Dark Matter in 60 minutes.
By: Albert van der Sel
Date: 1 March, 2015
Remark: Please refresh the page to see any updates.
Chapter 1. The "strong CP problem" and Dark Matter like the Axion.
Chapter 2. The "branon" as a Dark Matter candidate from Brane World scenario's.
Chapter 3. Supersymmetry and Dark Matter candidates like the neutralino or gluino.
Chapter 4. The Anapole Dark Matter proposal.
Chapter 5. Doubts on Dark Matter.
Chapter 6. Some alternative theories.
First, I want you to know that this is a very "lightweight note".
But, I hope that it is informative anyway, or at least, "nice to read"... If so, that would be something !
Secondly, although Dark Matter is (generally) quite accepted in the scientific community, some physicists still have serious doubts
on it's existence, and up to now, we only have astronomical observations, and still no real "hard" evidence.
Here we will discuss the following Dark Matter candidates:
(1) the "axion" as a result of the strong CP problem,
(2) "branons" as a consequence of Brane world scenario's.
(3) superpartner particles like the "neutralino", or special "squarks", or the "gluino"
(4) As a new "kid on the block", the "anapole" Dark Matter theory (as of 2012), might be a strong one too.
There are at least 4 (accepted) theories that have the potential to explain Dark Matter, and how it comes
- The "strong CP problem" which might lead to a new particle, called the "axion".
The axion is supposed to be a very light particle (but very abundant), with almost no interaction
with regular matter (except for the "weak interaction" and gravitation).
But some physicists say that the "strong CP problem" can be addressed within the framework of a Supersymmetry.
- "Brane World scenario's". These are probably the most exotic theories, probably having the least of
of support by the majority of physicists. A Brane (or our World manifold) oscillates in the "Bulk",
and that might produce "branons". These are dark too, since there is almost no interaction with regular matter.
- Supersymmetry theory, which might lead to particles like the "neutralino's" or the "chargino's".
The "neutralino" is dark, since it too would have almost no interaction with regular matter.
In this Supersymmetry framework, the best candidates are likely to be the lightest particles, since they will not decay any further.
- In the "anapole" Dark Matter theory, majorana like particles are put forward, having donut shaped electric/magnetic dipole moments,
so that the "cross-section" is very small, so that would be the basis for the fact that interaction with
external Electromagnetic fields and Standard Matter, would be very low.
About those theories: they are all serious, and scientifically accepted.
Ofcourse, maybe one of those is truly correct, or maybe none of them is, and explaining Dark Matter
has to be done using another "path"...
Research in so many aspects of physics is in full progress, and the number of scientific articles is astounding.
Since testing and research on "Supersymmetry" is on the "to do" list of the new upgraded LHC, it might well be
that new insights on Dark Matter is matter of months (or a few years).
Some physicists even think that the "strong CP problem" might be solved by some "Supersymmetry" variant,
and if that would be true, it follows that we only have three theories instead of four.
For now, I will treat this matter as four seperate parts of physics.
In the four sections below, I will quickly spend a few words on the "core" of each of those theories.
Before we go to the short descriptions of the theories, we first need to know a few facts, like why "Dark Matter" is called "dark",
and what the generic term WIMP means, and the difference between hot- and cold "Dark Matter" .
- Why Dark Matter is called "dark":
Dark Matter is assumed to have no (or very little) interaction with regular matter, except for gravity, and possibly the "weak force".
So, not even ElectroMagnetic fields will have any influence on it, which makes it "transparant".
Yes, you can look "through" it, as if it's not there at all. Photons (or light) will just go through.
Because the interaction with "normal" matter is absent (or very little), it is called "dark".
So, here you must interpret "dark" as being "invisible".
As another phrase, scientists often say that "Dark Matter" is (very) "weakly coupled" to regular matter.
Except for gravity (on a large scale), it's very likely that the familiar "weak interaction" will be in effect too,
between Dark- and regular matter.
On a Cosmic scale, gravitational effects have already been observed. In quite some occasions, when astronomers observe the sky,
sometimes they see galaxies "doubled" as if the light was bended (due to gravity of Dark Matter between the observer and those galaxies).
This effect is called gravitational lensing.
Other observations include the fact that the radial velocities of stars and dust in the Galactic disks
of spiral Milky ways, are more uniform than expected. This can be explained by a large distribution of invisible matter
around those Milky ways (in the "halo").
By now, almost all cosmologists are convinced that Dark Matter plays a role in the "large scale structure" of the Universe,
meaning the observations of the distribution of galaxies in clusters, superclusters, and filaments of clusters.
It's possible that other observed data has it's origin from Dark Matter as well. For example, some extremely high energy cosmic rays
(UHECR), were detected, and those still are a large mystery for physicists and astronomers.
Some suspect that this data has it's origin in neutralino annihilation.
- About WIMPs:
WIMP is short for "Weakly Interacting Massive Particles".
It is just a generic term for Dark Matter particles, like for example the "axion" (if it would exists).
As we shall see below, another candidate could be the "neutralino", which is classified as a WIMP too.
So WIMP is a "catch all" classifier. But it's a very effective term. Note the "Weakly Interacting" part.
As we have seen above, the "Weakly Interacting" part is precisely why we call Dark Matter "dark".
The other part "Massive Particles" should express the fact that "matter/mass" must be associated with "Dark Matter",
since strong gravitational effects have been observed in the Universe.
In general, the "weakly interacting" phrase means a weak coupling with normal (standard model) matter.
However, a referal to the "weak force" (or weak interaction) is another interpretation. Remember that the forces we know of are (at least)
the strong nuclear, the weak nuclear, the electromagnetic, and gravity.
Dark Matter will have interaction through gravity, but very likely through the "weak interaction" too.
To be clear on this: presently this would be the first noticable "coupling" of Dark Matter to regular matter in small to medium
sized scales. "In the very large scale", large lumps of Dark matter would primarily interact to regular matter, through gravity.
- Hot and Cold Dark Matter:
This is just no more in a differentiation in the relative speed of the dark matter candidates.
"Cold" means very slow moving stuff, compared to the speed of light. You can imaging that cold dark matter,
might form heavy (transparant) clouds, or forming "lumps" in some hierarchical way. Since gravity still "works" with dark matter,
that's not a silly assumption.
"Hot" means that we have particles, zipping around near, or at. the speed of light (or very fast anyway). So, this will probably
not form "lumps".
"Hot" dark matter was never very appealing to physicists, while "cold" dark matter is.
Chapter 1. The "strong CP problem" and Dark Matter
There are a number of observations on elementary particles, which are (still) a bit puzzling.
For example, the "weak nuclear" interaction exhibits a "CP violation". The weak interaction is thought to be responsible
for nuclear Beta decay and other processes. In "CP violation", the "P" violation means that a process is not symmetrical
if it's spatially inverted, or mirrored.
The "C" violation means that a process is not symmetrical if the charges are inverted.
It may not strike you as very relevant, but for physicists this "bias" is a big deal. What makes the "dice" cheating.?
Is there some property of SpaceTime, or the vacuum, possibly "coupled" to the particles involved, that is responsible for that bias?
For years (until the fifties of the former century) it was assumed that charge conjugation and parity were symmetries in effect
in elementary processes, determined by the electromagnetic-, strong nucler, and weak interaction. For example, if you mirror a nuclear process,
the same effects are observed. Well, later on that turned out to be false with the weak nuclear interaction.
Physicists also have determined that a "strong nuclear" interaction exists, which rules "between" quarks (and thus between protons,
neutrons, and other particles). QCD is the branch of physics, that specifically focusses on the "strong nuclear" interactions.
A CP violation has not been observed, to a very high accuracy.
However, if you would write down the Energy equation (the Lagrangian), there are multiple "components" which strongly suggests a "phase shift"
which would lead to "C/P" violations.
This is known as the "strong CP problem". Why don't we have a violation here?
A solution can be, if a field or process would exists, that "sort of" counteracts the expected violation.
Serveral solutions have been put forward. In terms of "Dark Matter", R. D. Peccei (one of the originators of Axions)
proposed a scalar field, called the "axions field" (with a corresponding axion particle), that might solve the CP problem.
Ofcourse, the articles introducing the "axion" are quite extensive (it's not just "another idea"), and they are certainly a bit "involved".
However, by now, the "axion" is a dark matter candidate, since it is matter (although very light), and has hardly
any interaction with "normal" matter.
It's a serious candidate. Research to "detect" them is in full progress. If the axion indeed exists, it could well be, that within
a few years (or even shorter), a positive or negative result is found.
If you like to see some more information on the "search for the axion", you might like to see the following articles:
The Axion Dark Matter eXperiment (ADMX) (phys.washington.edu)
Axion Particle Dark Matter (phys.org)
(Mainly) axion dark matter (arxiv.org)
Chapter 2. The "branon" as a Dark Matter candidate from Brane World scenario's.
I'am not sure you know about Superstring Theory and M-Theory (and later theories). Anyway, it's fair to say that "Brane World Cosmology"
has (at least partly) it's roots in M-Theory.
In Superstring Theory / M-Theory, elementary particles are "strings" on a extremely small scale. Here, the vibrating mode of the string, determines
the type of particle. This is quite a uniform statement, and is very appealing to physicists.
There are "open" strings, and "closed" strings. The open strings are the elementary particles we know so well, while an "entity" as the graviton
(force carrier of gravity) is believed to be a closed string.
Remarkably, from mathematical requirements, M-Theory requires 11 dimensions, that is, 10 spatial dimension and one time dimension.
Since our Universe really looks like a (3+1) SpaceTime Universe (that is: 3 spatial + 1 time dimension), the theory further states
that all other extra spatial dimensions are "compactified" (or "curled up"), and thats why they are "unnoticable" to us.
The metric of those "extra" dimensions is so extremely small (10-35 m), that effectively, they go completely undetected indeed.
At the time M-Theory and "Brane World Cosmology" developed, several important views came up:
=> Maybe it's not neccessary that all extra dimensions perse have to be "curled up".
=> There are reasons to believe that if such a large extra dimension exists, it might only be "noticable" by gravity (or the graviton).
Thus, indeed, here the assumption is that only 'gravity' is 'aware' of such an additional (large) dimension.
Then, for example, the Electromagnetic interaction, the strong nuclear force etc.., would not know of such an additional degree of freedom.
The same holds for true particles (open strings) which are 'unaware' of such an additional (large) dimension too.
=> A "Brane" might be viewed as a n-dimensional manifold (sort of subspace), in the larger space of extra dimensions.
=> It's quite tempting to view our Universe as a (3+1) Brane in the M-Theory framework.
=> The "open strings" (normal particles), have their "endpoints" on the Brane, and thus are therfore confined to the Brane.
=> In principle, a "closed string" (not having their "endpoints" on the Brane), is not confined onto the Brane, and may "dissapear" from the Brane.
=> If one or more larger extra dimensions exists, then we may view our Universe as a (3+1) Brane in the "Bulk", where the Bulk
is the space outside the Brane. In the theory sofar, only gravitions can move to the Bulk.
This may all strike you as pretty awkward, or weird even, but they are serious thoughts.
A specific model: The Randal & Sundrum model
First, a small remark. Although "Brane World" models are accepted as a possible solution for current problems (like the origin
of the Universe, Dark Matter, Dark Energy and others), many physicists are not too enthousiast for these theories.
As it is now, it seems that most physicists have more faith in the other two theories.
There are quite a few "Brane World" models actually.
Modern ones are based on the Randal & Sundrum model, which is a (3+1) Brane in a 5 dimensional Bulk.
Please note that "the hyperspace" here is (4+1), meaning 4 spatial and one time dimension. The Brane then would be (3+1) manifold
in that (4+1) space. Here, one additional "extra" dimension is used, which is not compactified, and is only "accessible"
Further details on such a model can be found in a classical article (1999):
"An Alternative to Compactification (1999)", which can be found here.
Essentially, the authors describe a single 3+1-brane with positive tension, embedded in a (4+1) (fivedimensional) Bulk spacetime.
Using "gravity" as the vehicle to explore the scene, and using wavefunctions, they argue that a graviton in the Bulk neccesarily
will be in a confined bound state. Initially, they make no assumptions on the additional spatial dimension rc, so
it could be compactified. Using math and letting rc grow to infinity, they manage to keep the "four-dimensional effective Planck scale",
to go to a "well-defined value" as well.
This means that a consistent 4 dimensional theory of gravity can be derived from the used model.
Such a 5 dimensional model, using no compactification, is a remarkable deviation from M-Theory based models.
Secondly, the bound state of the graviton as described in this article, actually means that they are close to (or near "at") the Brane,
and do not travel "freely" around.
Note that, in simple words, we can now speak of "distances" along that fourth spatial dimension. If you would insist, you can still use
more compactified dimensions. However, the one that (probably) only gravity is aware of, is that fourth spatial dimension.
Classical Article: "A new Dark Matter candidate in Low-Tension Brane Worlds".
Let's discuss the upper classical scientific article, which addresses "dark matter".
from J. A. R. Cembranos, A. Dobado, A. L. Maroto.
Ofcourse, countless articles have been published up to this date, but this one (from 2004) is rather nice in my opinion, to illustrate
a specific (classical) modern Brane World scenario.
Essentially, it discusses the Brane tensions (occilations), which is responsible for generating particles. If those particles do not couple to
ordinary "Standard Model" particles, we may have found a Dark Matter candidate.
I find that these sort of articles are quite hard to read and understand. It really takes time to go through them.
I will try to give a short understandable "discussion" (as far as I understand it).
Again, it assumed that the (ordinary) "Standard Model" particles (the ordinary matter: quarks, leptons) are confined to their Brane,
as they are open strings. Closed strings, in particular the graviton, can move (or leak out) to the Bulk.
But here, the emphasis here is not on "leaks to the Bulk", as it is here actually focussed on the dynamics of the Brane itself.
It's further assumed, due to good reasons, that Branes have a certain "tension distribution", which can be viewed as oscillators.
You may compare it to "stress-energy" which we can find in nummerous fields in physics, like fluid dynamics, relativity etc.. etc..
Such "stress-energy" might be interpreted as a flux or density of energy, in fact..., due to continuous "changes".
Now, also due to relativistic considerations seen over the metric (distances), the Brane releases energy, which is quantified in particles.
That is not so strange, since in Quantum Mechanics, energy absorption and release is always quantified, in quanta, or particles.
These particles are called "branons", with a clear reference in their name as to theirs origin: generated by Brane oscillations.
Most folks interpret the article in this way, that the fluctuations of the Brane are in the extra dimensions.
The creation of particles, or fields, from a fall of potential energy, is not new. For example, in certain inflationary theories,
it is argued that at a certain point, the false vacuum potential (top of the "mexican hat"), has dropped, which generated the Higgs field.
Also, closer to home, a transition from a higher quantum state to a lower quantum state, might produce a "free" quantum, like a photon,
which is quite a common event in atomic physics. There are many more examples to illustrate the key point.
The article is also partly based on the classical article from R. Sundrum (Phys. Rev. D59, 085009), called "Effective Field Theory for a Three-Brane Universe"
which can also be found here.
This article, decribes the dynamics of “3-brane”, which fluctuates in a higher-dimensional, gravitating spacetime (the Bulk).
Sundrum shows, that bosonic fields may arise in the 3-brane, in a background of the Bulk metric. But that article covers much more than that.
Under the assupmtion that "branons" do exist, it's now up to the authors to prove that they are "massive" and "dark".
For the latter, it means that branons have no, or neglegible interaction, with ordinary matter.
They try to do that by pondering on the equation of how branons would interact with Standard Model particles. As a result, it happens to have
a term 1/f 4 (where f is the tension in the Brane), thus meaning a very strong suppresion of that interaction.
In the same process, they show the branons are massive.
For an intuitive explanation for the strong suppresion: the branons are "sort of" strongly tied to the fluctuations themself, making their interaction
in the Brane with Standard Model particles negligible, except for their large mass, which can be noticed by the effects the astronomers see today.
Hence, a "dark matter" candidate, the branon, is proposed, from a Brane World scenario.
There are "variants" of this theory, which leads to a variant of the Branon, namely the "Kaluza Klein graviton" (KK graviton).
However, one good example (the branon) of a DM candidate from "Brane World scenario's" is good enough for the scope of this note.
A new dark matter candidate in low-tension brane-worlds.
An Alternative to Compactification.
- Detecting branons:
Brane-Worlds at the LHC: Branons and KK-gravitons.
Impact of DM direct searches and the LHC analyses on branon phenomenologys.
Chapter 3. Supersymmetry and Dark Matter candidates like the neutralino.
To be honest, I like Brane World scenario's a lot, since they might explain much more than Dark Matter only.
However, most physicists are not very keen on "Brane World scenario's", and they like theories
like the CP Problem (as a basis for the "axion"), or "Supersymmetry" (leading to DM superpartners), much more.
So, let's take a closer look at "Supersymmetry" and what it may tell us about Dark Matter.
The Standard Model of physics, has a "hypothetical" extension, called "supersymmetry".
Up to now, our fundamental particles can be devided into "fermions" ("normal" particles) and "bosons" (force carriers).
These two types of particles are very different in certain properties and behaviour.
Even now, you might already say that two such different "classes" is quite asymmetric, if you are a "purist".
Long time ago, a number of physicists thought the same way, and constructed a new theory: Supersymmetry.
There are many motivations for "Supersymmetry". Below, we will take a peek at a few of them.
1. Symmetric Transformations
In a constructed "space" of fermionic- and bosonic properties, mathematically it turned out that certain symmetric transformations
can be devised, which "maps" fermions to bosons, and the other way around.
This lead to the hypothesis, that fermions have "bosonic superpartners", and bosons have "fermionic superpartners".
For example, fermionic quarks are partners of bosonic squarks.
It's common (but not a rule) to place an "s" in front of the partner's name. Just like the example above, a selectron then would
be the superpartner of the electron.
As another interesting example: the bosonic gluons (the fields/force carrier between quarks of QCD) are partners of fermionic gluinos.
You might thus say, that Supersymmetry (SUSY), introduces a new symmetry between fundamental particles.
This symmetry might remind you of a similar relation found before, namely between matter and anti-matter.
This idea is not unlike to something that happened before in history. When Dirac tried to unify special relativity with quantum mechanics,
his equations implied a charge mirrorring principle. It actually meant that for example, an electron would have an "anti-particle",
and for all other fundamental particles, the same would apply.
The main difference is that the antiparticles would have their "charge" inverted.
As we know now, the "anti-particles" do indeed exist (for example, a "positron" is the anti-particle for the "electron").
All the anti-particles are collectively called "anti-matter", which is actually rather well-known by the general public.
You see the similarity? What we have now in Supersymmetry, looks quite a bit as what Dirac found.
Similarities (as illustrated in the note above) are "nice" ofcourse, but more importantly is the unifying principle in physics.
It has already long suspected, that what we see now, at particle/particle/field interactions, is that it actually is a manifestation of a low-energy
interactions, where there is no symmetry. For example, there are different fundamental forces, working on a different scales and entities.
AlMost all physicist, think that those forces would unify at higer energies.
It can be shown that Supersymmetry will help enormously in unification.
In other words, based on the Standard model alone, it is difficult (or impossible) to unify the different forces to unify
to one GUT-like force exactly. Using Supersymmetry, however, it works.
It is believed that there are several levels of "restored symmetries", depending on the Energy (or distance) between colliding particles.
At the very end, at an extremely high energey, all forces unite to one fundamental force. At such a scale, it's also assumed that
that Quantum effects and Gravitation, will converge as well, resulting to a sort of "Grand Unifying Theory".
At a lesser high energy, it is assumed that a sort of "medium" level of restored symmetry would be visible.
The superpartners of our familiar fermions and bosons would be detectable. It was calculated that this might happen
in the range of 1 TeV.
However, since the LHC became oprational, and at a certain point was capable of producing energies in the range of several TeV's,
it was concluded from the data that effects that might arise from "Supersymmetry" was not observed.
But, Higgs was found, and according to physicists, that would indicate that "Supersymmetry" may well be observed
when the "upgraded" LHC (using considerable higher energy) is in operation again (in the course of 2015).
The neutralino as the best DM candidate?
Quite a few physicists argue that the socalled "minimal supersymmetric extension of the standard model" (MSSM),
which is a well-accepted variant of Supersymmetry, can provide for a neutralino relic abundance in the Universe.
What they actually say here, is that "neutralino's" were formed in high abundance, in a phase quickly after the Big Bang,
where one of the "last symmetries" were broken. However, "relic", that is, still existing neutralino's, are present
in high quantaties.
Ofcourse, we have a few questions here. First: What is the "neutralino"?
In the Supersymmetry framework, all fermions and bosons have superpartners, as we already have seen.
If you would first focus on the partners of the "gauge bosons", we would get the list below.
By the way, the "gauge bosons" is a collective name for the usual force carrier bosons (like photons for the electromagnetic interaction,
W and Z bosons for the weak interaction, and gluons for the strong interaction).
So, you might say: don't we then have, as superpartner of the "gauge bosons":
- The photino as superpartner of the photon
- The gluino as superpartner of the gluon
- The gravitino as the superpartner of the graviton
- The winos are the superpartners of the W bosons
Yes indeed, those hypothetical partners would exist in Supersymmetry.
However, we also have the Higgs boson, which would have the "higgsino" as it's superpartner.
The people who are deep into this field, have already calculated and constructed, "decay" schemes and "mixing" schemes.
However, since "subtle" different lines of thoughts in Supersymmetry exists, different schemes exists as well.
Now the monkey will pop up out of the box:
Those upper superpartners will "mix" their states into particles called "charginos" and "neutralinos".
The MSSM model, contains two chargino and four neutralino states. Other Supersymmetry models use different numbers.
So, those "mixures" of states, could be rather complex. Anyway, the "neutralinos" are electrically neutral, while
the "charginos" are charged.
The more heavy states will rather easily interact with the "regular" W and Z bosons, meaning the can "decay"
into other products.
As it is theoretical found, this is the current situation:
- Heavy neutralinos might decay in the lightest neutralino + other stuff.
- Charginos might decay in the lightest neutralino + other stuff.
- The lightest neutralino is expected not to decay any further, and would be very abundant.
Hence, we have arrived as to an explanation as to why the "lightest" neutralino is the favourite Dark matter candidate
in Supersymmetry theories.
Some Relevant (classics) articles:
Supersymmetric Dark Matter (1995) (this "classic" is a very large article).
String Theory, Supersymmetry, Unification, and All That.
R-parity violating supersymmetry
Some other great articles:
Overview: Dark matter and cosmology (focusses on neutralino)
Chargino and neutralino production at the LHC in left-right supersymmetric models
Chapter 4. Anapole Dark Matter
The "crux" of "anapole" Dark Matter is this: Suppose we have a particle with complex "toroidal electric/magnetic moments", that is,
a sort of closed/twisted Electric/Magnetic fieldlines, while at the same time, the "cross section" would be so small, that it hardly
interacts with external Electromagnetic fields, then we would end up with a credible Dark matter candidate.
You can visualize such a particle as a donut (torus) shaped object, where the fieldlines essential follow that donut shape.
It was already experimentally found in the verly late '90s, and early 2000s, that the hyperfine structure of atomic 133 Cesium
could indeed be explained using an "anapole moment".
Think of an "anapole moment" a much more complex moment compared to the familiar dipole moment, or quadrupole moment.
But way before that (late 80s, early 90s), moleculair- and atomic physicists, "played" with models where several static charges
rotated in certain directions, not only resulting in familiar "dipolar" (north-south) moments, but also in "toroidal moments",
which go under the name anapole moment.
Chiu Man Ho and Robert J. Scherrer (2012), tried to incorporate such an anapole moment, in constructing a hypothetical WIMP-like particle.
As a result, they proposed a Majorana fermion having a "toroidal electric/magnetic "moment". The known standard model fermions (quarks, leptons),
are, let's say, well-described, while scientists are just beginning to explore Majorana particles (but already proposed by Ettore Majorana, in 1937).
A characteristic of Majorana particles, is that they are their "anti-particle" at the same time.
So, they should, for that reason, have no "charge", or, no "netto charge".
The great thing of the paper of Ho and Scherrer is, that they show that the reactive cross-section of the anapole moment
get smaller as the velocity of the particle decreases. Hence a good candidate for "cold Dark Matter".
Some great articles:
Anapole Dark Matter (Chiu Man Ho, Robert J. Scherrer, the originators of the theory).
5. Doubts on "Dark Matter"???:
Are there really doubts...??? Yes, there are.
Some physicists, astronomers and the like, have expressed serious doubts on the existence of, or the need for, Dark Matter.
For example, it is generally assumed (and almost accepted), that galaxies (milky ways), are embedded
in halo's of Dark Matter. However, some studies seem to point to the fact that the radial velocity of stars and dust in
the galactic disk, can be explained by baryonic matter (standard matter) after all.
Also, some studies to gravitational effects and matter in the Sun's local neighbourhood, seems to point to the fact
that it is (almost completely?) void of Dark Matter, since no gravitational effects could be observed, other than that
of luminous matter. Other types of studies go around as well, like some that seem to be motivated from "quantum gravity".
Although it's likely that Dark Matter exists, it is still somewhat "dangerous" to fully accept it "as is", without experimental
data (like from high-energy colliders as e.g. the LHC), or having a very good solid theoretical framework.
However, some exciting experiments are under way, and may provide more insight in the coming months or years.
For example, new experiments with the "upgraded" Large Hadron Collider (Cern), might shed light on "supersymmetric particles"
of which some of them might be Dark Matter candidates (for example: the "neutralino"?)
Or the ADMX experiments, which might show us if the "axion" could be "real" or not.
Or..., maybe nothing pointing to Dark Matter, will be experimentally found at all.
For now, a (smaller) group of scientists are still pretty much doubtfull on the Dark Matter business.
Here are a few scientific papers:
Gravitational lensing evidence against extended dark matter halos (arxiv.org).
Is Dark Matter just an illusion? (arxiv.org).
Deducing the nature of DM from direct and indirect detection experiments in the absence of collider signatures (arxiv.org).
No evidence of dark matter in the solar neighborhood (arxiv.org).
Chapter 6. Some alternative theories.
It might turn out, that none of the theories above, can explain the existence of Dark Matter.
New facts may surface, that conflicts with those theories, so, physicists will have to take an alternative route.
Indeed, some alternative ideas go around as well. Some can be dated from quite some time ago, while others are relatively new.
Characteristic of the three theories above, is that they support "WIMP like" particles (like the axion, or neutralino).
Some alternatives completely deviate from such thoughts, as you will see below.
One strong piece of evidence for the existence of Dark Matter, is the fact that stars in a Spiral Galaxy,
have a rather uniform radial velocity distribution, which was not expected.
It can be explained, if a large distribution of mass exists in the "Halo" (the outer rim) of such a Galaxy.
This Halo surrounds the "disk" of the Spiral Galaxy, and if a lot of mass is in that Halo, it will stronly effect
the matter in the galactic disk.
One old idea is, that extremely faint white dwars, or brown dwarfs exists in a relatively high concentration around the Galaxy.
It could indeed explain the observations of the largely uniform radial velocity distribution.
Here we already mentioned "white dwars", and "brown dwarfs", but other candidates are mentioned too (e.g. neutron stars).
Note that this theory does not use "extraordinary" WIMP-like particles: it's just ordinary (baryonic) matter, and it just happens to be
very faint and thus (almost) undetectable. Indeed, there is nothing extraordinary about a common white dwarf, although the
high abundance in the Halo still have to be explained.
These sort of objects are collectively called "MAssive Compact Halo Objects" or "MACHO's".
Although the idea of MACHO's might work for our Spiral Galaxy, for another observable effect, namely gravitational lensing,
some studies strongly suggest that MACHO'cannot explain that.
Support for MACHO's by physisics/astronomers, as the source of Dark Matter, has started to decline since the beginning of this century.
6.2 The Postulate of Dark Matter, only due to incomplete Physics:
The are three "main" (catagories of) theories of gravity:
(1) The classical theory of Sir Isaac Newton.
(2) Einstein's original General Theory of Relativity.
(3) Attempts to unify various modern theories, like "quantum gravity", Superstring/M-F theories etc..
Ofcourse scientist know that (1) and (2) are "incomplete", and cannot easily be reconciled with other famous theories, like e.g. Quantum Mechanics.
By the way, at "low" speeds and other moderate physical circumstances, Einsteins's theory will be quite "compatible"
to the classical one of Newton.
Still, focussed on gravity and descriptions of SpaceTime alone, Einstein's theory is the best we have right now.
But you cannot explain the "World" (the Universe), solely by Gravity alone.
Especially theoretical physicists work hard to unify Relativity and and other modern theories, like modern variants
of Quantum Mechanics (resulting for example in "quantum gravity") etc..
Now, we have seen some "reasonable" physical candidates for Dark Matter (like the axions, neutralino's, majorana particles etc..), and all
of them are back upped by some serious and reasonable theory, as you have read above.
But what if all that is wrong....?
If observations or experiments show that Dark matter simply cannot be explained by axions, neutralino's, majorana particles etc..
then simply another route has to be followed. That's not exactly new in science.
A very small minority of physicists could accept, or even find it plausible, that the observations that we now have attributed
to Dark Matter (like the rather uniform motion in the Galactic Disk, or gravitational lensing), is actually due to a serious
lack of knowledge of Gravity itself.
Who knows? Maybe gravity intrinsincally varies in strenght in SpaceTime (and not only due to mass), and only "acts to be" dense in some regions...
Allright, that seems not to be a good example.
What I really want to say here, is that some argue that we still have so many "missing links" in physics, so that what we now think
can be explained by Dark Matter, is actually due to our incomplete view in physics.
Most physicists don't think so..., but it cannot be denied that we have no complete idea on, for example, the Vacuum, or something like "Dark Energy", or
how exactly to unify theories that work great on atomic scales, with theories that work good on large scales, etc... etc...
But that is exactly what all physicist know is true.
However, most physicist/cosmologists believe that Dark Matter simply is real and that we are slowly building the picture
to understand it.
6.3 Cosmic Strings:
Once it was seen as a candidate for being the "motor" behind Galaxy formation. Then, since the late '90's or so, support for "Cosmic Strings"
has strongly declined too, as a result of more precise observations and theoretical developments.
The existence of Cosmic Strings is generally regarded as "very unlikely" today.
However, there still remains a small group of "die hards" in the scientific community.
A "Cosmic String" is a hypothetical object that falls into the class of "Topological Defects".
It was realized, that at various moments in the Inflationary Universe model (Big Bang), the vacuum has ondergone phase transitions,
where possibly "imperfections" may have arisen, resulting in Topological defects. A string-like defect would then be the
most likely one.
Just like when water freezes into ice (a phase transition), you may notice various "cracks" running along various directions.
If Cosmic Strings exists, some physicists see as the most likely scenario, that strings may have formed along the boundaries
of Hubble volumes. Some others think that it might be possible that a network of strings exists in our Universe, which is
responsible for the large scale structure of our Universe (large voids, with filaments of superclusters of Galaxies).
Cosmic String are thought to be extremely dense, and associated with enormous large gravity.
It is quite easy why they would work to explain gravitational lensing. It's a bit harder to see why it would work at the
uniform motion of the galactic disk.
But as already mentioned, Cosmic Strings are not "too lively" anymore in the minds of physicists and cosmologists.
So, what is "Dark Matter"? Nobody knows that right now. However, some exiting experiments are waiting just "around the corner",
to tell us more about the reality of the "axion" (like the ADMX experiment), or the new experiments at the LHC
(when it's ready to operate at 6.5 TeV per beam), which might reveal supersymmetric "partner" particles, or even provide
clues on "branons" or "majorana" particles.
We just have to wait a bit longer....
1. Something about Galaxy formation.
One modern idea is, that Dark Matter "lumps and halo's", sort of "herds" standard matter into relatively
confined area's, thereby (sort of) "promoting" Galaxy formation.
At least, that's a leading idea, in a nutshell. A couple of hunderds of millions of years after the Big Bang,
the halo's (quasi-spherical conglomerations) pulled together huge amounts of atoms, creating regions dense enough
for ultimately Galaxy formation to start.
Dark Matter is stuff that does not interact with Standard Matter (the normal matter), except for gravity.
However, modern ideas express the fact that Dark Matter and Standard Matter might have more flavours of interaction,
albeit extremely weak. But at least gravity, is a strong component.
It's believed that the first galaxies were of the "eleptical" types (and later, after collisions, the spiral ones formed).
Many astronomers also think that in the early Universe, close fly-bys, sometimes colliding, sometimes merging of Galaxies
happened more often than previously thought (as we even can witness today).
Dark matter halos provided (and still provide) most of the gravitation within which "relatively stable structures" formed in the universe.
which are the galaxies, and it is believed that those dark matter halos still preserve these galaxies, groups, and clusters.
This model is also quite reasonable, as another Universal component, namely "Dark energy", tries to tear apart "space itself",
and which is responsible for the accelerated expansion of the Universe..
To model galaxy formation in the Universe, is a formidable task, ofcourse. I can' help notice that that the last few years
studies in this topic seems to have intensified, as can be decuced from the number of papers in this subject.
"Cold" Dark matter seems (or seemed) to be the best candidate for the effect as described above, but many modern papers
nowadays use "hot" DM as well.
That's it. Hope you liked it!
Ofcourse, it's free for use anyway you want.