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0001 <sect1 id="ai-darkmatter">
0003 <sect1info>
0004 <author>
0005 <firstname>Jasem</firstname>
0006 <surname>Mutlaq</surname>
0007 <affiliation><address>
0008 </address></affiliation>
0009 </author>
0010 </sect1info>
0012 <title>Dark Matter</title>
0013 <indexterm><primary>Dark Matter</primary>
0014 </indexterm>
0016 <para>
0017 Scientists are now quite comfortable with the idea that 90% of the
0018 mass in the universe is in a form of matter that cannot be seen.
0019 </para>
0021 <para>Despite comprehensive maps of the nearby universe that cover
0022 the spectrum from radio to gamma rays, we are only able to account of
0023 10% of the mass that must be out there.  As Bruce H. Margon, an
0024 astronomer at the University of Washington, told the New York Times in
0025 2001: <quote>It's a fairly embarrassing situation to admit that we
0026 can't find 90 percent of the universe.</quote>  </para>
0028 <para>The term given this <quote>missing mass</quote> is
0029 <firstterm>Dark Matter</firstterm>, and those two words pretty well
0030 sum up everything we know about it at this point.  We know there is
0031 <quote>Matter</quote>, because we can see the effects of its
0032 gravitational influence.  However, the matter emits no detectable
0033 electromagnetic radiation at all, hence it is <quote>Dark</quote>.
0034 There exist several theories to account for the missing mass ranging
0035 from exotic subatomic particles, to a population of isolated black
0036 holes, to less exotic brown and white dwarfs.  The term <quote>missing
0037 mass</quote> might be misleading, since the mass itself is not
0038 missing, just its light.  But what is exactly dark matter and how do
0039 we really know it exists, if we cannot see it?  </para>
0041 <para>
0042 The story began in 1933 when Astronomer Fritz Zwicky was studying the
0043 motions of distant and massive clusters of galaxies, specifically the
0044 Coma cluster and the Virgo cluster.  Zwicky estimated the mass of each
0045 galaxy in the cluster based on their luminosity, and added up all of
0046 the galaxy masses to get a total cluster mass.  He then made a second,
0047 independent estimate of the cluster mass, based on measuring the
0048 spread in velocities of the individual galaxies in the cluster.
0049 To his surprise, this second <firstterm>dynamical mass</firstterm>
0050 estimate was <emphasis>400 times</emphasis> larger than the estimate
0051 based on the galaxy light.
0052 </para>
0054 <para>
0055 Although the evidence was strong at Zwicky's time, it was not until
0056 the 1970s that scientists began to explore this discrepancy
0057 comprehensively.  It was at this time that the existence of Dark
0058 Matter began to be taken seriously.  The existence of such matter
0059 would not only resolve the mass deficit in galaxy clusters; it
0060 would also have far more reaching consequences for the evolution and
0061 fate of the universe itself.
0062 </para>
0064 <para>
0065 Another phenomenon that suggested the need for dark matter is the
0066 rotational curves of <firstterm>Spiral Galaxies</firstterm>.  Spiral Galaxies
0067 contain a large population of stars that orbit the Galactic center on
0068 nearly circular orbits, much like planets orbit a star.  Like
0069 planetary orbits, stars with larger galactic orbits are expected to
0070 have slower orbital speeds (this is just a statement of Kepler's 3rd Law).
0071 Actually, Kepler's 3rd Law only applies to stars near the perimeter of a Spiral
0072 Galaxy, because it assumes the mass enclosed by the orbit to be
0073 constant.
0074 </para>
0076 <para>
0077 However, astronomers have made observations of the orbital speeds of
0078 stars in the outer parts of a large number of spiral galaxies, and
0079 none of them follow Kepler's 3rd Law as expected.  Instead of falling
0080 off at larger radii, the orbital speeds remain remarkably constant.
0081 The implication is that the mass enclosed by larger-radius orbits
0082 increases, even for stars that are apparently near the edge of the
0083 galaxy.  While they are near the edge of the luminous part of the
0084 galaxy, the galaxy has a mass profile that apparently continues well
0085 beyond the regions occupied by stars.
0086 </para>
0088 <para>
0089 Here is another way to think about it: Consider the stars near the
0090 perimeter of a spiral galaxy, with typical observed orbital
0091 velocities of 200 kilometers per second.  If the galaxy consisted of
0092 only the matter that we can see, these stars would very quickly fly
0093 off from the galaxy, because their orbital speeds are four times
0094 larger than the galaxy's escape velocity.  Since galaxies are not seen
0095 to be spinning apart, there must be mass in the galaxy that we are not
0096 accounting for when we add up all the parts we can see.
0097 </para>
0099 <para>Several theories have surfaced in literature to account for the
0100 missing mass such as <acronym>WIMP</acronym>s (Weakly Interacting
0101 Massive Particles), <acronym>MACHO</acronym>s (MAssive Compact Halo
0102 Objects), primordial black holes, massive neutrinos, and others; each
0103 with their pros and cons.  No single theory has yet been accepted by
0104 the astronomical community, because we so far lack the means to
0105 conclusively test one theory against the other.</para>
0107 <tip>
0108 <para>
0109 You can see the galaxy clusters that Professor Zwicky studied to
0110 discover Dark Matter.  Use the &kstars; <guilabel>Find Object</guilabel> window
0111 (<keycombo action="simul">&Ctrl;<keycap>F</keycap></keycombo>) to
0112 center on <quote>M 87</quote> to find the Virgo Cluster, and on
0113 <quote>NGC 4884</quote> to find the Coma Cluster.  You may have to
0114 zoom in to see the galaxies.  Note that the Virgo Cluster appears to
0115 be much larger on the sky.  In reality, Coma is the larger cluster;
0116 it only appears smaller because it is further away.
0117 </para>
0118 </tip>
0119 </sect1>