<sect1 id="ai-darkmatter">

<sect1info>
<author>
<firstname>Jasem</firstname>
<surname>Mutlaq</surname>
<affiliation><address>
<email>mutlaqja@ku.edu</email>
</address></affiliation>
</author>
</sect1info>

<title>Dark Matter</title>

<para>
Scientists are now quite comfortable with the idea that 90% of the
mass is the universe is in a form of matter that cannot be seen.
</para>

<para>
Despite comprehensive maps of the nearby universe that cover the
spectrum from radio to gamma rays, we are only able to account of 10%
of the mass that must be out there.  As Bruce H. Margon, an astronomer
at the University of Washington, told the New York Times in 2001:
"It's a fairly embarrassing situation to admit that we can't find 90
percent of the universe".
</para>

<para>
The term given this "missing mass" is <firstterm>Dark
Matter</firstterm>, and those two words pretty well sum up everything
we know about it at this point.  We know there is "Matter", because we
can see the effects of its gravitational influence.  However, the
matter emits no detectable electromagnetic radiation at all, hence it
is "Dark".  There exist several theories to account for the missing
mass ranging from exotic subatomic particles, to a population of
isolated black holes, to less exotic brown and white dwarfs.  The
term 'missing mass' might be misleading, since the mass itself is not
missing, just its light.  But what is exactly dark matter and how do
we really know it exists, if we can't see it?
</para>

<para>
The story began in 1933 when Astronomer Fritz Zwicky was studying the
motions of distant and massive clusters of galaxies, specifically the
Coma cluster and the Virgo cluster.  Zwicky estimated the mass of each
galaxy in the cluster based on their luminosity, and added up all of
the galaxy masses to get a total cluster mass.  He then made a second,
independent estimate of the cluster mass, based on measuring the
spread in velocities of the individual galaxies in the cluster.
To his suprise, this second <firstterm>dynamical mass</firstterm>
estimate was <emphasis>400 times</emphasis> larger than the estimate
based on the galaxy light.
</para>

<para>
Although the evidence was strong at Zwicky's time, it was not until
the 1970s that scientists began to explore this discrepancy
comprehensively.  It was at this time that the existence of Dark
Matter began to be taken seriously.  The existence of such matter
would not only resolve the mass deficit in galaxy clusters; it
would also have far more reaching consequences for the evolution and
fate of the universe itself.
</para>

<para>
Another phenomenon that suggested the need for dark matter is the
rotational curves of <firstterm>Spiral Galaxies</firstterm>.  Spiral Galaxies
contain a large population of stars that orbit the Galactic center on
nearly circular orbits, much like planets orbit a star.  Like
planetary orbits, stars with larger galactic orbits are expected to
have slower orbital speeds (this is just a statement of Kepler's 3rd Law).  
Actually, Kepler's 3rd Law only applies to stars near the perimeter of a Spiral
Galaxy, because it assumes the mass enclosed by the orbit to be
constant.
</para>

<para>
However, astronomers have made observations of the orbital speeds of
stars in the outer parts of a large number of spiral galaxies, and
none of them follow Kepler's 3rd Law as expected.  Instead of falling
off at larger radii, the orbital speeds remain remarkably constant.
The implication is that the mass enclosed by larger-radius orbits
increases, even for stars that are apparently near the edge of the
galaxy.  While they are near the edge of the luminous part of the
galaxy, the galaxy has a mass profile that apparently continues well
beyond the regions occupied by stars.
</para>

<para>
Here is another way to think about it: Consider the stars near the
perimeter of a spiral galaxy, with typical observed orbital
velocities of 200 kilometers per second.  If the galaxy consisted of
only the matter that we can see, these stars would very quickly fly
off from the galaxy, because their orbital speeds are four times
larger than the galaxy's escape velocity.  Since galaxies are not seen
to be spinning apart, there must be mass in the galaxy that we are not
accounting for when we add up all the parts we can see.
</para>

<para>
Several theories have surfaced in literature to account for the
missing mass such as WIMPs (Weakly Interacting Massive Particles),
MACHOs (MAssive Compact Halo Objects), primordial black holes, massive
neutrinos, and others; each with their pros and cons.  No single
theory has yet been accepted by the astronomical community, because
we so far lack the means to conclusively test one theory against the
other.
</para>

<tip>
<para>
You can see the galaxy clusters that Professor Zwicky studied to
discover Dark Matter.  Use the KStars Find Object Window
(<keycombo><keycap>Ctrl</keycap><keycap>f</keycap></keycombo>) to
center on "M 87" to find the Virgo Cluster, and on "NGC 4884" to find
the Coma Cluster.  You may have to zoom in to see the galaxies.
Note that the Virgo Cluster appears to be much larger on the
sky.  In reality, Coma is the larger cluster; it only appears smaller
because it is further away.
</para>
</tip>
</sect1>
