What is the Nature of Dark Matter and How Are Scientists Trying to Discover It?

[Phy6explorer]

A high tech search is going on around the globe to spot what is known as dark matter about which the scientists admittedly are still in the dark.Certainly, whoever discovers the nature of the exotic dark matter would solve one of the greatest mysteries of modern science.Some say that they are tiny, exotic particles left over from the Big Bang some 13.7 billion years ago, but what actually is it?

 

[cristo]

Well, the short answer is that no-one really knows. John Baez's Physics FAQ might give you a little more info.

 

[Phy6explorer]

I just came to know a bit of information about this. It seems that the prevailing theory about the dark matter is that it is made up of tiny exotic particles left over from the Big Bang some 13.7 billion years ago. I also read that the dark matter might have got its name because it does not give off either heat or light. Astronomers know it exists because of its gravitational tug-of-war with stars and galaxies.

 

[EL]

I just came to know a bit of information about this. It seems that the prevailing theory about the dark matter is that it is made up of tiny exotic particles left over from the Big Bang some 13.7 billion years ago. I also read that the dark matter might have got its name because it does not give off either heat or light. Astronomers know it exists because of its gravitational tug-of-war with stars and galaxies.

That's a good summary!

 

[Chronos]

I concur. Dark matter is because observational evidence insists it must exist. The evidence is overwhelming.

 

[Fluxman]

I concur. Dark matter is because observational evidence insists it must exist. The evidence is overwhelming.

The book "Dark Side of the Universe", subtitled "Dark Matter, Dark Energy, and the Fate of the Cosmos" is a super discussion of this topic.

 

[turbo]

Dark matter probably does not exist. It is postulated to protect the model of gravitation put forth by Einstein in his theory of General Relativity. This model was an improvement on the Newtonian model of gravitation in some subtle ways, but Einstein was never happy with the implementation of GR and worked for a long time to improve it, without success. Does anybody here think that if Einstein had known about the existence of extra-galactic spiral galaxies, and their anomalous flat rotation curves that he would have invented DM to cover the failure of his model? I think not. I believe that he would have returned to the fundamentals of gravitation instead of inventing superfluous entities.

 

[matt.o]

You can't put words into Einstein's mouth and use it as an argument against mainstream astronomy, though can you? That doesn't really help.

 

[turbo]

You can't put words into Einstein's mouth and use it as an argument against mainstream astronomy, though can you? That doesn't really help.

Mainstream astronomy is an entirely observational exercise because we cannot interact with the bodies being observed. I am not putting words into Einstein's mouth, since he wrote very extensively in his 1920 book on relativity, his 1920 Leiden essay on the the ether, and his 1924 essay "On the Ether" putting forth ideas that are not welcome today. If you are not familiar with these writings, I can help you out. I think that he was on the right path.

 

[Blueprint]

I thought dark matter was obscured matter from our view.. take a nebula like the horsehead for example... all but the brightest stars behind it are obscured from our viewpoint.

This is not dark matter. Dark matter seems to be the mass of invisible sub-atomic particles (virtual particles?) like neutrinos that we can't even detect yet. They are out there, and in number, and we can infer their existence from the rate of expansion of the universe. I think this played a hand in the cosmological constant?

 

[Wallace]

Mainstream astronomy is an entirely observational exercise because we cannot interact with the bodies being observed. I am not putting words into Einstein's mouth, since he wrote very extensively in his 1920 book on relativity, his 1920 Leiden essay on the the ether, and his 1924 essay "On the Ether" putting forth ideas that are not welcome today. If you are not familiar with these writings, I can help you out. I think that he was on the right path.

We've moved beyond science being the process of extracting meaning out of the words of a single ancient figure (such as Aristotelean philosophy as it proceeded for over two millennium). Einstein was a genius who gave us amazing insight, however there are many many people, some of whom were contemporaries of Einstein and many of whom came after, that have shaped out view of relativity. We will do best to work with the observations and ideas we have now rather than trying to imaging 'what would Einstein think?'.

 

[turbo]

We've moved beyond science being the process of extracting meaning out of the words of a single ancient figure (such as Aristotelean philosophy as it proceeded for over two millennium). Einstein was a genius who gave us amazing insight, however there are many many people, some of whom were contemporaries of Einstein and many of whom came after, that have shaped out view of relativity. We will do best to work with the observations and ideas we have now rather than trying to imaging 'what would Einstein think?'.

Would you like to explain how you can infer the existence of Dark Matter from any of Einstein's work? Please remember that Einstein's work ~100 years ago did not envision any kind of universal expansion, so you'll have to explain that, too.

 

[Wallace]

Isn't that my point? We can't infer dark matter, dark energy and plenty more 'from Einsteins work'. That's why we have to remember that hundreds of people have worked on relativity, not just Einstein. Fortunately using much of this work, we can say a good deal about those topics. I repeat what I said earlier, you seem to be focused on things only being true if they were said by Einstein and if he said something was false then it must be. This is not how science has worked since the 1600's.

As for universal expansion, I can deal with that. It is a pop-sci mythconception, not science so don't get into knots about that one.

 

[turbo]

Let me try to back up another step. When Einstein (and others) laid the groundwork for the relativistic model of gravitation, the nature of our visible universe was very poorly understood, and neighboring galaxies were still called "nebulae" and their natures were unknown. Newtonian gravitation explained the behavior of solar-system bodies pretty darned well, and the relativistic interpretation was marginally better.

Here we are ~100 years on, and we have lots of wonderful observations. Vera Rubin discovered from spectroscopic studies that the rotation curves of spiral galaxies are much flatter than can be explained by GR's gravitational model. The MOND folks introduced modifications to the Newtonian gravitational model that not only fit the flat rotation curves very well, but also predicted the behavior of low-surface brightness galaxies over a decade before they were actually observed. This does not prove that MOND is correct, just that is predictive and therefor somewhat useful.

On larger (cluster) scales we have observed that the gravitational binding forces are larger than can be expected from a GR model using visible matter. Zwickey knew this a long time back, so this is not a new problem. Lensing effects are also much stronger than we might expect by considering only the visible matter.

We also have the Pioneer anomalies, showing unmodeled Sun-ward accelerations in spin stabilized spacecraft . Is it possible that neither the Newtonian nor relativistic models for gravitation are accurate on some scales? I think that we should be open to the possibility that these gravitational models might have a limited scale of applicability. Invoking the existence of exotic undetectable matter that only interacts gravitationally is a kludge. Even if it were a viable fix, it is advisable to look at what the MOND folks have done and ask what we could do to modify current gravitational theory to more closely accord with observations.

I do not have a blind allegiance to Einstein or his work, despite what you might think. I am trying to point out that what he accomplished has to be seen in the context of scientific knowledge at that time, and inventing new entities (DM for one) to try to keep his relativistic model of gravitation predictive is short-sighted and likely counter-productive. Reading Einstein's tribute to Ernst Mach, I get the feeling that he would be very uncomfortable with this situation.

"How does it happen that a properly endowed natural scientist comes to concern himself with epistemology? Is there no more valuable work in his specialty? I hear many of my colleagues saying, and I sense it from many more, that they feel this way. I cannot share this sentiment. ... Concepts that have proven useful in ordering things easily achieve such an authority over us that we forget their earthly origins and accept them as unalterable givens. Thus they come to be stamped as 'necessities of thought,' 'a priori givens,' etc. The path of scientific advance is often made impassable for a long time through such errors. For that reason, it is by no means an idle game if we become practiced in analyzing the long common place concepts and exhibiting those circumstances upon which their justification and usefulness depend, how they have grown up, individually, out of the givens of experience. By this means, their all-too-great authority will be broken."

 

[cristo]

But.. doesn't MOND require dark matter anyway? See Phys. Rev. Lett., Vol. 100 (2008) 031302 (preprint http://arxiv.org/abs/0709.3189).

 

[turbo]

But.. doesn't MOND require dark matter anyway? See Phys. Rev. Lett., Vol. 100 (2008) 031302 (preprint http://arxiv.org/abs/0709.3189).

Milgrom formulated MOND to obviate the need for DM. Here is an interview from a few years ago in which he describes his motivations and the resistance he encountered.

http://www.americanscientist.org/template/AssetDetail/assetid/16363

 

[cristo]

Milgrom formulated MOND to obviate the need for DM. Here is an interview from a few years ago in which he describes his motivations and the resistance he encountered.

http://www.americanscientist.org/template/AssetDetail/assetid/16363

But that's an article from five years ago? The paper I linked to is from late last year, and was published in January this year. Here's a quote from the introduction:

Here we calculate within MOND the deflection angles for two generic density profiles and compare them with those predicted in standard lensing. We calculate the mass of the lenses and estimate the amount of dark matter required. We find that despite the alternative gravitational fall-off, the masses predicted by MOND are very similar to those predicted within standard gravitational lensing theory. We conclude that MOND within galactic scales needs a considerable amount of dark matter.

It doesn't make sense to counter a recent paper with an interview given five years ago!

 

[Kurdt]

I don't think worrying about whether Einstein would have liked dark matter or not is answering the question in the thread which is (with slight modification), "What are the candidates for dark matter?".

 

[turbo]

But that's an article from five years ago? The paper I linked to is from late last year, and was published in January this year. Here's a quote from the introduction:

It doesn't make sense to counter a recent paper with an interview given five years ago!

Newer does not equate to "more correct", especially in theoretical fields like cosmology. If you read through the Milgrom interview, you will see that theoreticians were trying to accommodate DM in MOND for years. The paper that you linked finds that the visible mass in their candidate galaxies is insufficient to explain the lensing observed. This would not be at all surprising if our gravitational theories need modification. The result goes hand-in hand with the stronger than expected cluster binding and stronger than expected cluster lensing.

I do not believe that MOND is correct or universally applicable, though on galactic scales it has been remarkably predictive. Its real value may be in quantifying the failure of GR gravity on galactic scales.

 

[turbo]

I don't think worrying about whether Einstein would have liked dark matter or not is answering the question in the thread which is (with slight modification), "What are the candidates for dark matter?".

True, but Cristo answered that as well as can be expected in his reply. Nobody knows what particles could be undetectable and weakly interactive, except gravitationally. We can talk about the fanciful names that have been made up - like WIMP for weakly interacting massive particles - but putting a name to an undetectable entity is not the same as defining its nature.

 

[nuby]

Maybe it can be looked at it this way. What is matter? It's the stuff which physical objects are composed of, right?

But then.. What is "darkness" officially in physics? You'll probably say it is the lack of light/photons. But can opposite of one thing really be the "lack of" that thing? In math that would be like saying the opposite of 5 is 0.

 

[turbo]

For the OP, scroll down on this page and you can view streaming video of lectures on dark matter, dark energy, dark matter candidates, etc. I highly recommend the lectures by Rock Kolb. He is involved in the search for DM candidate particles, but he is quite pragmatic about it and does not pretend that we know more than we do or that detection is "just around the corner" as we have heard so many times regarding the Higgs Boson.

http://www-conf.slac.stanford.edu/ssi/2003/program_post.htm

 

[denise]

I don't think worrying about whether Einstein would have liked dark matter or not is answering the question in the thread which is (with slight modification), "What are the candidates for dark matter?".

I read somewhere that dark matter is made up of anti-neutrons, positrons(positively-charged electrons) and anti-protons(which I believe are negatively charged protons). However, i do not fully understand the concept behind antimatter. If scientists have discovered antiparticles, and antimatter will annihilate in contact with matter, how can they conduct experiments with or even observe these dangerous particles?

 

[Astronuc]

I read somewhere that dark matter is made up of anti-neutrons, positrons(positively-charged electrons) and anti-protons(which I believe are negatively charged protons). However, i do not fully understand the concept behind antimatter. If scientists have discovered antiparticles, and antimatter will annihilate in contact with matter, how can they conduct experiments with or even observe these dangerous particles?

Anti-matter is not dangerous (to humans or life-forms) as long as it does not contact matter (atoms of protons, neutrons, electrons).

DM is not anti-matter, since antimatter interacts by EM and weak force (or electro-weak) as matter does. Basically, dark matter is a hypothetical form of matter that "does not emit/absorb or reflect (i.e. interact with) enough electromagnetic radiation to be observed directly." Ostensibly, this means it is not charged (?).

 

[Jimmy41]

A theeery darnit

Could dark matter (my cartoon of which is that we see massive things acting in a way that can't be explained by their mass) be the projection of inter-membrane gravitational tension (mass/Energy-structure) into our n-brane?

In other words if the effects of massive objects with respect to each other cannot be explained by their mass as we "see it", maybe the missing peices of those effects come from "things" that are in some strange way "massive" (gravitationally interacting) in other n-branes.

If so then the behavior of "mass_x" witnessed by the weirdly curious in other n-branes would also be unexplained by their local model, because they can't "see" in any physical sense, the structure of the piece we have here, that participates in the whole, a structure that exists in the container of both our n-branes.

My post on "String Theery, Planetary Rings, and sourceless gravitational waves" is related to this proposed explanation, and a half baked way to ask if it is possible.

What keeps haunting me is, does the structure of the missing piece (dark matter or - the quirkiness of mass) provide us with a veiled view of a neighboring brane, a void, that is actually... a silhouette?

Please debunk!

 

[Phy6explorer]

I am not sure about the 100% stuff, but I think you are right about gravitons because scientists say that dark matter have gravitational properties. But I have a question.Do gravitons have mass? Because scientists have also said that 25% percent of the total mass of the universe is made up by dark matter.

 

[Phy6explorer]

Maybe it can be looked at it this way. What is matter? It's the stuff which physical objects are composed of, right?

But then.. What is "darkness" officially in physics? You'll probably say it is the lack of light/photons. But can opposite of one thing really be the "lack of" that thing? In math that would be like saying the opposite of 5 is 0.

You are right about the matter part.But it is called dark, plainly because it is non-luminous.

I don't really understand what you mean by "But can opposite of one thing really be the "lack of" that thing?"

 

[cristo]

Dark Matter is 100% pure gravitons, that is the only characteristic it shows!.

I am not sure about the 100% stuff, but I think you are right about gravitons because scientists say that dark matter have gravitational properties. But I have a question.Do gravitons have mass? Because scientists have also said that 25% percent of the total mass of the universe is made up by dark matter.

It's extremely confusing to make comments like "dark matter is 100% pure gravitons," without putting this in context.

Firstly, I should remind everyone that idle speculation is not permitted on these forums. If in doubt, please check the guidelines in my signature. Secondly, I presume that obione is talking about some sort of braneworld scenario, whereby matter on an adjacent brane acts gravitationally with matter on our brane, thus looking like some sort of dark matter, however I would kindly request that obione both explains his thoughts in more detail and provides us links to published, peer-reviewed articles on the topic.

You are right about the mass of the graviton: it is a hypothetical particle that carries the gravitational force, and thus must be massless.

 

[Phy6explorer]

Thanks for clarifying my doubt about the mass of gravitons, Crist! But then dark matter do have gravitational properties and thus have gravitons and is confirmed to make up 25% of the total mass of the universe.How can that be if gravitons are mass-less?

 

[Jimmy41]

I don't know whether or not anything serious has been done on the "braneworld scenario whereby matter on an adjacent brane acts gravitationally with matter on our brane" (pretty succinct) though I'd be really suprised if not. I was hoping to find pointers to that stuff here.

I did propose something a bit more concrete (though not a lot) and related to this in my post "Please help: String Theery, Planetary Rings, and Sourceless Gravitational Waves", a thought experiment that I would really like to see if I can know more about (ie. could it be modeled on a computer? Has anything like it been done?)


My dad sent me this link, totally pop - not at all the real references I'm looking for, but I was going to head over there an poke around to see if there are other links to be found. If I find any I'll post.

Can't post url (15 post rule): but it's cosmiclog dot msn.com /archive/2008/05/09/1002136 dot aspx

 

[Fluxman]

It's extremely confusing to make comments like "dark matter is 100% pure gravitons," without putting this in context.

Firstly, I should remind everyone that idle speculation is not permitted on these forums. If in doubt, please check the guidelines in my signature. Secondly, I presume that obione is talking about some sort of braneworld scenario, whereby matter on an adjacent brane acts gravitationally with matter on our brane, thus looking like some sort of dark matter, however I would kindly request that obione both explains his thoughts in more detail and provides us links to published, peer-reviewed articles on the topic.

You are right about the mass of the graviton: it is a hypothetical particle that carries the gravitational force, and thus must be massless.

Is the amount of matter in the universe now equal or very close to equal the amount of matter at the beginning of the universe?

 

[Phy6explorer]

Matter is the substance of which physical objects are composed of. I find no meaning in your question!Matter cannot be physically measured, so the word"amount"is inappropriate.

 

[malawi_glenn]

Phy6explorer: Depends on how you define matter. On for example distinguish from the radiation domiated universe and matter dominated universe and so on.

 

[Phy6explorer]

You are quite right, but how else can we define matter?

 

[malawi_glenn]

You are quite right, but how else can we define matter?

leptons and baryons, is called baryonic matter.

 

[Phy6explorer]

But that is to be specific. But in general they are matter and matter is of what physical objects are composed of, isn't it? But I am saying this with reference to what Fluxman asked,if in general I am certainly wrong.

 

[malawi_glenn]

But that is to be specific. But in general they are matter and matter is of what physical objects are composed of, isn't it? But I am saying this with reference to what Fluxman asked,if in general I am certainly wrong.

In plain physics language, matter = baryonic matter.

Energy is constant in the universe, so it is a accepted question to ask if there was more matter bakc then with respect to today- since kinetic energy is not matter.

Photons are not regarded as matter in cosmology either, at least I have encountered thoses definitions in my studies, perhaps there are other conventions.

But most of the time, one knows what the term "matter" is referring to, at least cosmologists ;)

 

[Phy6explorer]

In plain physics language, matter = baryonic matter.

What about the exceptions of Non-baryonic matter such as neutrinos, that is, free electrons, black holes and dark matter?(For more info on dark matter go to "Dark matter-What is it" in the same forum.

 

[malawi_glenn]

that is why you introduce dark matter, you separate it from the ordinary matter, i.e the baryonic matter.

I know quite much about dark matter, and this IS the thread "Dark matter - what is it"..

I just wanted to stress that Fluxman's question is correct, since the total energy is the thing that is conserved, matter is not conserved. And if you want to call radiation matter or not is a 'matter' of taste. In cosmology, one usally distinguish from matter and radiation.

 

[Phy6explorer]

Oh,I got too involved and I forgot that this is the Dark matter thread.Now I have my doubt clarified.But why did people start using the term baryonic matter?

 

[Phy6explorer]

I meant no rudeness.I mentioned the place where we get info about DM b'cuz sometimes when I mention DM without any bacground info people get a bit disinterested.

 

[malawi_glenn]

Oh,I got too involved and I forgot that this is the Dark matter thread.Now I have my doubt clarified.But why did people start using the term baryonic matter?

recall that also electrons (in atoms and plasmas) are under the baryonic matter definition. It is a historical name which is not 100% logic (as many other things in astrophysics). I have no idea why and when one introduced the term.

But the definition today is that baryonic matter is particles that obeys a certain density relation with respect to time and shell factor. Photons and neutrinos does not fall under that definition.

 

[Phy6explorer]

Doesn't that mean we ar also baryonic matter?

 

[malawi_glenn]

Doesn't that mean we ar also baryonic matter?

well of course, has anyone said something else?

 

[Fluxman]

Matter is the substance of which physical objects are composed of. I find no meaning in your question!Matter cannot be physically measured, so the word"amount"is inappropriate.

Well, no disrespect intended, but I guess I am going with the idea that the quantity of a collection of matter can be measured by its mass.

 

[scupydog]

Well, no disrespect intended, but I guess I am going with the idea that the quantity of a collection of matter can be measured by its mass.

Perhaps DM should be called Dark Mass, since it is mass that is supposed to be missing from the critical density.

 

[Kurdt]

Protons, neutrons and electrons are considered baryons to a cosmologist. Neutrinos and radiation make up relativistic material (unless neutrinos have sufficient mass). Dark matter is non-baryonic, just to summarise what malawi glenn said.

 

[nikiforos]

What is Dark Matter?

Dark Matter must exist because too many parameters (some very fundamental) and many observations depend on its existence. Having said that: What is it?
There is a cosmology hypothesis (very extensive and several decades old “CCA”) from which one can calculate a plethora of parameters and including the determination of how much DM is there in the universe and a possibility for what DM is. {For example, by the CCA hypothesis, the DM states are, unlike any up to now hypothesized state, a stable member of a family of “archaic” particles (identified and well defined, all bosons) that are “totally blind” to “all but” gravitational field (perhaps also to a scalar). They are not of leptons or quarks but actually in the evolution of the universe, are “progenitors” of both leptons and quarks. These DM stable states currently have a number density ½ that of baryonic matter. If this is valid, then their “equivalent rest mass”, mo, from the above DM cosmic value and Nucleosynthesis and WMAP inferred age can be found from the amount of DM (Mdm) in the “accessible” universe which is:

Mdm = 6.41x10^55 g. and its “equivalent rest mass”, mo ~ (Mdm)/(3.5x10^78) = 1.82x10^-23 g. This is about 11 times more massive than the proton.
(From this, CMB details can also come about and can be calculated).

Why not discovered? Among other peculiarities, not “easily produced”. Very weak interacting (if these statements so far have not shocked everyone, the next statement, no doubt, will) “photon-blind antimatter”; no annihilation, but a “destructive” interaction shows photons up to energies of neutral kaon and neutral D-meson decays. They also, have cosmic behavior that causes non-linear oscillations (fast, across strong gravitational field, “hot” regions) to form (slow moving) concentrations at the most, weak field, “cold” regions of large structures. This means small number densities within visible matter concentrations}
Just to show that this is not, ad hoc and how the rest of the components of the universe fit with this, the densities D (for the total critical, the Dark Energy (vacuum energy) Dark Matter and Baryonic Matter respectively) are calculated and given to be compared with observations.
Dc = Dde + Ddm + Dbm
9.60 x10^-30 = 7.0 x10^-30 +2.16 x 10^-30 + 4.16 x10^-31 g-cm^-3
For the total critical value of omega = 1, or (100%)
100% ~ 73.6% + (22.2% + 4.3%)
and can be compared to the published values.
How does this fit with GR? The Ricci Tensor cannot vanish (no such thing as “empty space”). A different metric (non-Minkowski, and with past-future asymmetry). The Equivalence Principle needs modification. Not a “pure” Cosmological “Constant”. Otherwise, GR is ok.

 

[gravity guru]

turbo-1, i like the way you think. here"s something to consider that may pertain to DM it"s only conceptual in nature and might only be enteresting and nothing more. if out in space away from objects a static charge was made to spin where would the magnetic field that it produces come from? might that be DM that is being gathered by the spinning charge and formed into a magnetic field around the charge? since DM lacks energy of it"s own perhaps space is filled with it in varying degrees of densities and is manifested only when acted upon by moving charge. just a thought. gravity guru.

 

[Phy6explorer]

I don't think DM needs to gather gravitational field.DM itself has gravitational properties!