What is Dark Matter?

[Kleonaptra]

Hi Guys,
Tonight I caught a doco on SBS2 called 'is everything we know about the universe wrong?' I came in after it had started and hadnt planned on watching it, but I was utterly fascinated by the idea of dark matter.

Ive heard it said on this forum that ignoring reality is taking away from its beauty, that not understanding how it really is means a lesser existance to that individual. One of the scientists on this doco actually said, in the invention of dark matter, they were ignoring what was really happening.

I agree the title of this doco was inflammatory, the whole idea was skewed towards disproving the big bang theory, and I understand there is plenty of evidence to suggest the existance of dark matter and dark energy. I just want to understand it. If it is invisible on every spectrum, and intangible enough to go through matter, how does it have enough mass to do what it needs to do to make the equation work?

I saw the experiments where they were trying to detect it - is this even possible? If it is what we percieve it to be, what we need it to be for the theory, can it be detectable at all?

Im really bad at maths, so if anyone wants to explain it with numbers, Im going to get lost quickly.

Any thoughts are welcome - I know only what I saw on this doco and openly admit my ignorance. Tell me what you know!

 

[Pixel42]

If it is invisible on every spectrum, and intangible enough to go through matter, how does it have enough mass to do what it needs to do to make the equation work?

I think of it as being a bit like the nitrogen in the atmosphere, which makes up the bulk of it but is mostly inert.

 

[Zeuzzz]

Dark matter is basically something we have invented so established gravitational theories can still work, as they fall short of explaining the rotation curves of galaxies. We have no real way to experimentally test its existence, its all inferred, and for current theories to work it has to be there.

Its called a fudge factor.

 

[Pixel42]

There's certainly a whiff of epicycles about it.

 

[edd]

Hi Guys,
Tonight I caught a doco on SBS2 called 'is everything we know about the universe wrong?' I came in after it had started and hadnt planned on watching it, but I was utterly fascinated by the idea of dark matter.

Ive heard it said on this forum that ignoring reality is taking away from its beauty, that not understanding how it really is means a lesser existance to that individual. One of the scientists on this doco actually said, in the invention of dark matter, they were ignoring what was really happening.

I agree the title of this doco was inflammatory, the whole idea was skewed towards disproving the big bang theory, and I understand there is plenty of evidence to suggest the existance of dark matter and dark energy. I just want to understand it. If it is invisible on every spectrum, and intangible enough to go through matter, how does it have enough mass to do what it needs to do to make the equation work?

There's no particular reason to associate mass with tangibility.

I saw the experiments where they were trying to detect it - is this even possible? If it is what we percieve it to be, what we need it to be for the theory, can it be detectable at all?

There's a number of possible theoretical candidates for dark matter, and although they all have to interact with light very weakly and not bounce off matter very often, it doesn't preclude some very occasional interaction - much as neutrinos flood through the earth in enormous quantities but still very rarely interact. It's quite reasonable to try to detect it, although obviously it's very difficult.

 

[Dancing David]

Hi Guys,
Tonight I caught a doco on SBS2 called 'is everything we know about the universe wrong?' I came in after it had started and hadnt planned on watching it, but I was utterly fascinated by the idea of dark matter.

Ive heard it said on this forum that ignoring reality is taking away from its beauty, that not understanding how it really is means a lesser existance to that individual. One of the scientists on this doco actually said, in the invention of dark matter, they were ignoring what was really happening.

I agree the title of this doco was inflammatory, the whole idea was skewed towards disproving the big bang theory, and I understand there is plenty of evidence to suggest the existance of dark matter and dark energy. I just want to understand it. If it is invisible on every spectrum, and intangible enough to go through matter, how does it have enough mass to do what it needs to do to make the equation work?

I saw the experiments where they were trying to detect it - is this even possible? If it is what we percieve it to be, what we need it to be for the theory, can it be detectable at all?

Im really bad at maths, so if anyone wants to explain it with numbers, Im going to get lost quickly.

Any thoughts are welcome - I know only what I saw on this doco and openly admit my ignorance. Tell me what you know!

dark matter is hypothesized to explain the observed speed of rotation of galaxies.

It is like neutrinos but more massive in theory.

 

[ben m]

I saw the experiments where they were trying to detect it - is this even possible? If it is what we percieve it to be, what we need it to be for the theory, can it be detectable at all?

Good question.

The astronomical data---which comes from multiple, independent sources which all seem to tell the same story---tell us only that dark matter is pretty weakly interacting. If it were something with a collision-cross-section as large as that of an ordinary atom, it would have behaved differently in galaxies and in the CMB plasma.

The astrophysics data, with a handful of exceptions, would be perfectly happy with strongly interacting dark matter. Imagine something that looked almost exactly like a neutron, only (a) stable and (b) immune to bound states. (Real neutrons, if not tied up in an appropriate nucleus, beta-decay with t1/2 = 12 minutes). Let's call it a Neutralon. Neutralons would be very "dark" and immune to most photon emission/absorption. Neutralons would fly straight through galaxies and galaxy clusters. Neutralons would decouple from the Cosmic Microwave Background plasma and undergo dark-matter-like acoustic oscillations. They'd be a fine dark matter candidate.

But they'd be easy to detect, if they existed; if you launch a dedicated neutron-sensitive detector into space, or into the upper atmosphere, you'd see this population of neutron-like particles colliding with your detector.

(This experiment has been done, see review at http://arxiv.org/abs/astro-ph/0105567 and it looks unlikely that this sort of particle exists.)

But yeah, there's no conflict between "weak enough to agree with cosmology data" and "strong enough to discover in an appropriate detector". But Nature doesn't necessarily care how good our detectors are---she could have pushed that "strong" threshhold way, way, way down. Dark matter made of GeV-scale neutrinos would have evaded discovery until the early 2000's. (It's now basically ruled out.) Dark matter made of axions might be discovered (or ruled out) in the next 5 years. Dark matter made of SUSY WIMPs might be discovered (or ruled out) any time in the next ~15 years. There are some reports of "discoveries" (but not very confident ones) of a lightweight dark matter particle colliding with the CoGeNT detector. To reiterate: any of the above would be consistent with the cosmology data.

 

[Kleonaptra]

Great ideas, wow!

Ok....I know Im gonna cop it for this one....but isnt the whole idea of dark matter like a leap of faith? I can see why in the equation it must exist, but so far, it doesnt. Its just a theory. To me its like saying "The ghost is there because it has to be, I just cant prove it" why exactly is belief in dark matter different?

Im not trolling, and Im not here to spout my beliefs at you, I just want to know why something so intangible is ultimately believed in while other intangibles are not. I know dark matter has plenty of other reasons other than faith to support it, but to the person seeing the ghost, they do too.

Isnt it possible that dark matter is spirit/conciouness/thought/soul energy? They have similar descriptions.

And finally.....If there is no dark matter at all....What are other explanations for the speed of galaxies? Is there any theory other than dark matter that would explain it?

 

[Kleonaptra]

There's no particular reason to associate mass with tangibility.There's a number of possible theoretical candidates for dark matter, and although they all have to interact with light very weakly and not bounce off matter very often, it doesn't preclude some very occasional interaction - much as neutrinos flood through the earth in enormous quantities but still very rarely interact. It's quite reasonable to try to detect it, although obviously it's very difficult.

Why? Dont we usually do so? In reality, to have mass, it must be tangible. Please explain if this isnt so, its just my understanding of it.

'Tangible - perceptable by touch; definite, clearly intelligible,not illusive or visionary. Corporeal.'

From the dictionary.

 

[ben m]

Ok....I know Im gonna cop it for this one....but isnt the whole idea of dark matter like a leap of faith? I can see why in the equation it must exist, but so far, it doesnt. Its just a theory. To me its like saying "The ghost is there because it has to be, I just cant prove it" why exactly is belief in dark matter different?

It's not that it has to exist "because it's in an equation". There's obviously something out there which behaves differently than normal matter. We see it causing gravitational lensing. We see galaxies and stars accelerate under its gravitational pull. We see it compressing and flowing in the waves of the Cosmic Microwave Background. It's obviously something that gravitates.

It's true that we wouldn't be able to be so precise about it if we weren't using equations. If you had shown a Cosmic Microwave Background map to a medieval astronomer, they would have been able to say "huh, it's got some waves" and that's it. A modern astronomer can say "... and the waves show increased power at a multipole of l=200" and other precise (and interesting, and useful-to-follow-up-on) statements. But you wouldn't say that they're 'just in the equation'.

Likewise: do you believe that the Earth has an iron core and a molten mantle? You've never seen them, and neither has anyone else. All you see are equations which say "This mathematical feature of the data would not be there if the mantle were solid, or if the core were silicate".

Isnt it possible that dark matter is spirit/conciouness/thought/soul energy? They have similar descriptions.

No. Dark matter is meant to be ordinary "stuff" obeying ordinary physical laws---no more exotic, and no more mysterious, than the neutrino or the proton or the tau lepton. Just harder to experiment on for technical reasons, i.e. hard enough that we haven't discovered it directly.

And finally.....If there is no dark matter at all....What are other explanations for the speed of galaxies? Is there any theory other than dark matter that would explain it?

The most serious hypothesis is called MOND. This is the hypothesis that General Relativity, which is a well-tested theory of gravity under many circumstances, is invalid under some other (not terribly well understood) circumstances, and maybe this means that ordinary matter's gravity just so happens to add up to explain the data. It's worth pursuing, but no one has quite made it work yet and there are data (the Bullet Cluster) that make it look very unlikely.

 

[Roboramma]

Great ideas, wow!

Ok....I know Im gonna cop it for this one....but isnt the whole idea of dark matter like a leap of faith? I can see why in the equation it must exist, but so far, it doesnt. Its just a theory. To me its like saying "The ghost is there because it has to be, I just cant prove it" why exactly is belief in dark matter different?

Everything we observe is observed based on it's interactions with other things. For instance, when I look out my window at the building across the street I don't have some mystical direct interaction with the building: light that interacted with ("bounced off) the building hits my eyes and interacts with them, and it's the pattern of that light that I use to deduce the presence of the building.

Dark matter doesn't interact with light (hence "dark"), but it does interact with gravity. So in much the same way that I deduce the presence of the building by the way it affects the light that strikes it, we can deduce the presence of dark matter by the way it affects the other matter in it's neighborhood (through gravitation).

Isnt it possible that dark matter is spirit/conciouness/thought/soul energy? They have similar descriptions.

What's similar in their descriptions? They may be similar in what they aren't (ie. interacting with electromagnetism), but I don't see any similarity in what they are (for instance, dark matter can't think).

But it's odd to consider things to be similar because they are lacking a particular quality: both the planet Jupiter and I don't have red hair, for instance, but that doesn't make me particularly similar to Jupiter.

And finally.....If there is no dark matter at all....What are other explanations for the speed of galaxies? Is there any theory other than dark matter that would explain it?

If there is no dark matter then general relativity needs to be revised.

 

[ben m]

Why? Dont we usually do so? In reality, to have mass, it must be tangible. Please explain if this isnt so, its just my understanding of it.

'Tangible - perceptable by touch; definite, clearly intelligible,not illusive or visionary. Corporeal.'

From the dictionary.

Nope. Counterexample: the neutrino is intangible (10^6 of them pass through every centimeter of your body every second, but you've only got a 50-50 shot of having one interact with you during your lifetime) but it's a real thing with mass.

"Tangible" has to do with the forces that allow matter to interact with your senses. It has nothing to do with whether or not something has mass, or can be called matter, or exists.

 

[Kleonaptra]

It's not that it has to exist "because it's in an equation". There's obviously something out there which behaves differently than normal matter. We see it causing gravitational lensing. We see galaxies and stars accelerate under its gravitational pull. We see it compressing and flowing in the waves of the Cosmic Microwave Background. It's obviously something that gravitates.

It's true that we wouldn't be able to be so precise about it if we weren't using equations. If you had shown a Cosmic Microwave Background map to a medieval astronomer, they would have been able to say "huh, it's got some waves" and that's it. A modern astronomer can say "... and the waves show increased power at a multipole of l=200" and other precise (and interesting, and useful-to-follow-up-on) statements. But you wouldn't say that they're 'just in the equation'.

Likewise: do you believe that the Earth has an iron core and a molten mantle? You've never seen them, and neither has anyone else. All you see are equations which say "This mathematical feature of the data would not be there if the mantle were solid, or if the core were silicate".



No. Dark matter is meant to be ordinary "stuff" obeying ordinary physical laws---no more exotic, and no more mysterious, than the neutrino or the proton or the tau lepton. Just harder to experiment on for technical reasons, i.e. hard enough that we haven't discovered it directly.



The most serious hypothesis is called MOND. This is the hypothesis that General Relativity, which is a well-tested theory of gravity under many circumstances, is invalid under some other (not terribly well understood) circumstances, and maybe this means that ordinary matter's gravity just so happens to add up to explain the data. It's worth pursuing, but no one has quite made it work yet and there are data (the Bullet Cluster) that make it look very unlikely.

I can only speak in broad terms - ie, in the equation - because my knowledge is limited to the doco I watched last night, and I dont know any of the equations. Im counting on you guys to know them.

I really like that statement Ive highlighted, a lot of the scientists in the show used similar words.

I believe the earth has an iron core, but I understand its represented in different ways. We dont agree on its specifics. Dark matter strikes me as the same thing - way into the grey area.

 

[Kleonaptra]

Everything we observe is observed based on it's interactions with other things. For instance, when I look out my window at the building across the street I don't have some mystical direct interaction with the building: light that interacted with ("bounced off) the building hits my eyes and interacts with them, and it's the pattern of that light that I use to deduce the presence of the building.

Dark matter doesn't interact with light (hence "dark"), but it does interact with gravity. So in much the same way that I deduce the presence of the building by the way it affects the light that strikes it, we can deduce the presence of dark matter by the way it affects the other matter in it's neighborhood (through gravitation).

What's similar in their descriptions? They may be similar in what they aren't (ie. interacting with electromagnetism), but I don't see any similarity in what they are (for instance, dark matter can't think).

But it's odd to consider things to be similar because they are lacking a particular quality: both the planet Jupiter and I don't have red hair, for instance, but that doesn't make me particularly similar to Jupiter.



If there is no dark matter then general relativity needs to be revised.

How do you know dark matter doesnt think?

The soul energy or counciousness doesnt think either - humans think, imagine, create. This creates the thought energy. I take your point, I get what you are saying, but I think we have different definitions of thought energy, or akasha.

Say that certain people believe in a spirit world, which is the same as this one, existing in the same place, an exact replica of everything physical, only invisible, would that fit dark matter better than 'thought energy' or 'conciousness'?

 

[Kleonaptra]

Nope. Counterexample: the neutrino is intangible (10^6 of them pass through every centimeter of your body every second, but you've only got a 50-50 shot of having one interact with you during your lifetime) but it's a real thing with mass.

"Tangible" has to do with the forces that allow matter to interact with your senses. It has nothing to do with whether or not something has mass, or can be called matter, or exists.

My dictionary is from 1930....So we have a new definition of 'tangible'? I thought it meant literally 'what you can feel and touch' not just 'what you could interact with without knowing you are interacting with it'

Wow, I really dont want to debate the english language, there are other forums for that, but Im lost.

 

[Roboramma]

How do you know dark matter doesnt think?

Thought is a type of information processing. The only way that we know of to do information processing is the complex physical interactions: ie. the set-up in the brain, the mechanisms in your computer, etc. Dark matter, because it doesn't interact electro-magnetically, is unlikely to be capable of forming the type of complex parts necessary to form an information processing system, thus thought is ruled out.

Of course, it's concievable that there are some other forces through which dark matter interacts that we're not aware of, but we so far have no reason to posit such. Further, even if there were, it's likely that like normal matter the vast majority of dark matter would not be in the form of thinking systems.

Say that certain people believe in a spirit world, which is the same as this one, existing in the same place, an exact replica of everything physical, only invisible, would that fit dark matter better than 'thought energy' or 'conciousness'?

No, because dark matter is simply made up of particles that have different properties than regular matter. It doesn't form "an exact replica of everything physical", and its distributions and interactions are very very different from that of regular matter, because the forces acting on it are different (ie. basically just gravity), and because it's mass, velocity, etc. are also different.

 

[Roboramma]

My dictionary is from 1930....So we have a new definition of 'tangible'? I thought it meant literally 'what you can feel and touch' not just 'what you could interact with without knowing you are interacting with it'

Wow, I really dont want to debate the english language, there are other forums for that, but Im lost.

He's not debating the english language: you said: "In reality, to have mass, it must be tangible. Please explain if this isnt so, its just my understanding of it."

But a neutrino has mass and yet doesn't fit your definition of tangible:
'Tangible - perceptable by touch; definite, clearly intelligible,not illusive or visionary. Corporeal.'

So it's not true that "to have mass it must be tangible".

 

[Cheetah]

Kleonaptra,
I can see these posters are just confusing you because they are throwing you in at the deep end and telling you how things are, talking about weird sounding particles and things, without explaining why. Once you understand why, it will make a lot more sense. So let me give you a quick and dirty rundown on why dark matter is 'dark' and difficult to see.

There are 4 fundamental forces of nature: gravity, weak nuclear, strong nuclear and electromagnetic.
Matter particles interact with each other through these forces and each have a sort of 'strength of interaction' property for each of these (and a force particle mediating it).
For gravity we call it 'mass' and for the electromagnetic force 'charge'.
Matter particles differ in their 'strength of interaction' for each of these forces, and if the gravity property is zero, the particle has no mass and if it's strong, its heavy (and the same principal holds for the other forces).
Photons (light) are force particles of zero mass (but are easy to see ) and the mediators of the electromagnetic force (EF from now).

Almost everything that happens around us on a day to day basis happens due to the EF and gravity, but we observe things exclusively using only one of these, the EF, literally EVERYTHING (Hope I'm correct, or else I'm gonna get nailed ).
Touch something, look at something, taste, hear etc. they are all mediated via the EF. Even weighing something and measuring gravity our observations are mediated via the EF, because it is what makes things able to touch each other.

Since our observations are mediated via the EF, if a particle has a EF 'strength of interaction' of zero, it is extremely hard to notice and we have to use indirect methods and, once again, do it via the EF.

Since 'normal matter' is made of atoms which are mostly empty space with a tiny little nucleus in there, and held together via the EF, a particle with a charge of zero will just fly right through it and we will never know, unless it happens to hit that tiny little nucleus and causes an effect we can observe, via the EF.

So we are effectively 'blind' to all but the good old EF. That means there can be (and are) plenty of particles flying around everywhere, even really heavy ones, and we are all but oblivious to them.

I hope that makes it somewhat clearer for you.

Cheers
Cheetah

(This is just my layman's understanding, off the top of my head, so if I have made any errors, please help out/correct/add/clarify)

 

[Ziggurat]

Say that certain people believe in a spirit world, which is the same as this one, existing in the same place, an exact replica of everything physical, only invisible, would that fit dark matter better than 'thought energy' or 'conciousness'?

That would not fit at all. The whole point about dark matter is that its distribution does NOT match the distribution of ordinary matter.

Oh, and "though energy" is meaningless.

 

[Dancing David]

How do you know dark matter doesnt think?

The soul energy or counciousness doesnt think either - humans think, imagine, create. This creates the thought energy. I take your point, I get what you are saying, but I think we have different definitions of thought energy, or akasha.

Say that certain people believe in a spirit world, which is the same as this one, existing in the same place, an exact replica of everything physical, only invisible, would that fit dark matter better than 'thought energy' or 'conciousness'?

No, thoughts are NOT energy they are biochemical exchanges.



Repeat THOUGHTS ARE NOT ENERGY they are complex cascades of biochemical events.



Neurons are comparable to biochemical switches, not to electrical components.

Now all matter and energy are transferable.