What Is Dark Matter?

[DaTario]

Hi All,

I would like to know what is dark matter.

Thanks

DaTario

 

[Naty1]

It's a name for the cosmoligical constant from general relativity.

Try this discussion: https://www.physicsforums.com/showthread.php?p=2642299#post2642299

and check if you like dark matter in a search (as at the top of this page) for other discussions.

 

[Vanadium 50]

That's not correct. That's dark energy - maybe.

Dark matter is matter that we can see only via it's gravitational effects. It does not interact strongly with ordinary matter.

 

[bapowell]

It's a name for the cosmoligical constant from general relativity.

The cosmological constant has nothing necessarily to do with dark matter. Furthermore, dark energy may or may not be a cosmological constant.

I think questions as broad as "What is dark matter" are best researched on one's own outside the forum. When you have a basic understanding, come back and ask more specific questions. Then we'd be happy to help.

 

[Chalnoth]

When you just want to get started on a topic, and don't yet know much, Wikipedia is almost always a good place to do that.

 

[The legend]

Well Chalnoth is right.
Wikipedia is really a great database. Actually the thing is even I didnt know much about dark matter myself. Wiki helped!

Here's the definition from wikipedia:

In astronomy and cosmology, dark matter is a conjectured form of matter that is undetectable by its emitted electromagnetic radiation, but whose presence can be inferred from gravitational effects on visible matter and background radiation.

For more info go to
http://en.wikipedia.org/wiki/Dark_matter

 

[DaTario]

Well Chalnoth is right.
Wikipedia is really a great database. Actually the thing is even I didnt know much about dark matter myself. Wiki helped!

Here's the definition from wikipedia:


For more info go to
http://en.wikipedia.org/wiki/Dark_matter

Ok Thanks for redirecting me to Wikipedia. But now I would like to ask (only to those who is interested in giving answers, of course) if this emission criteria for defining dark matter may be interpreted as celestial bodies far enough and emitting so small an amount of radiation (planets, for instances) that we cannot detect it due to scattering or absortion in the midway.
It seems that there is another possibility, namely: a different patern (species) of matter.


Thank you all

Best Wishes

DaTario

 

[DaTario]

The cosmological constant has nothing necessarily to do with dark matter. Furthermore, dark energy may or may not be a cosmological constant.

I think questions as broad as "What is dark matter" are best researched on one's own outside the forum. When you have a basic understanding, come back and ask more specific questions. Then we'd be happy to help.

So I am inclined to think that questions as broad as
what is time?
what is space?
what is energy?
what is momentum?

may also be first looked up in wikipedia, am I correct?

Best wishes, Mr. Censor-148

DaTario

 

[bapowell]

So I am inclined to think that questions as broad as
what is time?
what is space?
what is energy?
what is momentum?

may also be first looked up in wikipedia, am I correct?

Yes.

 

[Chalnoth]

Ok Thanks for redirecting me to Wikipedia. But now I would like to ask (only to those who is interested in giving answers, of course) if this emission criteria for defining dark matter may be interpreted as celestial bodies far enough and emitting so small an amount of radiation (planets, for instances) that we cannot detect it due to scattering or absortion in the midway.
It seems that there is another possibility, namely: a different patern (species) of matter.

This was an early proposed solution, but detailed investigation showed that it didn't work. In particular, dark matter has a very different distribution from the normal matter: normal matter can lose energy by radiating, and so collapses into things like stars and galaxies. By contrast, dark matter loses very little energy with time, and so doesn't tend to collapse nearly as much as normal matter.

There's also the issue that before the emission of the cosmic microwave background, the normal matter experienced pressure because it could interact with the photons, while the dark matter did not. This different behavior leads to exceedingly different signatures in the CMB, and because of this we are very sure that it's not just a matter of the dark matter being normal matter we can't see: it actually has to be stuff not made out of protons, neutrons, and electrons.

 

[DevilsAvocado]

I would like to know what is dark matter.

And the simple answer is – No one knows, it's a mystery!

(And that's why they tell you to go investigate it yourself! )

I guess you did read http://en.wikipedia.org/wiki/Dark_matter" , and knows that it started with the "missing mass" in the orbital velocities of galaxies in clusters, including the rotational speeds of galaxies.

The best proof for the existence of Dark Matter is the Bullet Cluster:

http://imgsrc.hubblesite.org/hu/db/images/hs-2006-39-a-xlarge_web.jpg"

This picture shows the formation after the collision of two large clusters of galaxies (the most energetic event known in the universe since the Big Bang).

http://hubblesite.org/newscenter/archive/releases/2006/39/image/a"

Hot gas detected by Chandra in X-rays is seen as two pink clumps in the image and contains most of the "normal," or baryonic, matter in the two clusters. The bullet-shaped clump on the right is the hot gas from one cluster, which passed through the hot gas from the other larger cluster during the collision. An optical image from Magellan and the Hubble Space Telescope shows the galaxies in orange and white. The blue areas in this image depict where astronomers find most of the mass in the clusters. The concentration of mass is determined by analyzing the effect of so-called gravitational lensing, where light from the distant objects is distorted by intervening matter. Most of the matter in the clusters (blue) is clearly separate from the normal matter (pink), giving direct evidence that nearly all of the matter in the clusters is dark.

The hot gas in each cluster was slowed by a drag force, similar to air resistance, during the collision. In contrast, the dark matter was not slowed by the impact because it does not interact directly with itself or the gas except through gravity. Therefore, during the collision the dark matter clumps from the two clusters moved ahead of the hot gas, producing the separation of the dark and normal matter seen in the image. If hot gas was the most massive component in the clusters, as proposed by alternative theories of gravity, such an effect would not be seen. Instead, this result shows that dark matter is required.

Comparing the optical image with the blue emission shows that the most of the galaxies in each cluster are located near the two dark matter clumps. This shows that the galaxies in each cluster did not slow down because of the collision, unlike the hot gas.

Here's a video from NOVA scienceNOW, explaining The Dark Matter Mystery:

https://www.youtube.com/watch?v=<object width="480" height="385"><param name="movie" value="http://www.youtube.com/v/nJN2X3NrQAE&hl=en_US&fs=1&rel=0&color1=0x006699&color2=0x54abd6"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/nJN2X3NrQAE&hl=en_US&fs=1&rel=0&color1=0x006699&color2=0x54abd6" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="480" height="385"></embed></object>

The Large Hadron Collider that started (world record) collisions at 7TeV yesterday, hopefully will https://www.physicsforums.com/showthread.php?t=390908".


(Chalnoth, if you read this – sorry for taking so long to https://www.physicsforums.com/showthread.php?p=2648574#post2648574".)

 

[Orion1]

The current leading candidate for dark matter is supersymmetry (SUSY) particles, called neutralinos.

In supersymmetry models, all Standard Model particles have partner particles with the same quantum numbers except for the quantum number spin, which differs by 1/2 from its partner particle. Since the superpartners of the Z boson (zino), the photon (photino) and the neutral higgs (higgsino) have the same quantum numbers, they can mix to form four eigenstates of the mass operator called "neutralinos".

The exact properties of each neutralino will depend on the details of the mixing (e.g. whether they are more higgsino-like or gaugino-like), but they tend to have masses at the weak scale (100 GeV - 1 TeV) and couple to other particles with strengths characteristic of the weak interaction. In this way they are phenomenologically similar to neutrinos, and so are not directly observable in particle detectors at accelerators.

As a heavy, stable particle, the lightest neutralino is an excellent candidate to comprise the universe's cold dark matter. In many models the lightest neutralino can be produced thermally in the hot early universe and leave approximately the right relic abundance to account for the observed dark matter. A lightest neutralino of roughly 10-10000 GeV is the leading weakly interacting massive particle (WIMP) dark matter candidate.

In particle physics, supersymmetry (often abbreviated SUSY) is a symmetry that relates elementary particles of one spin to other particles that differ by half a unit of spin and are known as superpartners. In a theory with unbroken supersymmetry, for every type of boson there exists a corresponding type of fermion with the same mass and internal quantum numbers, and vice-versa

If supersymmetry exists close to the TeV energy scale, it allows for a solution of the hierarchy problem of the Standard Model, i.e., the fact that the Higgs boson mass is subject to quantum corrections which — barring extremely fine-tuned cancellations among independent contributions — would make it so large as to undermine the internal consistency of the theory. In supersymmetric theories, on the other hand, the contributions to the quantum corrections coming from Standard Model particles are naturally canceled by the contributions of the corresponding superpartners. Other attractive features of TeV-scale supersymmetry are the fact that it allows for the high-energy unification of the weak interactions, the strong interactions and electromagnetism, and the fact that it provides a candidate for Dark Matter and a natural mechanism for electroweak symmetry breaking.

Other candidates are called supersymmetry weakly interacting massive particles or SUSY WIMPS.
[/Color]
Reference:
http://en.wikipedia.org/wiki/Supersymmetry"
http://en.wikipedia.org/wiki/Neutralino"
http://en.wikipedia.org/wiki/Weakly_interacting_massive_particle"

 

[DevilsAvocado]

The current leading candidate for dark matter is supersymmetry (SUSY) particles, called neutralinos.

Thanks Orion1 for a thorough explanation.
(Maybe DM is only 'mysterious' if you don't know what you are talking about... )

Correct me if I'm wrong, but as I understand you there is some 'incompatibility' between SUSY and the Higgs boson (mass)?

If supersymmetry exists close to the TeV energy scale, it allows for a solution of the hierarchy problem of the Standard Model, i.e., the fact that the Higgs boson mass is subject to quantum corrections which — barring extremely fine-tuned cancellations among independent contributions — would make it so large as to undermine the internal consistency of the theory. In supersymmetric theories, on the other hand, the contributions to the quantum corrections coming from Standard Model particles are naturally canceled by the contributions of the corresponding superpartners.

Does this mean that if the LHC finds evidence for SUSY, we will not find the Higgs boson, and vice versa?

If I understood you wrong (there is compatibility SUSY/Higgs) – Does the Higgs boson interact with DM to give it mass? And if so – Will we then have an indirect 'link' to DM through Higgs? And if so – Why does DM interact with the Higgs boson, and no other boson? If not so – what gives DM mass?

(Interesting times... LHC -> Higgs -> SUSY -> DM -> GUT -> Extra Dimensions -> Strings=GR=QM=TOE)


------------------------------------------------------------------
Footnote
There are a lot of abbreviations out there, and I'm happy no one came up with one for "Particles with Unbroken SuperSymmetrY"... My God, life can be tough enough for a little WIMP between Black Holes and MACHOs!

 

[Chalnoth]

Correct me if I'm wrong, but as I understand you there is some 'incompatibility' between SUSY and the Higgs boson (mass)?

Oh, no, not at all. SUSY models most definitely include Higgs bosons.

 

[DevilsAvocado]

Oh, no, not at all. SUSY models most definitely include Higgs bosons.

Hi Chalnoth! What's up (with "planck s")!

So what do you say about this:
Does the Higgs boson interact with DM to give it mass?
And if so – Will we then have an indirect 'link' to DM through Higgs?
And if so – Why does DM interact with the Higgs boson, and no other boson?
If not so – What gives DM mass?

(Edit: It's real late here, I'll be back tomorrow, GN)

 

[Chalnoth]

So what do you say about this:
Does the Higgs boson interact with DM to give it mass?
And if so – Will we then have an indirect 'link' to DM through Higgs?
And if so – Why does DM interact with the Higgs boson, and no other boson?
If not so – What gives DM mass?

Well, since we don't know what the dark matter particle is, we obviously can't say for sure. However, that said, in quantum field theory, even SUSY, there remains a fundamental problem: if you insert a non-zero fundamental mass for any particle in the theory, it leads to a mathematical contradiction. This means that all masses must arise from interactions. For single particles, that interaction is modeled by an interaction with one or more Higgs fields. For composite particles (like protons and neutrons), it's a complex combination of interactions between the Higgs fields and the binding energy of the component particles.

Finally, if you think it strange that DM would interact with the Higgs and not other bosons, consider this: of the confirmed bosonic interactions, quarks interact with photons, gluons, and W/Z bosons. Electrons interact with only photons and W/Z bosons. Neutrinos only interact with the W/Z bosons.

 

[DevilsAvocado]

Thanks Chalnoth, interesting answers as always.

This means that all masses must arise from interactions. For single particles, that interaction is modeled by an interaction with one or more Higgs fields.

I know that the Higgs boson is expected at LHC, but also that prominent scientist like http://vimeo.com/4062801" – "Well, I think it'll be a lot more exciting if we don't find it."

I have absolutely no clue about the complicated math behind all this. But I do know there are some 'difficulties' in getting all 'pieces in place', like the measured cosmological constant, that is smaller than the calculated quantum field vacuum energy by a factor of 10^-120. And we don't know what DM really is. And 90% of the mass in nucleons comes from quantum fluctuations (virtual particles), etc.

To me this looks like there must be some 'BIG Answers' to dig out from the quantum field vacuum... it looks like all is 'connected' through this 'spooky stuff'... DE, DM, mass, etc...? Or is this wacky? Could DM get most of its mass from quantum fluctuations, like nucleons??

Finally, if you think it strange that DM would interact with the Higgs and not other bosons, consider this:

Okay, but isn't it 'weird' that DM does not interact directly with itself (except through gravity)? Electrons and photons (must?) do. Well, maybe neutrinos don't...

 

[Chalnoth]

Thanks Chalnoth, interesting answers as always.


I know that the Higgs boson is expected at LHC, but also that prominent scientist like http://vimeo.com/4062801" – "Well, I think it'll be a lot more exciting if we don't find it."

Well, yes, in part because it would point us in an entirely new, unexpected direction of high-energy physics. Finding new information is always interesting, but finding things that nobody expected are often more interesting.

Though I should mention that there do exist some specific models that have no Higgs, I'm not familiar with them, and as far as I know they tend to be rather less well-motivated than SUSY.

I have absolutely no clue about the complicated math behind all this. But I do know there are some 'difficulties' in getting all 'pieces in place', like the measured cosmological constant, that is smaller than the calculated quantum field vacuum energy by a factor of 10^-120. And we don't know what DM really is. And 90% of the mass in nucleons comes from quantum fluctuations (virtual particles), etc.

Sadly I worry that the energy available at the LHC may simply be dramatically insufficient to say much about dark energy, or even about much of high-energy physics. As for dark matter, the LHC just won't be good at either producing or detecting such particles, so it's somewhat unlikely that we'll see them.

The mass of nucleons, on the other hand, is, I believe, a rather well-understood consequence of quantum chromodynamics.

 

[bapowell]

Okay, but isn't it 'weird' that DM does not interact directly with itself (except through gravity)? Electrons and photons (must?) do. Well, maybe neutrinos don't...

If dark matter is a WIMP -- a weakly interacting particle, then it interacts through the weak force as well as gravitationally. Thus, it would couple to W/Z as well as with itself through neutral currents.

 

[Chalnoth]

If dark matter is a WIMP -- a weakly interacting particle, then it interacts through the weak force as well as gravitationally. Thus, it would couple to W/Z as well as with itself through neutral currents.

So far as I know, the "weak" in WIMP doesn't specifically relate to the weak nuclear force. Rather it's just a statement that its interactions with itself and other matter, whatever they may be, are rather weak. We don't yet know what those interactions are: they could be the weak nuclear force, they could be something else. But they aren't electromagnetism or the strong nuclear force (as with those interactions they'd simply interact too strongly with normal matter).

 

[DevilsAvocado]

Thanks bapowell, but if I’m not totally lost, one of these must be wrong:

Thus, it would couple to W/Z as well as with itself through neutral currents.

http://hubblesite.org/newscenter/archive/releases/2006/39/image/a"

The hot gas in each cluster was slowed by a drag force, similar to air resistance, during the collision. In contrast, the dark matter was not slowed by the impact because it does not interact directly with itself or the gas except through gravity.

 

[Chalnoth]

That NASA quote could be accurately modified to state that it doesn't interact much with itself.

 

[DevilsAvocado]

The mass of nucleons, on the other hand, is, I believe, a rather well-understood consequence of quantum chromodynamics.

Okay, so if LHC find (confirm) the Higgs boson, we can say nucleons get 90% of its mass from Quantum Chromodynamics, and 10% from Higgs.

Would it then be 'safe' to predict that the 'voluminous' Dark Matter gets 100% of its mass from Higgs?

 

[Chalnoth]

Okay, so if LHC find (confirm) the Higgs boson, we can say nucleons get 90% of its mass from Quantum Chromodynamics, and 10% from Higgs.

Would it then be 'safe' to predict that the 'voluminous' Dark Matter gets 100% of its mass from Higgs?

Probably, as electrons, neutrinos, and the W/Z bosons are expected to.

 

[DevilsAvocado]

... it doesn't interact much with itself.

Okay, thanks Chalnoth.

 

[DevilsAvocado]

Probably, as electrons, neutrinos, and the W/Z bosons are expected to.

Interesting! So the reason nucleons get 'fatter' with the help of Quantum Chromodynamics – is because they are much heavier, right?

But, correct me if I’m wrong – Isn't DM one of the heaviest stuff we know in universe??

 

[bapowell]

So far as I know, the "weak" in WIMP doesn't specifically relate to the weak nuclear force. Rather it's just a statement that its interactions with itself and other matter, whatever they may be, are rather weak. We don't yet know what those interactions are: they could be the weak nuclear force, they could be something else. But they aren't electromagnetism or the strong nuclear force (as with those interactions they'd simply interact too strongly with normal matter).

Thanks for clarifying, but my point is simply that several WIMP candidates do interact through the weak nuclear force. Up to this point in the thread, this has not been addressed.

 

[Chalnoth]

Interesting! So the reason nucleons get 'fatter' with the help of Quantum Chromodynamics – is because they are much heavier, right?

Well, it all has to do with how the binding energies work out in QCD. Consider an electron and proton that are far away. The total mass of the electron and proton will just be the sum of the masses of the two. But, if you bring the two together to form a hydrogen atom, this changes: bringing the electron and proton together lowers their electrical potential energy (making the electrical potential energy negative). This means that the Hydrogen atom is actually very slightly less massive than the sum of the masses of an electron and a proton individually. Of course, the magnitude of this potential is actually very small compared to their masses, so it's a pretty negligible effect.

But for nucleons, the situation is entirely different. The strong nuclear force, instead of dying away with distance, actually gets stronger. This means that if you hold a quark and anti-quark some distance apart, the strong force between them produces a very large positive potential energy, rather like if they were held together by a spring under tension. In fact, if you pull the quark and anti-quark too far away from one another, the tension between them gets so great, that a quark/anti-quark pair pops out of the vacuum between them and instead of a pair of quarks, you now have a pair of mesons.

Anyway, so if you have three quarks, two ups and a down, sort of the minimum-distance configuration they can have is actually so that these gluon "springs" between them actually have quite a lot of tension. So much, in fact, that protons aren't just made out of three quarks, but three quarks plus a whole bunch of quark/anti-quark pairs continually popping in and out of the vacuum. Of course, energy is conserved when these quark/anti-quark pairs pop in and out, so you can think of the extra energy as coming from the tension of these gluon "springs" between the quarks.

But, correct me if I’m wrong – Isn't DM one of the heaviest stuff we know in universe??

The total aggregate mass of dark matter is many times that of the normal matter. This doesn't mean that the particles themselves need to be particularly massive. Though many dark matter candidates are very massive, axions, for instance, are not.

 

[DaTario]

Is it possible that DM be associated with the mass of quantum vacuum ?

 

[bapowell]

Is it possible that DM be associated with the mass of quantum vacuum ?

The quantum vacuum doesn't have a mass, it has an energy. The energy associated with the vacuum is very strange indeed. It has a constant energy density and a negative pressure. Vacuum energy doesn't behave like matter. For one thing, it doesn't clump like dark matter appears to do. Also, it causes the expansion of the universe to accelerate. One possibility for the accelerated expansion that we observe today is called 'dark energy', and might be due to some form of vacuum energy.