies.

It has zero to do with black holes. The leading candidate for dark matter is WIMPS.

Black holes evaporate so slowly that they will be the last large objects to exist in the universe.

 

[ZapperZ]

My Google searches are result in either something too complicated, or confusing non scientific journal articles.

I always thought black holes were what people were referring to by dark matter. And I also was under the impression that small black holes should have all evaporated by now if they ever existed.

Dark matter is some new kind of particle? What's the justification for adding a new particle? I read something that said we could be certain dark matter doesn't exist in supervoids because an overall lower temperature was measured there. But if dark matter isn't emitting light, how can it emit heat (I can't imagine things colliding much in regions that large)?

Zz.

 

[adjacent]

TheWikipedia article is really good.You can get a lot of information from there.

Dark matter is a type of matter hypothesized in astronomy and cosmology to account for a large part of the mass that appears to be missing from the universe. Dark matter cannot be seen directly with telescopes; evidently it neither emits nor absorbs light or other electromagnetic radiation at any significant level.Instead, the existence and properties of dark matter are inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe. According to the Planck mission team, and based on the standard model of cosmology, the total mass–energy of the known universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy.Thus, dark matter is estimated to constitute 84.5% of the total matter in the universe and 26.8% of the total content of the universe.

However,

Although the existence of dark matter is generally accepted by the mainstream scientific community, there is no generally agreed direct detection of it. Other theories, including MOND and TeVeS, are some alternative theories of gravity proposed to try to explain the anomalies for which dark matter is intended to account.

 

[tolove]

Dark matter can't emit a photon of any wavelength? If it only interacts through gravitational forces, where does the heat come from?

If it interacts with itself gravitationally, why doesn't it form clumps?

Is it possible for their to be a light cloud of dark matter throughout our solar system and we don't know it?

What is dark energy?

I'm sorry for all the questions, but this just doesn't make any sense!

edit: To make the heat question more clear, I'm speaking of the thing I read that said supervoids in space lack even dark matter because of lower temperatures.

 

[phinds]

Dark matter can't emit a photon of any wavelength? If it only interacts through gravitational forces, where does the heat come from?

There is no heat associated with dark matter.

If it interacts with itself gravitationally, why doesn't it form clumps?

Because it ONLY interacts gravitationally. Regular matter particles have friction among themselves, dark matter does not. This is why dark matter creates halos at the outer areas of galaxies --- if it move into the center, it just keeps going and comes back out to the halo and because of the equations of such motion, it spends most of its time in the halo.

Is it possible for their to be a light cloud of dark matter throughout our solar system and we don't know it?

no, that would, by definition, not BE dark matter

What is dark energy?

whole different topic. Look it up

 

[tolove]

..

Check out my edit for the heat question!

And with dark matter present in our solar system, I didn't mean "light" as in luminescent, I meant it as in a small amount. If there's so much of this substance, why is it only present at the edges of galaxies?

Is this similar to the idea of a graviton?

Thank you very much for your time!

 

[phinds]

Check out my edit for the heat question!

And with dark matter present in our solar system, I didn't mean "light" as in luminescent, I meant it as in a small amount. If there's so much of this substance, why is it only present at the edges of galaxies?

Is this similar to the idea of a graviton?

Thank you very much for your time!

Has nothing to do with gravitons.

Small amounts of light emitted by something rules it out as dark matter.

I already explained why it is MAINLY (not ONLY) present at the outer regions (not edges) of galaxies.

The fact, if true, that some regions don't have dark matter has nothing to do with any heat associated with dark matter because there IS no heat associated with dark matter.

 

[tolove]

I already explained why it is..

This is why dark matter creates halos at the outer areas of galaxies --- if it move into the center, it just keeps going and comes back out to the halo and because of the equations of such motion, it spends most of its time in the halo.

Oh, wow. I was thinking in terms of stars and rings around planets, but with no interactions they go all the way out the the edge of a galaxy. That's pretty amazing.

Are there any more basic properties?

1. It doesn't interact with anything outside gravitational forces.
2. It carries mass.
3. It does not emit light.

I don't get how something can carry mass and not bump into things. That just.. doesn't make sense? Is there something I'm missing?

edit:

I think I'm confused with exactly what the four fundamental forces do. If we take a planet's worth of hydrogen and break it up into components of electrons, protons, neutrons, then give them random motion in close proximity, what happens?

With all four forces present, a planet and maybe some rings will eventually form, right?

What happens when we take away one particular fundamental force and keep the other three? Is this a good approach to understanding why dark matter is behaving like it does?

 

[Chrispen Evan]

I don't get how something can carry mass and not bump into things. That just.. doesn't make sense? Is there something I'm missing?

Look at Neutrinos. they have mass but rarely interact with other particles. Thinking of particles "bumping into each other" is not the most accurate way of visualising these events. Interacting is probably a better way. Or so I believe.

 

[phinds]

I don't get how something can carry mass and not bump into things. That just.. doesn't make sense? Is there something I'm missing?

No, you're not missing anything, that's just the way it is.

There are numerous things in quantum mechanics (the very small) and cosmology (the very large) that don't "make sense" to us because they are totally outside the realm of the things that we have evolved being familiar with. Throwing a spear to hit a moving animal leads to LOTS of things for example, but NOTHING leads to any intuitive feel for the double slit experiment.

 

[Bobbywhy]

Dark matter may “clumpâ€. There have been many dark matter collisional cross-section studies. One recent paper, “A STUDY OF THE DARK CORE IN A520 WITH HUBBLE SPACE TELESCOPE: THE MYSTERY DEEPENS†describes their research and its puzzling results.
From the conclusion:
“Dark matter self-interaction cross-section must be at least _6 _ larger than the upper limit _1cm2g􀀀1 determined by the Bullet Cluster observation. Therefore, it is difficult to attribute the feature to dark matter self-interaction without falsifying the weak lensing analysis of the Bullet Cluster (Clowe et al. 2006), which does not show any significant mass clump between the two dominant mass peaks.
Despite our solid confirmation on the presence of the dark core, we conclude that it is yet premature to single out the most probable cause of the dark core from the above scenarios.â€

http://arxiv.org/pdf/1202.6368.pdf

 

[sophiecentaur]

It is so nice to read a thread which is 13 whole posts long yet contains a civilised interchange of ideas with not a hint of cranky magic / Teslaism or a groundless 'but surely'. Perhaps it's too early in the life of Dark Matter for the BS bug to have found it.
It's what PF is all about.

 

[Bobbywhy]

It is so nice to read a thread which is 13 whole posts long yet contains a civilised interchange of ideas with not a hint of cranky magic / Teslaism or a groundless 'but surely'. Perhaps it's too early in the life of Dark Matter for the BS bug to have found it.
It's what PF is all about.

Oh, sophiecentaur! I agree that here in our PF there is great opportunity for learning and spreading scientific knowledge.

Is there some irony in your invocation of "the BS bug"? Is it possible you are a "secret DM denier"?

Here are two articles on the same "LUX" experiment results just released. They've not detected one single "WIMP"! Might DM consist of axions? Or, some other unnamed "particle"? The mystery continues...and the deniers continue to look for alternate explanations to explain the observations.

https://www.sciencenews.org/article/candidates-dark-matter-particles-bite-dust
http://www.scientificamerican.com/a...means-search-may-soon-become-more-challenging

 

[sophiecentaur]

Oh, sophiecentaur! I agree that here in our PF there is great opportunity for learning and spreading scientific knowledge.

Is there some irony in your invocation of "the BS bug"? Is it possible you are a "secret DM denier"?

Here are two articles on the same "LUX" experiment results just released. They've not detected one single "WIMP"! Might DM consist of axions? Or, some other unnamed "particle"? The mystery continues...and the deniers continue to look for alternate explanations to explain the observations.

https://www.sciencenews.org/article/candidates-dark-matter-particles-bite-dust
http://www.scientificamerican.com/a...means-search-may-soon-become-more-challenging

Oh ye of little faith . No hint of irony in my message. Give me dark matter every time.

 

[Chronos]

In the Dirac-Milne model, dark matter and dark energy are not required - re: http://arxiv.org/abs/1110.3054, Introducing the Dirac-Milne universe. On the downside, it requires antimatter with antigravity properties. Observational and theoretical constraints suggest that is improbable. While WIMPs are the leading DM candidate, other candidates remain viable, such as neutralinos, sterile neutrinos, axions, and gravitinos.

 

[Eirhead]

I was just reading this on Dark Matter...
http://en.wikipedia.org/wiki/Local_supercluster#Dark_matter

Gave me some better insight into the thought process of the idea. But I think I disagree with it. I believe near field gravitational lensing versus far field gravitational lensing could be used as an explanation for the disparity in mass to light ratios.Like... I think of the Earth's orbital speed around our Sun... Then I think of how the Orbital speed of the Sun is about 8 times that around the center of our galaxy. And I then I go back to thinking of some other cosmological models, like the plasma universe for example. I don't particularly subscribe to it - but I think of radiation pressure, gravitational lensing, and how everything interacts on a solar level, galactic level, universal level... and it just kind of all makes sense without the need for dark matter to me.

 

[Drakkith]

The problem is that when you get right down to it, the other cosmological models simply don't make as much sense as dark matter. By that I mean that they require even more changes to modern physics than dark matter does.

 

[Chronos]

DM could consist of a variety of particle, just like baryonic matter. A mix of warm and dark DM particles of various species appears possible. We already know of one DM particle called the neutrino. They are incredibly difficult to detect because they rarely interact with other particles. They can easily pass through several million miles of lead - re: http://www.Newton.dep.anl.gov/askasci/phy05/phy05165.htm. We have been able to confirm their existence only because they are believed to be the most abundant particle in the universe. Neutrinos would be considered hot dark matter, so we know they cannot be the only particle in the DM zoo.

 

[romanhoit]

Dark matter may “clumpâ€. There have been many dark matter collisional cross-section studies. One recent paper, “A STUDY OF THE DARK CORE IN A520 WITH HUBBLE SPACE TELESCOPE: THE MYSTERY DEEPENS†describes their research and its puzzling results.
From the conclusion:
“Dark matter self-interaction cross-section must be at least _6 _ larger than the upper limit _1cm2g􀀀1 determined by the Bullet Cluster observation. Therefore, it is difficult to attribute the feature to dark matter self-interaction without falsifying the weak lensing analysis of the Bullet Cluster (Clowe et al. 2006), which does not show any significant mass clump between the two dominant mass peaks.
Despite our solid confirmation on the presence of the dark core, we conclude that it is yet premature to single out the most probable cause of the dark core from the above scenarios.â€

http://arxiv.org/pdf/1202.6368.pdf

The link you mentioned above is so much helpful.
Thanks for such a nice post!

 

[Keln]

Just to be clear, I do not "believe" or "disbelieve" in DM. Those are dogmatic concepts anyway and have no place in science. Personally, I don't care if there is or isn't dark matter. Either way is interesting, and what is, is...there's no point in crying about it.

But on the dark matter question, I've wondered why a whole form of matter, and proposed to be the most common form at that, is necessary to explain gravitational interactions in the cosmos that we cannot currently explain. And further, this new matter cannot be described in any way other than that it doesn't interact with anything but gravity.

It seems to me that, if the one force at work is gravity, then it is our understanding of gravity that might want to be looked at too, perhaps before coming up with new forms of matter that are even less defined than what we know of gravity. Even today, some 90 odd years after GR was proposed, can anyone fully define gravity?

I'm not saying some alternative theory of gravity is "more correct", or even more likely than a new form of undetectable matter; I just don't understand the attitude towards working from the gravity angle instead of the matter angle. It seems reasonable that both are valid until something is proven.

 

[ZapperZ]

Just to be clear, I do not "believe" or "disbelieve" in DM. Those are dogmatic concepts anyway and have no place in science. Personally, I don't care if there is or isn't dark matter. Either way is interesting, and what is, is...there's no point in crying about it.

But on the dark matter question, I've wondered why a whole form of matter, and proposed to be the most common form at that, is necessary to explain gravitational interactions in the cosmos that we cannot currently explain. And further, this new matter cannot be described in any way other than that it doesn't interact with anything but gravity.

It seems to me that, if the one force at work is gravity, then it is our understanding of gravity that might want to be looked at too, perhaps before coming up with new forms of matter that are even less defined than what we know of gravity. Even today, some 90 odd years after GR was proposed, can anyone fully define gravity?

I'm not saying some alternative theory of gravity is "more correct", or even more likely than a new form of undetectable matter; I just don't understand the attitude towards working from the gravity angle instead of the matter angle. It seems reasonable that both are valid until something is proven.

Did you even see the video that I linked to in Post #3? It answers/addresses at least a couple of the questions you asked here, if not more.

And yes, we CAN "fully define" gravity, even though we continue to test the General Relativity picture of gravity. Don't you think it is rather silly to not know enough about gravity, and yet, we can predict celestial events with such accuracy? How many other things that you think you can define and have such amazing accuracy?

Zz.

 

[Drakkith]

Just to be clear, I do not "believe" or "disbelieve" in DM. Those are dogmatic concepts anyway and have no place in science. Personally, I don't care if there is or isn't dark matter. Either way is interesting, and what is, is...there's no point in crying about it.

Believing or disbelieving in something doesn't mean that it's something spiritual or religious. I believe that the BBT is accurate. I do not believe it is the final word in cosmological theories.

But on the dark matter question, I've wondered why a whole form of matter, and proposed to be the most common form at that, is necessary to explain gravitational interactions in the cosmos that we cannot currently explain. And further, this new matter cannot be described in any way other than that it doesn't interact with anything but gravity.

The short answer is that adding a new type of matter is LESS problematic and currently has MORE evidence in favor for it than theories involving modified gravity.

It seems to me that, if the one force at work is gravity, then it is our understanding of gravity that might want to be looked at too, perhaps before coming up with new forms of matter that are even less defined than what we know of gravity. Even today, some 90 odd years after GR was proposed, can anyone fully define gravity?

Various versions of a theory that modifies gravity have been developed. They simply don't match observations as well as dark matter.

See here: http://en.wikipedia.org/wiki/MOND#Consistency_with_the_observations

I'm not saying some alternative theory of gravity is "more correct", or even more likely than a new form of undetectable matter; I just don't understand the attitude towards working from the gravity angle instead of the matter angle. It seems reasonable that both are valid until something is proven.

Scientific theories are never "proven". The simplest ones that give us the most accurate predictions with the least amount of assumptions are generally taken as being the most "accurate" for the time being.

 

[Keln]

Don't you think it is rather silly to not know enough about gravity, and yet, we can predict celestial events with such accuracy?

Zz.

Wasn't that the point of introducing dark matter in the first place...that certain celestial events, such as the rotation of galaxies, was not, in fact, accurately predicted by our understanding of gravity and known matter, so that more matter than was observed was hypothesized? It was either gravity or matter or both that was not totally known, and the decision was apparently made that it was matter alone. Darkkith alludes to good reasons for that, so I'll just accept it since astrophysics isn't my area.

Really my question/concern is the seemingly angry attitude many seem to exhibit about even questioning the existence of dark matter. I'm always skeptical about anything that is treated as above any questioning, especially when those who do are called "deniers" or the like. Sure, we like to say "never question the data" here where I work, but realistically, we even question that too, all of the time.

I'll watch the video you mentioned when I get to a computer that can play videos later today.

Thanks.

 

[Drakkith]

Really my question/concern is the seemingly angry attitude many seem to exhibit about even questioning the existence of dark matter. I'm always skeptical about anything that is treated as above any questioning, especially when those who do are called "deniers" or the like. Sure, we like to say "never question the data" here where I work, but realistically, we even question that too, all of the time.

I'm not sure what attitudes you are referring to. Here on PF? As far as I have seen, practically all reasonable questions about dark matter are answered in a good manner with little to no "angry" attitude. Usually the first few posts answer the original question and present both the dark matter theories and, if applicable, mention the modified gravity theories. Things only seem to go downhill if the original poster, or someone else, decides to not be civil about it.

 

[phinds]

Things only seem to go downhill if the original poster, or someone else, decides to not be civil about it.

Things also go downhill if someone persistently refuses to believe what more knowledgeable folks are telling them.

 

[Chronos]

It is difficult to convince an 'unbeliever' in the reality of dark matter. The evidence appears overwhelming to many of us, but, there are still scientists out there championing causes like MOND. I certainly understand resistance to invisible fairy explanations, but, we routinely accept other things we cannot perceive with our own senses. Confirmation of the neutrino is [in my mind] a convincing demonstration of a form of dark matter. I'm exposing my bias here, but, the case for the sterile neutrino as a dominant form of dark matter remains strong. A good paper that received several citations in the latter half of 2013 is http://arxiv.org/abs/1306.4954, Next decade of sterile neutrino studies.

 

[sophiecentaur]

If it looks like a duck, swims like a duck and quacks like a duck, then why not call it a duck.
Enough of this nonsense about 'what it really is'. What is anything, 'really'.

 

[phinds]

... there are still scientists out there championing causes like MOND ...

Yeah, I've seen that, but I don't get how MOND explains the bullet cluster, for example.

 

[Jonathan Scott]

Yeah, I've seen that, but I don't get how MOND explains the bullet cluster, for example.

Even if MOND only gets individual galaxies right every time, and does not have very good explanations for colliding galaxies or clusters, it's still an interesting puzzle how dark matter could manage in that case to systematically reproduce a simple rule that only depends on the visible matter.

 

[julcab12]

If it looks like a duck, swims like a duck and quacks like a duck, then why not call it a duck.
Enough of this nonsense about 'what it really is'. What is anything, 'really'.

... But taste like chicken for now. They haven't found/cook the exact duck 'flavor' yet. ^^

 

[mesa]

"...I certainly understand resistance to invisible fairy explanations.

I would think this is the major reason why some people have so much resistance to the idea. There is a certain uneasiness to 'pixie dust' explanations in Science and this isn't the first time we have tried to 'solve' problems by inserting invisible matter we can not see or test in order to make a model work (e.g. aether, caloric).

Questioning thinking is how Science advances and so I say bring it (just have something better to bring to the table than 'I don't like this explanation!').

 

[ZapperZ]

I would think this is the major reason why some people have so much resistance to the idea. There is a certain uneasiness to 'pixie dust' explanations in Science and this isn't the first time we have tried to 'solve' problems by inserting invisible matter we can not see or test in order to make a model work (e.g. aether, caloric).

Questioning thinking is how Science advances and so I say bring it (just have something better to bring to the table than 'I don't like this explanation!').

But you also need to look at the OTHER side of it. Neutrino WAS invented exactly that way, to account for the missing momentum in beta decay. It was years before its existence was verified.

People seem to forget, or simply don't understand, the process of discovery. When you see something new, the first thing you try to do is to try and figure it out using the available tools and knowledge that you know works. It is when, after you try and try, that it can't work, and then somebody comes along and proposed something new that not only explain the new discovery, but also all the previous existing understanding, that's when we have expanded our knowledge!

Is there compelling evidence for the existence of dark matter? Sure! But is dark matter verified? No! If it is, then we will have zero reason to spend all the millions of dollars/euros/yen/etc. in the numerous different experiments to detect dark matter. I wish the General Public has a bit more of a sense that we are STILL in the middle of the discovery phase in understanding this. This could change, and in fact, I am sure that things WILL change as we learn more and more about how to explain the observations that we currently have.

But does not detract for the fact that, while we have not make any discovery yet on the existence of dark matter, based on what we have observed, we know what it has and doesn't have. We have a list of characteristics, a phenomenological model, of this dark matter. So if someone asks "what is dark matter", the appropriate response will be to describe what we know about it so far!

Zz.

 

[mesa]

But you also need to look at the OTHER side of it. Neutrino WAS invented exactly that way, to account for the missing momentum in beta decay. It was years before its existence was verified.

People seem to forget, or simply don't understand, the process of discovery. When you see something new, the first thing you try to do is to try and figure it out using the available tools and knowledge that you know works. It is when, after you try and try, that it can't work, and then somebody comes along and proposed something new that not only explain the new discovery, but also all the previous existing understanding, that's when we have expanded our knowledge!

Is there compelling evidence for the existence of dark matter? Sure! But is dark matter verified? No! If it is, then we will have zero reason to spend all the millions of dollars/euros/yen/etc. in the numerous different experiments to detect dark matter. I wish the General Public has a bit more of a sense that we are STILL in the middle of the discovery phase in understanding this. This could change, and in fact, I am sure that things WILL change as we learn more and more about how to explain the observations that we currently have.

But does not detract for the fact that, while we have not make any discovery yet on the existence of dark matter, based on what we have observed, we know what it has and doesn't have. We have a list of characteristics, a phenomenological model, of this dark matter. So if someone asks "what is dark matter", the appropriate response will be to describe what we know about it so far!

Zz.

Absolutely agree, this is the best we currently have and it will likely change with time, my apologies if my post came across the wrong way.

 

[Vanadium 50]

MOND has a problem with the Bullet Cluster - LCDM does a better job with its mass and its distribution. But the collisional velocity is something that MOND does better than LCDM. Also, Abell 520 is sort of an anti-bullet cluster in that the mass coincides with the has and not the galaxies.

What I find annoying is that many of the same people (some of whom are friends) who say that the Bullet Cluster is the clear smoking gun for LCDM and that no alternative is possible also say when asked about Abell 520 that you can't tell anything from a single pair of interacting galaxies because things are all weird and confusing.

What I think is a fair description of the situation is:

• LCDM is an ugly theory that does better than MOND at very large scales.
• MOND is a profoundly ugly theory - so ugly that its proponents often argue that it is but an approximation of some future theory - but does better than LCDM at galactic scales.

My conclusion from this is that MOND probably is telling us something about how galaxies form, evolve and behave, much like the Drude model tells us something about metals. But it's probably not how the universe is put together.