Modern Scientific Understanding

Dark matter is any form of matter that does not self-luminesce at any wavelength (Rubin 1, 73). The implications of dark matter are as far reaching as the overall shape and fate of the universe.
Discovery
    Dark matter was discovered in the 1950s by Vera Ruben when she began recording the Doppler shifts of the 21cm emission line in certain galaxies.  Radio astronomer Vic Carlson explained the 21cm emission line as being caused by Hydrogen's single electron flipping its spin because of its proton's magnetic field.  From these Doppler shifts Ruben was able to record velocity rotation curves of those galaxies.  She observed that, in spiral galaxies, hydrogen gas rotates about the galactic center with the same velocity regardless of its distance from the center. To say that dark matter was discovered is slightly misleading; the presence of dark matter can only be inferred from the motion of hydrogen in the galactic disk.  To understand how dark matter is inferred, compare the graph of velocity for planets in our solar system in terms of semi-major axis with a similar graph for matter in the disk of a spiral galaxy.
    The graph of the solar system changes according to Kepler's laws: the velocity of rotation decreases exponentially as the distance from the center increases.  If galaxies act in a similar way the graph of their rotation velocity would drop off like the graph of local rotation velocity, yet, this clearly does not happen on the galactic level.  For an explination of Kepler's thrid law go here.

Adaped from graphs made by Vera Rubin in Bubles, voids and bumps in time pages 86 and 91

    The graph of rotation curves made by Rubin for galaxies NGC 1417, 1085, 7083, 3145 show no decrease in velocity at any distance from the center (Rubin 1, 91).  Either Kepler's laws fall apart on large scales or the luminous matter in these spiral galaxies does not accurately represent all the matter.  A hypothetical shell of dark matter that permeated and extended around the luminous boundaries of these galaxies could have enough collective gravity to adjust the rotation of matter in the disk to the observed quantities (Begelman & Rees 88).  The primary models of this dark matter shell that I have researched are baryonic and particle based theories that involve actual dispersed matter in various quantity, size and type.  More complicated theories that involve strong magnetic fields and dispersed gravitational fields have also been proposed also.
Candidates
    Baryonic
    Baryonic matter can be defined as any thing composed of neutrons and protons (Hawley & Holcomb, 364).  However, for the purposes of this paper baryonic will indicate the larger types of dark matter: brown dwarfs, white dwarfs and any MAssive Compact Halo Objects.  The baryonic types of dark matter have an advantage over other types because baryonic matter becuase they can be detected using simple methods.  Simplicity, however, may not be enough to save the baryonic theories.
 
    Understanding star formation will aid in understanding the origins of brown and white dwarfs.  In a galactic disk, stars are formed from Barnard objects, regions of slightly denser gas that gravitationally accretes surrounding gas (Kaufmann & Freedman, 498).  These protostars are massive objects that contract under their own gravity, heating their core to extreme temperatures.  At about a few million degrees kelvin the star's core begins thermo-nuclear reactions (Kaufmann & Freedman, 501).  Stars that have regular thermo-nuclear reactions range from 15 to 0.8 solar masses (Kaufmann & Freedman, 501).  However, not all protostars can accrete enough mass to begin thermo-nuclear reactions.  These hypothetical stars become brown dwarfs with masses around 0.07 solar masses (Begelman & Rees, 91).

 Image of brown dwarf orbiting Gliese 229, taken from Universe page 184

    White dwarfs are stars old burnt out stars that shed their outer layers rather than supernova.  What remains is a dense core of carbon, glowing much like the embers of camp fire.  Only through a telescope can these low luminosity stars be detected (Kaufmann & Freedman, 475).  The upper range of mass for white dwarfs is about 1.4 solar masses (Begelman & Rees, 91).  Then, how can dark brown dwarfs and dim white dwarfs be detected?
    Fritz Zwicky suggests that a dim or dark object that passes directly in front a star will bend the background light with a characteristic pattern (Zwicky, 215).  These patters, if observed, would then indicate the presence of some dark baryonic object passing between the observer and the star.  Astronomers have been able estimate the potential number of observable gravitational lensing events in the Large Magelanic Cloud, and they have found the actual observed events to be significantly less than predicted (Wambsganss, 248).  While baryonic matter may not account for all dark matter, the observed lensing events do prove that some of the dark matter is baryonic.  The remaining dark matter could be sub-atomic particles.
Candidates
    Neutrinos
    There have been multiple particle candidates: gravitinos, axions, photinos and others, nevertheless, neutrinos have the distinct advantage of being detectable (Rubin 2, 127).  Neutrinos are weakly interacting particles with a neutral charge and massive abundance--in the universe's current state 109 neutrinos exist for every nuclear particle (Weinberg, 117).  Until recently, neutrinos have existed only as a tool to conserve the lepton number of physical reactions.  In the first stage of the proton-proton chain reaction that powers a star's core, a neutrino is employed to conserve the lepton number of the reaction:
                                                              H1 + H1 - --H2 + Positron + Neutrino
                                             charge        +1     +1       +1        +1               0
                                            baryon #      +1     +1       +2          0               0
                                             lepton #        0       0         0         -1             +1
In one ten second period, supernova 1987A ejected 10 to the 58 power neutrinos into space with a total energy of 10 to the 46 power joules, 100 times more total energy the sun has emitted in its life (Kaufmann & Freedman, 556).  Some neutrinos directed towards Earth were detected by various neutrino detectors around the world. From information obtained by the neutrino detectors, particle physicists were able to determine that not only are neutrinos real but they also have a minute mass.  Physicists were able to determine the non-zero rest mass of neutrinos by timing the space between the arrival of photons from 1987A and the arrival of neutrinos from the same source.  Currently the rest mass of the neutrino is unknown, however, the upper limit is around 15eV (Kaufmann &Freedman, 557).  15eV is extremely small, for comparison, the electron weights about 511,000eV and the baryonic proton weighs about 938,260,000,000eV (Kaufmann & Freedman, 736).  Weinberg's neutrino to nuclear particle ratio indicates that neutrinos with a weight of about 15eV will contribute about as much mass to the universe as nuclear particles.
    Though neutrinos are nearly mass less their abundance and weak interaction make neutrinos an excellent dark matter candidate.  Calcluations made by E.J. Zita, asuming a reasonable distribution of neutrinos and critical density equal to 10 to the minius 26 power of kg per square meter, show that neutrinos could account for all dark matter in a closed universe if their weight was only 1eV.
For more information on neutrino detection, visit the Super Kaminkande web page at http://www.phys.washington.edu/~superk/.
How does dark matter effect astronomy and cosmology?
    Astronomy has long been considered the study of the heavens.  However, the advent of dark matter has revealed that astronomers are only studying the 1% of the heavens that are self-luminescent.  Some astronomers have even refused to accept the quest for dark matter because observation is the only valid way to know something (Begelman & Ress, 94).  The overall effect of dark matter on astronomy is small. The 1% of the universe that is self-luminescent still remains to be known.
    Cosmology, the study of the universe, is directly concerned with dark matter for its role in structure formation of the universe and the shape of the universe.  In the standard model of universal genesis, the amount and type of dark matter in the beginning affects the structure formation and age of the resulting universe (Schramm, 1).  The structure formation and age of the universe are primary aspects of cosmology.  In cosmology, omega is the actual density of the universe over the critical density where critical density is the amount of matter that will halt the expansion of the universe.  If ommega=1 the universe is flat and expansion halts, if omega>1 the universe is spherical and will eventually contract, and if omega<1 the universe is hyperbolic and will expand forever.  In a universe without dark matter omega<1.  The search for dark matter is also the search for a stable universe that could exist in its present state forever.


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