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dark matter

Dark matter refers to non-luminous matter particles whose presence is suggested because of the gravitational effects on the rotation rate of visible matter such galaxies and the presence of clusters of galaxies.When the Universe was young, it was nearly smooth and featureless. As it grew older and developed, it became organized. We know that our solar system is organized into planets (including the Earth!) orbiting around the Sun. On a scale much larger than the solar system (about 100 million times larger!), stars collect themselves into galaxies. Our Sun is an average star in an average galaxy called the Milky Way. The Milky Way contains about 100 billion stars. Yes, that’s 100,000,000,000 stars! On still larger scales, individual galaxies are concentrated into groups, or what astronomers call clusters of galaxies.

There are many reasons to believe that the universe is full of “dark matter”, matter that influences the evolution of the universe gravitationally, but is not seen directly in our present observations.

Hot X-ray gas in a cluster of galaxies
An overlay of an optical image of a cluster of galaxies
with an x-ray image of hot gas lying within the cluster

The cluster includes the galaxies and any material which is in the space between the galaxies. The force, or glue, that holds the cluster together is gravity — the mutual attraction of everything in the Universe for everything else. The space between galaxies in clusters is filled with a hot gas. In fact, the gas is so hot (tens of millions of degrees!) that it shines in X-rays instead of visible light. In the image above, the hot X-ray gas (shown in pink) lying between the galaxies is superimposed on an an optical picture of the cluster of galaxies. By studying the distribution and temperature of the hot gas we can measure how much it is being squeezed by the force of gravity from all the material in the cluster. This allows scientists to determine how much total material (matter) there is in that part of space.

Remarkably, it turns out there is five times more material in clusters of galaxies than we would expect from the galaxies and hot gas we can see. Most of the stuff in clusters of galaxies is invisible and, since these are the largest structures in the Universe held together by gravity, scientists then conclude that most of the matter in the entire Universe is invisible. This invisible stuff is called ‘dark matter’, a term initially coined by Fritz Zwicky who discovered evidence for missing mass in galaxies in the 1930s. There is currently much ongoing research by scientists attempting to discover exactly what this dark matter is, how much there is, and what effect it may have on the future of the Universe as a whole.

Hot Dark Matter and Cold Dark Matter

Discussions of dark matter typically consider two extremes

  • Hot Dark Matter
  • Cold Dark Matter

Hot dark matter is composed of particles that have zero or near-zero mass (the neutrinos are a prime example). The Special Theory of Relativity requires that massless particles move at the speed of light and that nearly massless particles move at nearly the speed of light. Thus, such very low mass particles must move at very high velocities and thus form (by the kinetic theory of gases) very hot gases.On the other hand, cold dark matter is composed of objects sufficiently massive that they move at sub-relativistic velocities. They thus form much colder gases. The difference between cold dark matter and hot dark matter is significant in the formation of structure, because the high velocities of hot dark matter cause it to wipe out structure on small scales.

Tutorial on Current Status of Dark Matter

The following is a brief tutorial on this issue 

  1. If inflation is correct the density of the Universe should be exactly the closure density. Luminous stars and galaxies contribute only about 0.5% of the closure density, so 99% of the Universe is in the form of dark matter. We may speculate on what particles could make up this dark matter.
  2. The known neutrinoes have problems as candidates for dark matter because they are relativistic (hot dark matter) and therefore they erase fluctuations on small scales. Thus, relativistic neutrinos could form large structures like superclusters, but would have trouble forming smaller structures like galaxies. These arguments might be at least partially invalidated if one of the types of neutrinos (the tau neutrino is the obvious candidate) is considerably more massive than the electron or muon neutrino.
  3. On smaller scales such as galaxies and clusters of galaxies, dynamical estimates of the mass based on rotation curves or velocity dispersions of galaxies indicate that 90% (not 99%) of the total mass is not seen (“sub-luminous”). This implies that the mass density of the Universe is 10% of the closure density. In this case, the sub-luminous mass could be normal (baryonic) and be locked up in stellar remnants (white dwarfs, neutron stars, black holes) or just in very dim stars called “Brown Dwarfs”. There is recent evidence for possible observation of one of these very dim Brown Dwarfs.
    1. The adjacent image exhibits one recent piece of evidence for undetected matter: the hot gas seen in the X-ray spectrum would have dispersed if it were held in place only the by gravity of the mass that is producing light in this image (the so-called “luminous mass”). The nature of this dark matter, and the associated “missing mass problem”, is one of the fundamental cosmological issues of modern astrophysics.
      1. Although inflation demands that the Universe have a density equal to its critical density (and inflation is necessary to solve certain problems of the standard big bang model like the horizon problem) there has never been any observational evidence to support this high of mass density. Most dynamical studies suggest values of 10-20% of closure density. These studies are based on large scale deviations from Hubble expansion velocities (peculiar velocities).
      2. Large scale structure (e.g. the distribution of galaxies) is very hard to understand, particularly in light of the relatively smooth microwave background as measured by the COBE satellite. One way to accomodate this is to go to a mixed dark matter model in which you have some hot dark matter (for the large scale) and some cold dark matter to act as a seed for galaxy formation. None of those models, however, fit the data using the critical density. The best models to date suggest mixed dark matter and an overall cosmological mass density of 20-30% of closure. Hence, to retain inflation, with its inescapable prediction that the Universe must be flat, requires re-invoking Einstein’s cosmological constant – meaning the universe has vacuum energy (negative pressure) and is currently accelerating. This makes our cosmology complicated but much data is pointing this way.
      3. Supernova 1987a neutrino time of flight studies as well as the Solar Neutrino experiment are consistent with the neutrino having a mass, but a very small mass, not one that can cosmologically dominate. We cannot currently test for various supersymmetric particles which would only be created at very high energy (e.g. the early universe) – so there remain many viable potential particles that are consistent with the Standard Model of particle physics, that would remain unnoticed in any accelerator experiments.

      Searches for Dark Matter Candidates

      Here are links to two experimental searches for dark matter candidates that could be made of ordinary matter (what astromomers call baryonic matter):For a more extensive discussion of dark matter, see this reference. These particular searches make use of the principle of gravitational lensing in the theory of General Relativity. Finally, do not confuse the term “dark matter” with the term “antimatter”. Here is a discussion of the difference.

      Starting from the number of stars’ and galaxies’ revolutions (on the cluster level), it is possible to measure the mass of the dark matter, and to deduce its distribution. A great quantity of this matter should be within the galaxies, not in the galactic disc but in the form of a halo including the galaxy. Indeed, this configuration allows a stability of the galactic disc. Moreover, certain galaxies have rings perpendicular to the disc and composed of gas, dust and stars. There, the halo of matter would explain the formation and stability required. On the other hand, it is impossible that the dark matter be in the galactic disc, due to the fact that we shoudl then observe an oscillation perpendicular to the disc in the stars movement (which is not the case).

      The study of satellite galaxies (small galaxies turning around other galaxies) obliges to think of very wide halos: approximately 200 or 300 kpc. By comparison, the Sun is located at approximately 8,6 kpc center of our galaxy. The galaxy of Andromède – galaxy nearest to us – is located at 725 kpc, that is to say a little more of the double of the halo radius of our galaxy. These halos could be common between close galaxies.

      In 1933, Swiss astronomer Fritz Zwicky of CalTech decided to study a small group of seven galaxies in the Coma Cluster. Its objective was to calculate the total mass of this cluster by studying the speed (or rather the dispersion speeds) of these seven galaxies. By using Newton laws, he calculated its mass ‘dynamic mass’, then compared it with the ‘luminous mass’, which is the mass calculated from the quantity of light emitted by the cluster (by making to the assumption of a raisonable distribution of the star population in the galaxies).

      The dispersion speeds (or in other words, how the speed of these 7 galaxies differed from each other) is directly related to the cluster’s mass. In fact, a star cluster can be compared with a gas, where the particles would be galaxies. If the gas is hot and light, the dispersion speed of the particles is high. In the extreme case, the particles which have a sufficient speed leave the gas (evaporation). If the gas is cold and heavy, the dispersion speed is weak.

      Zwicky was surprised to note that the speeds observed in the Coma Cluster were very high. The dynamic mass was 400 times larger than the luminous mass! At the time, the methods and the precision of measurements were not accurate enough to be neglected. Moreover, massive objects such as brown dwarf, white dwarf, neutron stars , black holes and in general of poorly non radiating objects were little known. And same for interstellar dust and molecular gas.

      Zwicky announced its observation to its fellows, but they were not interested. Zwicky’s reputation was not so good due to a strong character and its measurements were criticized due to measurement uncertainties.

      The same phenomenon was again observed in 1936 by Sinclair Smith during the calculation of the Virgo Cluster’s total dynamic mass. This one was 200 times more important than Edwin Hubble’s estimate. According to Smith, is could be explained by the presence of matter between the galaxies of the cluster. Moreover, the galaxy clusters were still considered by a great number of astronomers as of temporary structures rather than of stable structures. This explanation was enough to justify excessive speeds.

      At the time, astronomers had other ‘more imporant’questions to solve (such as the expansion of the Universe) and the question of this difference between the dynamic and luminous mass was let aside for several decades.

      The neutrino is a particle introduced for the first time in 1930 by Wolfgang Pauli, before the discovery of the neutron (one year later), and which was detected in 1956 by Frederick Reines and Clyde Cowan. This particle – insensitive to electromagnetic forces and strong nuclear force – is emitted during a beta desintregration, along with an electron. The neutrino doesn’t interacts much with other particles, which makes it a good candidate for dark matter.

      The mass of the neutrino was considered very small, even null. With the problem of the missing mass of the Universe, the physicists wondered if the neutrino would have a nonzero mass. Especially as the neutrino is the most abundant particle in the universe, after the photon. However, the experiments Super-Kamiokande and SNO (Sudbury Observatory Neutrino) revealed a too small mass to consider that this particle would constitute the dark matter. The neutrinos could represent, at best, 18% of the niverse’s total mass.

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