What Are We Missing?


Why is it important to actually search for and to identify the dark matter? Of course it is intrinsically interesting to know what the primary constituent of the Universe consists of, but also until we know the dark matter identity, there will always be the doubt that there is no dark matter, and instead there is some flaw in ourknowledge of fundamental physics.


THE STANDARD MODEL


http://www.united-academics.org/space-physics/latest-theory-of-everything-to-hit-the-physics-shelves/

 The Standard Model explains how the basic building blocks of matter interact, governed by four fundamental forces.

 The theories and discoveries of thousands of physicists since the 1930s have resulted in a remarkable insight into the fundamental structure of matter: everything in the universe is found to be made from a few basic building blocks called fundamental particles, governed by four fundamental forces. Our best understanding of how these particles and three of the forces are related to each other is encapsulated in the Standard Model of particle physics. Developed in the early 1970s, it has successfully explained almost all experimental results and precisely predicted a wide variety of phenomena. Over time and through many experiments, the Standard Model has become established as a well-tested physics theory.

Matter Particles
  All matter around us is made of elementary particles, the building blocks of matter. These particles occur in two basic types called quarks and leptons. Each group consists of six particles, which are related in pairs, or “generations”. The lightest and most stable particles make up the first generation, whereas the heavier and less stable particles belong to the second and third generations. All stable matter in the universe is made from particles that belong to the first generation; any heavier particles quickly decay to the next most stable level. The six quarks are paired in the three generations – the “up quark” and the “down quark” form the first generation, followed by the “charm quark” and “strange quark”, then the “top quark” and “bottom (or beauty) quark”. Quarks also come in three different “colours” and only mix in such ways as to form colourless objects. The six leptons are similarly arranged in three generations – the “electron” and the “electron neutrino”, the “muon” and the “muon neutrino”, and the “tau” and the “tau neutrino”. The electron, the muon and the tau all have an electric charge and a sizeable mass, whereas the neutrinos are electrically neutral and have very little mass.

Forces and carrier particles
  There are four fundamental forces at work in the universe: the strong force, the weak force, the electromagnetic force, and the gravitational force. They work over different ranges and have different strengths. Gravity is the weakest but it has an infinite range. The electromagnetic force also has infinite range but it is many times stronger than gravity. The weak and strong forces are effective only over a very short range and dominate only at the level of subatomic particles. Despite its name, the weak force is much stronger than gravity but it is indeed the weakest of the other three. The strong force, as the name suggests, is the strongest of all four fundamental interactions.
  Three of the fundamental forces result from the exchange of force-carrier particles, which belong to a broader group called “bosons”. Particles of matter transfer discrete amounts of energy by exchanging bosons with each other. Each fundamental force has its own corresponding boson – the strong force is carried by the “gluon”, the electromagnetic force is carried by the “photon”, and the “W and Z bosons” are responsible for the weak force. Although not yet found, the “graviton” should be the corresponding force-carrying particle of gravity. The Standard Model includes the electromagnetic, strong and weak forces and all their carrier particles, and explains well how these forces act on all of the matter particles. However, the most familiar force in our everyday lives, gravity, is not part of the Standard Model, as fitting gravity comfortably into this framework has proved to be a difficult challenge. The quantum theory used to describe the micro world, and the general theory of relativity used to describe the macro world, are difficult to fit into a single framework. No one has managed to make the two mathematically compatible in the context of the Standard Model. But luckily for particle physics, when it comes to the minuscule scale of particles, the effect of gravity is so weak as to be negligible. Only when matter is in bulk, at the scale of the human body or of the planets for example, does the effect of gravity dominate. So the Standard Model still works well despite its reluctant exclusion of one of the fundamental forces.
So far so good, but...
...it is not time for physicists to call it a day just yet. Even though the Standard Model is currently the best description there is of the subatomic world, it does not explain the complete picture. The theory incorporates only three out of the four fundamental forces, omitting gravity. There are also important questions that it does not answer, such as “What is dark matter?”

  
Unlike normal matter, dark matter does not interact with the electromagnetic force. This means it does not absorb, reflect or emit light, making it extremely hard to spot. In fact, researchers have been able to infer the existence of dark matter only from the gravitational effect it seems to have on visible matter. Dark matter seems to outweigh visible matter roughly six to one, making up about 27% of the universe. Here's a sobering fact: The matter we know and that makes up all stars and galaxies only accounts for 5% of the content of the universe! But what is dark matter?




  We are much more certain what dark matter is not than we are what it is. First, it is dark, meaning that it is not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the Universe to make up the 27% required by the observations. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter. Finally, we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. High concentrations of matter bend light passing near them from objects further away, but we do not see enough lensing events to suggest that such objects to make up the required 25% dark matter contribution.

  However, at this point, there are still a few dark matter possibilities that are viable. Baryonic matter could still make up the dark matter if it were all tied up in brown dwarfs or in small, dense chunks of heavy elements. These possibilities are known as massive compact halo objects, or "MACHOs". But the most common view is that dark matter is not baryonic at all, but that it is made up of other, more exotic particles like axions or WIMPS (Weakly Interacting Massive Particles). It could contain "supersymmetric particles" - hypothesized particles that are partners to those already known in the Standard Model.




  

POSSIBILITIES FOR DARK MATTER



  
  The two main categories of objects that scientists consider as possibilities for dark matter include MACHOs, and WIMPs. These are acronyms which help us to remember what they represent. Listed below are some of the pros and cons for the likelihood that they might be a component of dark matter.



MACHOs (MAssive Compact Halo Objects): MACHOs are objects ranging in size from small stars to super massive black holes. MACHOS are made of ordinary matter (like protons, neutrons and electrons). They may be black holes, neutron stars, or brown dwarfs.
  Neutron Stars and Black Holes are the final result of a supernova of a massive star. They are both compact objects resulting from the supernovae of very massive stars. Neutron stars are 1.4 to 3 times the mass of the sun. Black holes are greater than 3 times the mass of the sun. Because a supernova usually leaves behind a remnant cloud of gas, these objects must travel far from the remnant to be "hidden."
upPros:Neutron stars are very massive, and if they are isolated, they both can be dark.


downCons:Because they result from supernovae, they are not necessarily common objects. As a result of a supernova, a release of a massive amount of energy and heavy elements should occur. However, there is no such evidence that they occur in sufficient numbers in the halo of galaxies.



  Brown Dwarfs have a mass that is less than eight percent of the mass of the Sun, resulting in a mass too small to produce the nuclear reactions that make stars shine.
  Astronomers have been detecting MACHOs using their gravitational effects on the light from distant objects. In formulating his theory of gravity, Einstein discovered that the gravitational attraction of a massive object can bend the path of a light ray, much like a lens does. So when a massive object passes in front of a distant object (e.g. a star or another galaxy), the light from the distant object is "focused" and the object appears brighter for a short time. Astronomers search for MACHOs (usually brown dwarfs) in the halo of our galaxy by monitoring the brightness of stars near the center of our galaxy and of stars in the Large Magellanic Cloud.
  The MACHO Project, one of the groups using this "gravitational lens" technique, observed about 15 lensing events toward the LMC over a span of 6 years of observations. They set a limit of 20% as the contribution to the dark matter in our Galaxy due to objects with mass less than 0.5 that of the sun.
upPros:Astronomers have observed objects that are either brown dwarfs or large planets around other stars using the properties of gravitational lenses.
downCons:While they have been observed, astronomers have found no evidence of a large enough population of brown dwarfs that would account for all the dark matter in our Galaxy.

WIMPs (Weakly Interacting Massive Particles): WIMPs are the subatomic particles which are not made up of ordinary matter. They are "weakly interacting" because they can pass through ordinary matter without any effects. They are "massive" in the sense of having mass (whether they are light or heavy depends on the particle). The prime candidates include neutrinos, axions, and neutralinos.
  Neutrinos were first "invented" by physicists in the early 20th century to help make particle physics interactions work properly. They were later discovered, and physicists and astronomers had a good idea how many neutrinos there are in the universe. But they were thought to be without mass. However, in 1998 one type of neutrino was discovered to have a mass, albeit very small. This mass is too small for the neutrino to contribute significantly to the dark matter.
  Axions are particles which have been proposed to explain the absence of an electrical dipole moment for the neutron. They thus serve a purpose for both particle physics and for astronomy. Although axions may not have much mass, they would have been produced abundantly in the Big Bang. Current searches for axions include laboratory experiments, and searches in the halo of our Galaxy and in the Sun.
  Neutralinos are members of another set of particles which has been proposed as part of a physics theory known as supersymmetry. This theory is one that attempts to unify all the known forces in physics. Neutralinos are massive particles (they may be 30x to 5000x the mass of the proton), but they are the lightest of the electrically neutral supersymmetric particles. Astronomers and physicists are developing ways of detecting the neutralino either underground or searching the universe for signs of their interactions.
upPros:Theoretically, there is the possibility that very massive subatomic particles, created in the right amounts, and with the right properties in the first moments of time after the Big Bang, are the dark matter of the universe. These particles are also important to physicist who seek to understand the nature of sub-atomic physics.
downCons:The neutrino does not have enough mass to be a major component of Dark Matter. Observations have so far not detected axions or neutralinos.
  There are other factors which help scientists determine the mix between MACHOs and WIMPs as components of the dark matter. As referred before, recent results by the WMAP satellite show that our universe is made up of only 4% ordinary matter. This seems to exclude a large component of MACHOs. About 23% of our universe is dark matter. This favors the dark matter being made up mostly of some type of WIMP. However, the evolution of structure in the universe indicates that the dark matter must not be fast moving, since fast moving particles prevent the clumping of matter in the universe. So while neutrinos may make up part of the dark matter, they are not a major component. Particles such as the axion and neutralino appear to have the appropriate properties to be dark matter. However, they have yet to be detected.

http://imagine.gsfc.nasa.gov/educators/galaxies/imagine/dark_matter.html
  
Specific WIMP theories include:
  • Supersymmetry: Perhaps the most popular extension to the Standard Model, supersymmetry predicts that each particle in the Standard Model has a heavier partner of different spin but similar interactions. The lightest of these particles (the lightest superpartner or LSP) is stable in many cases (if a symmetry called "R-parity" is exact), and is often a neutralino(one of the partners of the Z boson, photon, and Higgses) - an excellent dark matter candidate.
  • Extra Dimensions: Many theorists also suggest that our universe may have more spatial dimensions than the three we are familiar with. A fourth one may be curled up very small, for example, as if each point in our familiar space were actually a tiny ring which a particle could run around. Particles moving around such rings would look like more massive versions of the Standard Model particles, and the lightest of these (the lightest Kaluza-Klein particle or LKP) is often stable as well (and a good dark matter candidate).

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