Dark Matter: What Is It & How Was It Formed?

[Baboon]

What is dark matter? How was dark matter formed? Any replies would be greatly appreciated.

 

[fatra2]

Simply put, dark matter is matter that is dark. hahaha!

A bit more details would probably be helpful. Let's start from the beginning. By looking at galaxies around us, we can see them spinning around a center. The speed of rotation depends on the amount of matter in the galaxy. The more matter the faster the spinning. Nothing too complicated up to now. Problem comes here. Calculating the rotational speed of galaxies from the only mass we see does not explain the speed observed in the telescops. Two solutions are possible: 1. our laws of phyiscs are wrong, 2. some matter is hidden somewhere. Of course, we could never imagine that we made a mistake drawing the laws of physics, therefore we had to look into matter that is not seen (therefore dark matter).

Does that explain your question?

 

[Baboon]

What about ...Dark matter is matter that cannot be detected by its emitted radiation but whose presence can be inferred from gravitational effects on visible matter such as stars and galaxies. Estimates of the amount of matter in the universe based on gravitational effects consistently suggest that there is far more matter than is directly observable.

 

[fatra2]

...Dark matter is matter that cannot be detected by its emitted radiation

That's the beauty of this subject. By definition of "dark", dark matter does not emit any radiation.

Now the gravitational pull is the reason why we started wondering about dark matter. Since then, we came a long way. We know that there are many candidates that could explain this missing matter. First, let's not forget the massive objects (starts) at the end of the life, like white dwarfs, neutron stars, black holes don't emit radiation (or very few). They partly explain this rotational overspeed. Therefore, scientists attention turned to find what is the rest. You have the choice, between neutrinos, WIMP (weakly interactive massive particles) and many more. Since this is not my field, I can only give few details.

Your next comment might be on the neutrinos, thinking "how can such small particles explains missing mass of the universe?" My answer would be quite simple. Of course one does make that much of a difference in our Universe. But remember that more than 10^{11} particle pass through every kg of your body every second of your life.

To make the little story complete. These neutrinos have no electric charge (no electric field, and don't interact with matter, except for direct collision), very little mass (from what I remember less than 1/1000 the mass of the electron). They seem to be very good candidate for this dark matter.

Cheers

 

[George Jones]

These neutrinos have no electric charge (no electric field, and don't interact with matter, except for direct collision), very little mass (from what I remember less than 1/1000 the mass of the electron).

The most massive neutrinos (there are three flavours) have mass less than 1/1000000 the mass of the electron (page 396 of the second edition of Introduction to Elementary Particles by David Griffiths).

They seem to be very good candidate for this dark matter.

Neutrinos likely account for only a small fraction of dark matter mass. Also, neutrino dark matter cannot account for structure formation in the early universe that leads to the galaxies and clusters of galaxies that we observe. Neutrinos move too fast to allow this to happen.

 

[fatra2]

Thank you for the clarifications on neutrinos. I gave the numbers from the top of my head.

You might be right to say that neutrino account for only a fraction of the dark matter. From my understanding, we are just at the beginning of this field, and discoveries will most likely enlighten us in the near future.

Facts are that dark matter seems to be out there. We just need to find the right place to look for it.

 

[Baboon]

Im the beginner in the physics and it is simply interesting to me
Forgive for a silly or simple question

tnx



http://www.u-n-i-v-e-r-s-e.com/the_Universe.html"

 

[Heisenberg.]

I have a question that regards to some of the comments posted above - According to string theory dark matter might possibly be a higher vibration of the superstring -Since string theory claims that us three dimensional beings can only see the lowest vibration of the superstring (e.g atoms, light) then dark matter might be the next set in vibrations - how popular is this string theory interpretation of dark matter, for I noticed it was not mentioned above? Is the reason neutrinos are so seemingly elusive to us the fact they have a higher vibration or are neutrions separate from string theory altogether?

 

[Chronos]

Dark matter is like neutrinos, we know its there but is mighty hard to directly detect. Most scientists doubt it is a Baskin-Robbins collage of neutrinos, rather suspecting it is a fundamentally different family of particles [I suspect there is more than one flavor, as is the case with neutrinos]. I usually generally avoid string discussions. The music is lovely, but, there are no lyrics.

 

[Hippasos]

For all those who wander in their imagination:

http://www.pnas.org/content/early/2009/08/14/0901627106

 

[Chronos]

Expanding on [repeating?] what George said, neutrinos travel nearly at the speed of light. This is not conducive to large scale structure formation. Dark matter appears to travel around the same speed as ordinary matter.

 

[ideasrule]

For all those who wander in their imagination:

http://www.pnas.org/content/early/2009/08/14/0901627106

I haven't read the paper, but it seems to address why the galaxies are accelerating away from us, and not why galaxies have the observed rotation curve.

these corrections to the Standard Model might be the source of the anomalous acceleration of the galaxies, an explanation not requiring the cosmological constant or dark energy

 

[George Jones]

I haven't read the paper, but it seems to address why the galaxies are accelerating away from us, and not why galaxies have the observed rotation curve.

Right, it discusses (the possible non-existence of) dark energy, not dark matter. See also the thread

https://www.physicsforums.com/showthread.php?p=2319754#post2319754.

 

[DaveC426913]

Dark matter is matter that cannot be detected by its emitted radiation

It might be more more intuitive to see it as a form of matter (it could be much like protons and electrons for all we know) that simply does not interact with photons - neither absorbing them nor emitting them.

If it does not intereact with EM radiation, then it is invisible to all our sensory apparati yet still interacts normally with gravity.

 

[kldickson]

What kind of technology would we need to positively detect dark matter and go beyond inferring its existence?

 

[DaveC426913]

What kind of technology would we need to positively detect dark matter and go beyond inferring its existence?

Why, a Dark Matter Detector of course. (Go head. Ask what a DMD is and how it works.)



Seriously. You do realize that, since we don't know what it is or why we can't see it, there is no way of knowing what it would take...

 

[Chronos]

Particle physics is the current search method. Even dark matter particles have a probability of interacting with normal matter, or other dark matter particles, if you observe a sufficient number of collisions.

 

[George Jones]

What kind of technology would we need to positively detect dark matter and go beyond inferring its existence?

I think that if the LHC finds evidence of supersymmetry, the case for non-baryonic dark matter will be greatly strengthened.

 

[kldickson]

Yes, dark matter particles don't emit electromagnetic radiation, but surely there are other ways of positively identifying them. What is known from their interactions with particles that do emit radiation?

 

[fatra2]

Yes, dark matter particles don't emit electromagnetic radiation, but surely there are other ways of positively identifying them.

The only way of identifying dark matter is through indirect effect. Like when a neutrino makes a direct hit with a nucleus, we can only measure the recoil of the nucleus and deduce that it was made by a neutrino.

If there would be a direct way of detecting dark matter, it would become "visible" in some way, and could not be called "dark" matter anymore.

Cheers

 

[azzkika]

Am i correct in thinking the term 'dark' infers no emission of EMF whatsoever?

If that is correct, does that imply that dark matter does not interact with 'fields' the same as normal matter if at all?

Referring to the neutrino discussion, does the faster a particle travels have an increase to it's gravitational effect when passing another particle? I'm very amateur to physics, but if this was so, then maybe neutrino's could constitute more of dark matter if they are all traveling in the right direction.

 

[DaveC426913]

If there would be a direct way of detecting dark matter, it would become "visible" in some way, and could not be called "dark" matter anymore.

Its behaviour is dictated by what we've given it as a nickname?

Seems kind of the tail wagging the dog wouldn't you say?

 

[Chronos]

'Dark' was coined in reference to its resistance to detection by means of kinetic reactions and EM emissions. DM is very much like neutrinos. It took us many years to confirm the existence of neutrinos, it will take us many more to detect DM particles. Neutrinos travel at nearly light speed, making them relatively easy to detect. DM does not, making it much harder to detect.

 

[_PJ_]

I must admit, I've never liked the ideas of Dark Matter (or Dark Energy for that matter - pun unintentional) However, neutrinos perhaps account for some of it. It has occurred to me that gravtiational effects of some bodies may be 'concealed' by other matter, for examplethe consensus that there are black holes at the centre of many galaxies (perhaps all), including the Milky Way. The Milky Way centre has what is called the Great Attractor near its centre, and around this, many stars and their associated families no doubt are pullled into tight orbits. All this mass of which we cannot detect individually, only infer by the motion and what radiation is emitted, may therefore be 'hiding' greater mass behind it?

 

[Chronos]

The 'bullet cluster' study is the smoking gun in the case for dark matter. See:
A direct empirical proof of the existence of dark matter
Douglas Clowe (1), Marusa Bradac (2), Anthony H. Gonzalez (3), Maxim Markevitch (4), Scott W. Randall (4), Christine Jones (4), Dennis Zaritsky (1) ((1) Steward Observatory, Tucson, (2) KIPAC, Stanford, (3) Department of Astronomy, Gainesville, (4) CfA, Cambridge)
http://arxiv.org/abs/astro-ph/0608407

We present new weak lensing observations of 1E0657-558 (z=0.296), a unique cluster merger, that enable a direct detection of dark matter, independent of assumptions regarding the nature of the gravitational force law. Due to the collision of two clusters, the dissipationless stellar component and the fluid-like X-ray emitting plasma are spatially segregated. By using both wide-field ground based images and HST/ACS images of the cluster cores, we create gravitational lensing maps which show that the gravitational potential does not trace the plasma distribution, the dominant baryonic mass component, but rather approximately traces the distribution of galaxies. An 8-sigma significance spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law, and thus proves that the majority of the matter in the system is unseen.

 

[DaveC426913]

The 'bullet cluster' study is the smoking gun in the case for dark matter...


A direct empirical proof of the existence of dark matter

...a unique cluster merger, that enable a direct detection of dark matter

Nitpick: this is not direct; it is still indirect detection. (We are still seeing only the smoke, not the bullet.)

 

[Vanadium 50]

I like Dark Matter. I am LCDM's biggest fan. But I still have to say that the Bullet Cluster as proof of Dark Matter has been oversold. It tells us that there is more mass in the Bullet Cluster than one gets from stars and x-ray gas, and it tells us that it is non-collisional. This is what's expected from CDM, so it's very tempting to say, "Aha! This is proof!"

Unfortunately, this is also what one expects from black holes, brown dwarfs, and even hot dark matter (e.g. massive neutrinos). And while none of these (apart from maybe black holes) can be the matter that closes the universe, there's nothing wrong with saying this particular cluster has most of its mass in black holes, brown dwarfs, or massive neutrinos.

So this is certainly evidence in favor of CDM. But proof? I don't think so.

 

[ranrod]

I have an admittedly uninformed question about this. So we observe that stars at the edge of galaxies are moving "too fast", that at those speeds they should escape the galaxy's pull on them unless there was some other unknown force holding them in. The same is observed in galaxy clusters spinning around each other. It resembles the effect of an object with gravity, but instead of concluding there must be a "dark matter" object creating extra gravity, why don't we conclude some of our equations for determining these things have errors or are incomplete? Maybe there's a point at which general relativity breaks down. Maybe at a galactic level, the effect of gravity on the space-time fabric is different than we expect. Maybe for an unknown reason a much larger concentration of neutrinos than expected accumulates in the halos of galaxies. Maybe an error or missing factor small enough to be unperceivable on relatively small things such as our solar system, but tremendously visible at the galactic level.

My question is this: Why not just say, "there's an unknown force or error in our understanding of the universe?" Why make it a definite object? Making it an object seems like a narrow-minded way of approaching an unknown problem and assumes too much.

 

[Chronos]

Good point, ranrod, and welcome to PF! How would you characterize dark matter as other than matter that does not interact with other particles? Scientists have pondered this question for over half a century and DM is their best guess at this point.

 

[Chronos]

Dave, I guess this is a question of what constitutes 'direct proof'. I sypathize with your objection, but the evidence remains in favor of the conventional definition of dark matter. See also:
http://arxiv.org/abs/astro-ph/0601489[/URL]
Title: Dark matter: A phenomenological existence proof
D. V. Ahluwalia-Khalilova
The non-Keplerian galactic rotational curves and the gravitational lensing data strongly indicate a significant dark matter component in the universe. Moreover, these data can be combined to deduce the equation of state of dark matter. Yet, the existence of dark matter has been challenged following the tradition of critical scientific spirit. In the process, the theory of general relativity itself has been questioned and various modified theories of gravitation have been proposed. Within the framework of the Einsteinian general relativity, here I make the observation that if the universe is described by a spatially flat Friedmann-Robertson-Walker cosmology with Einsteinian cosmological constant then the resulting cosmology predicts a significant dark matter component in the universe. The phenomenologically motivated existence proof refrains from invoking the data on galactic rotational curves and gravitational lensing, but uses as input the age of the universe as deciphered from studies on globular clusters.

 

[Vanadium 50]

I have an admittedly uninformed question about this...
My question is this: Why not just say, "there's an unknown force or error in our understanding of the universe?" Why make it a definite object? Making it an object seems like a narrow-minded way of approaching an unknown problem and assumes too much.

You're right. Your question is uninformed, and calling scientists "narrow-minded" based on your own ignorance says rather more about you than them.

As it happens, there is a theory, called MOND, which attempts to explain galaxy rotation curves by changing the gravitational force law (which is equivalent to adding another force). This theory has its proponents, but is not very popular. One problem is this theory is that different galaxies seem to have different proportions of dark matter. This is natural if dark matter is a substance, but it is decidedly unnatural - and hard to explain - if one tries to explain dark matter as MOND. MOND would suggest a universal curve, and that's not really what's seen.

 

[ranrod]

You're right. Your question is uninformed, and calling scientists "narrow-minded" based on your own ignorance says rather more about you than them.

I'm afraid I did sound rather disrespectful phrasing it that way, sorry about that. I was saying it's odd that the standard response is not, "we don't know", but instead, "it's this object we just made up called dark matter that we can't find". I got to learn how to rite more better & articulate me thoughts more proprly.

Even if it was a tremendously large concentration of WIMPS, for instance, wouldn't it be just as puzzling to figure out why they're hanging out around the halos of a galaxies?

 

[Galteeth]

I think dark matter/dark energy is a frustrating thing for lay people. The way its presented in popular science might have something to do with it. "Here is this stuff that comprises most of the universe, but there's no way to see it or detect it, and it doesn't interact with anything except by way of gravity."

I remember getting excited when it occurred to me that if the universe was spinning from the moment of the big bang, maybe it was just centrifical force. But I realized that if that were the case, somebody a lot smarter then me would have thought of it already.

While people without backgrounds can certainly question things, there are generally good reasons when physicists achieve some consensus, especially since its such a competitive field. I don't "like" dark matter either, but I realize I don't have the necessary background to form a coherent criticism.

 

[ranrod]

While people without backgrounds can certainly question things, there are generally good reasons when physicists achieve some consensus, especially since its such a competitive field. I don't "like" dark matter either, but I realize I don't have the necessary background to form a coherent criticism.

Thanks Galteeth. I want to apologize once again for sounding condescending. That wasn't my intention, and I'm sorry it came off that way. I meant it when I said I need to work on my writing skills. The work physicists do on this subject is amazing and I didn't mean to be dismissive.

I was trying to express surprise at the prevalency of the physical object "dark matter" theory as an explanation for things, and seeing if anyone would explain more background on that. I think I came off as condescending, criticizing and seemed to be offering crack-pot theories. Although I must say, I got a warm smile when I read Galteeth's idea about it :) I've had similar feelings before about other things, and it reminded me of it.