Dark matter

1. Introduction

    Astronomers know, from the way that galaxies move, that there is a great deal more dark matter in the Universe than there is bright stuff in the form of visible stars and galaxies. This dark matter is revealed by its gravitational influence on the bright stuff.
    Dark matter's existence is inferred from gravitational effects on visible matter and gravitational lensing of background radiation, and was originally hypothesized to account for discrepancies between calculations of the mass of galaxies, clusters of galaxies and the entire universe made through dynamical and general relativistic means, and calculations based on the mass of the visible "luminous" matter these objects contain: stars and the gas and dust of the interstellar and intergalactic medium.
    According to observations of structures larger than star systems, as well as Big Bang cosmology interpreted under the Friedmann equations and the Friedmann–Lemaître–Robertson–Walker metric, dark matter accounts for 26.8% of the mass-energy content of the observable universe. In comparison, ordinary (baryonic) matter accounts for only 4.9% of the mass-energy content of the observable universe, with the remainder being attributable to dark energy. From these figures, matter accounts for 31.7% of the mass-energy content of the universe, and 84.5% of the matter is dark matter.
    Dark matter plays a central role in state-of-the-art modeling of cosmic structure formation and Galaxy formation and evolution and has measurable effects on the anisotropies observed in the cosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than that which interacts with electromagnetic radiation.
    Important as dark matter is thought to be in the cosmos, direct evidence of its existence and a concrete understanding of its nature have remained elusive. Though the theory of dark matter remains the most widely accepted theory to explain the anomalies in observed galactic rotation, some alternative theoretical approaches have been developed which broadly fall into the categories of modified gravitational laws and quantum gravitational laws.

2. Categories

    Some of the dark matter may be made of baryons, the same sort of stuff that stars and galaxies (and people) are made of. But many cosmologists believe that the best way to explain how the Universe was born (a theory known as inflation) requires that there should be enough matter in the Universe to make it gravitationally ‘closed’, so that the present expansion will eventually be brought to a halt. If that is the case, there must be much more dark matter, and the amount of helium produced in the Big Bang and seen in old stars sets a limit on how much of this can be baryonic. At least ten times, and possibly 100 times, more matter must exist if the Universe is closed. Some of this non-baryonic matter, but not all, could be explained if neutrinos have a very small mass. At least two-thirds, however, must be in a form never detected on Earth, as particles of cold dark matter (also known as weakly interaction massive particles, or WIMPs). This is exciting for particle physicists, since their favoured grand unified theories predict the existence of just the right kind of particles, given names like axions, photinos and zinos, which would have been produced in large quantities in the Big Bang if these theories were correct. This marriage between particle physics and cosmology is one of the most important developments in science since the beginning of the 1980s.
    Historically, three categories of dark matter candidates had been postulated. The categories cold, warm, and hot refer to how far the particles could move due to random motions in the early universe, before they slowed down due to the expansion of the Universe – this is called the "free streaming length". Primordial density fluctuations smaller than this free-streaming length get washed out as particles move from overdense to underdense regions, while fluctuations larger than the free-streaming length are unaffected; therefore this free-streaming length sets a minimum scale for structure formation.
2.1 Cold dark matter
    Today, cold dark matter is the simplest explanation for most cosmological observations. "Cold" dark matter is dark matter composed of constituents with a free-streaming length much smaller than the ancestor of a galaxy-scale perturbation. This is currently the area of greatest interest for dark matter research, as hot dark matter does not seem to be viable for galaxy and galaxy cluster formation, and most particle candidates become non-relativistic at very early times, hence are classified as cold.
    The composition of the constituents of cold dark matter is currently unknown. Possibilities range from large objects like MACHOs (such as black holes) or RAMBOs, to new particles like WIMPs and axions. Possibilities involving normal baryonic matter include brown dwarfs, other stellar remnants such as white dwarfs, or perhaps small, dense chunks of heavy elements.
    Many supersymmetric models naturally give rise to stable dark matter candidates in the form of the Lightest Supersymmetric Particle (LSP). Separately, heavy sterile neutrinos exist in non-supersymmetric extensions to the standard model that explain the small neutrino mass through the seesaw mechanism.
2.2 Warm dark matter
     Warm dark matter refers to particles with a free-streaming length comparable to the size of a region which subsequently evolved into a dwarf galaxy. This leads to predictions which are very similar to cold dark matter on large scales, including the CMB, galaxy clustering and large galaxy rotation curves, but with less small-scale density perturbations. This reduces the predicted abundance of dwarf galaxies and may lead to lower density of dark matter in the central parts of large galaxies; some researchers consider this may be a better fit to observations. A challenge for this model is that there are no very well-motivated particle physics candidates with the required mass ~ 300 eV to 3000 eV.
    There have been no particles discovered so far that can be categorized as warm dark matter. There is a postulated candidate for the warm dark matter category, which is the sterile neutrino: a heavier, slower form of neutrino which does not even interact through the Weak force unlike regular neutrinos. Interestingly, some modified gravity theories, such as Scalar-tensor-vector gravity, also require that a warm dark matter exist to make their equations work out.
2.3 Hot dark matter
     Hot dark matter is particles that have a free-streaming length much larger than a proto-galaxy size. An example of hot dark matter is already known: the neutrino. Neutrinos were discovered quite separately from the search for dark matter, and long before it seriously began: they were first postulated in 1930, and first detected in 1956. Neutrinos have a very small mass: at least 100,000 times less massive than an electron. Other than gravity, neutrinos only interact with normal matter via the weak force making them very difficult to detect (the weak force only works over a small distance, thus a neutrino will only trigger a weak force event if it hits a nucleus directly head-on). This would classify them as Weakly Interacting Light Particles, or WILPs, as opposed to cold dark matter's theoretical candidates, the WIMPs.
     There are three different known flavors of neutrinos (i.e. the electron-, muon-, and tau-neutrinos), and their masses are slightly different. The resolution to the solar neutrino problem demonstrated that these three types of neutrinos actually change and oscillate from one flavor to the others and back as they are in-flight. It's hard to determine an exact upper bound on the collective average mass of the three neutrinos (let alone a mass for any of the three individually). For example, if the average neutrino mass were chosen to be over 50 eV/c2 (which is still less than 1/10,000th of the mass of an electron), just by the sheer number of them in the universe, the universe would collapse due to their mass. So other observations have served to estimate an upper-bound for the neutrino mass. Using cosmic microwave background data and other methods, the current conclusion is that their average mass probably does not exceed 0.3 eV/c2 Thus, the normal forms of neutrinos cannot be responsible for the measured dark matter component from cosmology.
    Hot dark matter was popular for a time in the early 1980s, but it suffers from a severe problem: since all galaxy-size density fluctuations get washed out by free-streaming, the first objects which can form are huge supercluster-size pancakes, which then were theorised somehow to fragment into galaxies. Deep-field observations clearly show that galaxies formed at early times, with clusters and superclusters forming later as galaxies clump together, so any model dominated by hot dark matter is seriously in conflict with observations.

3. Candidate

3.1 WIMPs
    In astrophysics, weakly interacting massive particles or WIMPs, are hypothetical particles serving as one possible solution to the dark matter problem. These particles interact through the weak force and gravity, and possibly through other interactions no stronger than the weak force. Because they do not interact through electromagnetism they cannot be seen directly, and because they do not interact through the strong nuclear force they do not interact strongly with atomic nuclei. This combination of properties gives WIMPs many of the properties of neutrinos, except for being far more massive and therefore slower. They don't interact with normal particles, but interact with other WIMPs. When this happens they emit gamma rays. This means they should be able to interact with the Higgs field.
    WIMP-like particles are predicted by R-parity-conserving supersymmetry, a popular type of extension to the standard model of particle physics, although none of the large number of new particles in supersymmetry have been observed. The main theoretical characteristics of a WIMP are: Interactions only through the weak nuclear force and gravity, or possibly other interactions with cross-sections no higher than the weak scale; Large mass compared to standard particles (WIMPs with sub-GeV masses may be considered to be light dark matter).
     Because of their lack of electromagnetic interaction with normal matter, WIMPs would be dark and invisible through normal electromagnetic observations. Because of their large mass, they would be relatively slow moving and therefore cold. Their relatively low velocities would be insufficient to overcome the mutual gravitational attraction, and as a result WIMPs would tend to clump together. WIMPs are considered one of the main candidates for cold dark matter, the others being massive compact halo objects (MACHOs) and axions. (These names were deliberately chosen for contrast, with MACHOs named later than WIMPs.) Also, in contrast to MACHOs, there are no known stable particles within the standard model of particle physics that have all the properties of WIMPs. The particles that have little interaction with normal matter, such as neutrinos, are all very light, and hence would be fast moving or hot.
     The WIMP fits the model of a relic dark matter particle from the early Universe, when all particles were in a state of thermal equilibrium. For sufficiently high temperatures, such as existed in the early Universe, the dark matter particle and its antiparticle would have been both forming from and annihilating into lighter particles. As the Universe expanded and cooled, the average thermal energy of these lighter particles decreased and eventually became insufficient to form a dark matter particle-antiparticle pair. The annihilation of the dark matter particle-antiparticle pairs, however, would have continued, and the number density of dark matter particles would have begun to decrease exponentially. Eventually, however, the number density would become so low that the dark matter particle and antiparticle interaction would cease, and the number of dark matter particles would remain (roughly) constant as the Universe continued to expand. Particles with a larger interaction cross section would continue to annihilate for a longer period of time, and thus would have a smaller number density when the annihilation interaction ceases. Based on the current estimated abundance of dark matter in the Universe, if the dark matter particle is such a relic particle, the interaction cross section governing the particle-antiparticle annihilation can be no larger than the cross section for the weak interaction. If this model is correct, the dark matter particle would have the properties of the WIMP.
Experimental detection
    Because WIMPs may only interact through gravitational and weak forces, they are extremely difficult to detect. However, there are many experiments underway to attempt to detect WIMPs both directly and indirectly. Halo WIMPs may, as they pass through the Sun, interact with solar protons and helium nuclei. Such an interaction would cause a WIMP to lose energy. The resulting slower WIMP would not have enough energy to escape the gravitational pull of the sun and thus would be "captured" by the Sun. As more and more WIMPs thermalize inside the Sun, they begin to annihilate with each other, forming a variety of particles including high-energy neutrinos. These neutrinos may then travel to the Earth to be detected in one of the many neutrino telescopes, such as the Super-Kamiokande detector in Japan. The number of neutrino events detected per day at these detectors depends upon the properties of the WIMP, as well as on the mass of the Higgs boson. Similar experiments are underway to detect neutrinos from WIMP annihilations within the Earth and from within the galactic center.
3.2 Axion
     Hypothetical subatomic particle required by some theories to explain details of the workings of quantum chromodynamics (QCD). If axions do exist, they each have a mass of around 10-5 eV, less than 10-12 of the mass of a proton, but there could be so many of them in the Universe in the space between the stars and galaxies, that they contribute a large proportion of the overall mass of the Universe, in the form of dark matter.
    The axion was postulated to explain why CP violation is not observed in interactions involving the strong force, although it should be according to simple versions of QCD. It enables the theory to take account of the way the particle world distinguishes between left and right in some interactions, but not others. This is described in terms of axial symmetry, hence the name.
3.3 Photino
    A particle predicted by the theory of supersymmetry, but not yet detected in any experiment. The photino is the supersymmetric counterpart of the photon, and would probably be the lightest supersymmetric particle. It would have spin 1/2, making it fermion.
3.4 MACHOs
    Massive astrophysical compact halo object, or MACHO, is a general name for any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a body composed of normal baryonic matter, which emits little or no radiation and drifts through interstellar space unassociated with any planetary system. Since MACHOs would not emit any light of their own, they would be very hard to detect. MACHOs may sometimes be black holes or neutron stars as well as brown dwarfs or unassociated planets. White dwarfs and very faint red dwarfs have also been proposed as candidate MACHOs. The term was coined by astrophysicist Kim Griest, in contrast to WIMPs, another proposed form of dark matter.
Detection
    A MACHO may be detected when it passes in front of or nearly in front of a star and the MACHO's gravity bends the light, causing the star to appear brighter in an example of gravitational lensing known as gravitational microlensing. Several groups have searched for MACHOs by searching for the microlensing amplification of light. These groups have ruled out dark matter being explained by MACHOs with mass in the range 10-8 solar masses to 100 solar masses. One group, the MACHO collaboration, claims to have found enough microlensing to predict the existence of many MACHOs with mass of about 0.5 solar masses, enough to make up perhaps 20% of the dark matter in the galaxy. This suggests that MACHOs could be white dwarfs or red dwarfs which have similar masses. However, red and white dwarfs are not completely dark; they do emit some light, and so can be searched for with the Hubble Telescope and with proper motion surveys. These searches have ruled out the possibility that these objects make up a significant fraction of dark matter in our galaxy. Another group, the EROS2 collaboration does not confirm the signal claims by the MACHO group. They did not find enough microlensing effect with a sensitivity higher by a factor 2. Observations using the Hubble Space Telescope's NICMOS instrument showed that less than one percent of the halo mass is composed of red dwarfs. This corresponds to a negligible fraction of the dark matter halo mass. Therefore, the missing mass problem is not solved by MACHOs.
Types of MACHOs
    MACHOs may sometimes be considered to include black holes. Black holes are truly black in that they emit no light and any light shone upon them is absorbed and not reflected. It is thought possible that there is a halo of black holes surrounding the galaxy. A black hole can sometimes be detected by the halo of bright gas and dust that forms around it as an accretion disc being pulled in by the black hole's gravity. Such a disk can generate jets of gas that are shot out away from the black hole because it cannot be absorbed quickly enough. An isolated black hole, however, would not have an accretion disk and would only be detectable by gravitational lensing. Cosmologists doubt they make up a majority of dark matter because the black holes are at isolated points of the galaxy. The largest contributor to the missing mass must be spread throughout the galaxy to balance the gravity. A minority of physicists, including Chapline and Laughlin, believe that the widely accepted model of the black hole is wrong and needs to be replaced by a new model, the dark-energy star; in the general case for the suggested new model, the cosmological distribution of dark energy would be slightly lumpy and dark-energy stars of primordial type might be a possible candidate for MACHOs.
     Neutron stars are somewhat like black holes, but are not heavy enough to collapse completely, instead forming into a material rather like that of an atomic nucleus (sometimes informally called neutronium). After sufficient time these stars could radiate away enough energy to become cold enough that they would be too faint to see. Likewise, old white dwarfs may also become cold and dead, eventually becoming black dwarfs, although the universe is not thought to be old enough for any stars to have reached this stage.
    The next candidate for MACHOs is the brown dwarfs mentioned above. Brown dwarfs are sometimes called "failed stars" as they do not have enough mass for nuclear fusion to begin and simply glow a dull brown. Hence, their only source of energy is released through their own gravitational contraction, and may therefore be faintly visible in some circumstances. Brown dwarfs are about thirteen to seventy-five times the mass of Jupiter.
Theoretical considerations
     Theoretical work simultaneously also showed that ancient MACHOs are not likely to account for the large amounts of dark matter now thought to be present in the universe. The Big Bang as it is currently understood could not have produced enough baryons and still be consistent with the observed elemental abundances, including the abundance of deuterium. Furthermore, separate observations of baryon acoustic oscillations, both in the cosmic microwave background and large-scale structure of galaxies, set limits on the ratio of baryons to the total amount of matter. These observations show that a large fraction of non-baryonic matter is necessary regardless of the presence or absence of MACHOs.
3.5 RAMBOs
    In astronomy, a RAMBO or robust association of massive baryonic objects is a dark cluster made of brown dwarfs or white dwarfs. RAMBOs were proposed by Moore and Silk in 1995. They may have an effective radius between 1 and 15 pc, with masses in the range 10–100,000 Mʘ (solar masses).The dynamics of these objects, if they do exist, must be quite different from that of standard star clusters. With a very narrow mass range (all brown dwarfs or white dwarfs), the evaporation rate of these RAMBOs should be very slow as predicted by the evolution of simulated mono-component cluster models. Theoretically, these very long-lived objects could exist in large numbers. The presence of a clustered thick disk-like component of dark matter in the Galaxy has been suggested by Sanchez-Salcedo (1997, 1999) and Kerins (1997).

4. Substitute

     Although the existence of dark matter is generally accepted by the mainstream scientific community, there is no generally agreed direct detection of it. Other theories, including MOND (Modified Newtonian Dynamics) and TeVeS (Tensor–vector–scalar gravity), are some alternative theories of gravity proposed to try to explain the anomalies for which dark matter is intended to account.