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120 lines
5.3 KiB
120 lines
5.3 KiB
<sect1 id="ai-darkmatter">
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<sect1info>
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<author>
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<firstname>Jasem</firstname>
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<surname>Mutlaq</surname>
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<affiliation><address>
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</address></affiliation>
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</author>
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</sect1info>
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<title>Dark Matter</title>
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<indexterm><primary>Dark Matter</primary>
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</indexterm>
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<para>
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Scientists are now quite comfortable with the idea that 90% of the
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mass is the universe is in a form of matter that cannot be seen.
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</para>
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<para> Despite comprehensive maps of the nearby universe that cover
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the spectrum from radio to gamma rays, we are only able to account of
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10% of the mass that must be out there. As Bruce H. Margon, an
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astronomer at the University of Washington, told the New York Times in
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2001: <citation>It's a fairly embarrassing situation to admit that we
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can't find 90 percent of the universe</citation>. </para>
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<para> The term given this <quote>missing mass</quote> is
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<firstterm>Dark Matter</firstterm>, and those two words pretty well
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sum up everything we know about it at this point. We know there is
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<quote>Matter</quote>, because we can see the effects of its
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gravitational influence. However, the matter emits no detectable
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electromagnetic radiation at all, hence it is <quote>Dark</quote>.
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There exist several theories to account for the missing mass ranging
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from exotic subatomic particles, to a population of isolated black
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holes, to less exotic brown and white dwarfs. The term <quote>missing
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mass</quote> might be misleading, since the mass itself is not
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missing, just its light. But what is exactly dark matter and how do
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we really know it exists, if we cannot see it? </para>
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<para>
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The story began in 1933 when Astronomer Fritz Zwicky was studying the
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motions of distant and massive clusters of galaxies, specifically the
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Coma cluster and the Virgo cluster. Zwicky estimated the mass of each
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galaxy in the cluster based on their luminosity, and added up all of
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the galaxy masses to get a total cluster mass. He then made a second,
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independent estimate of the cluster mass, based on measuring the
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spread in velocities of the individual galaxies in the cluster.
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To his suprise, this second <firstterm>dynamical mass</firstterm>
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estimate was <emphasis>400 times</emphasis> larger than the estimate
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based on the galaxy light.
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</para>
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<para>
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Although the evidence was strong at Zwicky's time, it was not until
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the 1970s that scientists began to explore this discrepancy
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comprehensively. It was at this time that the existence of Dark
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Matter began to be taken seriously. The existence of such matter
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would not only resolve the mass deficit in galaxy clusters; it
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would also have far more reaching consequences for the evolution and
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fate of the universe itself.
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</para>
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<para>
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Another phenomenon that suggested the need for dark matter is the
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rotational curves of <firstterm>Spiral Galaxies</firstterm>. Spiral Galaxies
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contain a large population of stars that orbit the Galactic center on
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nearly circular orbits, much like planets orbit a star. Like
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planetary orbits, stars with larger galactic orbits are expected to
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have slower orbital speeds (this is just a statement of Kepler's 3rd Law).
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Actually, Kepler's 3rd Law only applies to stars near the perimeter of a Spiral
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Galaxy, because it assumes the mass enclosed by the orbit to be
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constant.
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</para>
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<para>
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However, astronomers have made observations of the orbital speeds of
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stars in the outer parts of a large number of spiral galaxies, and
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none of them follow Kepler's 3rd Law as expected. Instead of falling
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off at larger radii, the orbital speeds remain remarkably constant.
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The implication is that the mass enclosed by larger-radius orbits
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increases, even for stars that are apparently near the edge of the
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galaxy. While they are near the edge of the luminous part of the
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galaxy, the galaxy has a mass profile that apparently continues well
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beyond the regions occupied by stars.
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</para>
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<para>
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Here is another way to think about it: Consider the stars near the
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perimeter of a spiral galaxy, with typical observed orbital
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velocities of 200 kilometers per second. If the galaxy consisted of
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only the matter that we can see, these stars would very quickly fly
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off from the galaxy, because their orbital speeds are four times
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larger than the galaxy's escape velocity. Since galaxies are not seen
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to be spinning apart, there must be mass in the galaxy that we are not
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accounting for when we add up all the parts we can see.
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</para>
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<para> Several theories have surfaced in literature to account for the
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missing mass such as <acronym>WIMP</acronym>s (Weakly Interacting
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Massive Particles), <acronym>MACHO</acronym>s (MAssive Compact Halo
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Objects), primordial black holes, massive neutrinos, and others; each
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with their pros and cons. No single theory has yet been accepted by
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the astronomical community, because we so far lack the means to
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conclusively test one theory against the other. </para>
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<tip>
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<para>
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You can see the galaxy clusters that Professor Zwicky studied to
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discover Dark Matter. Use the &kstars; Find Object Window
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(<keycombo action="simul">&Ctrl;<keycap>F</keycap></keycombo>) to
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center on <quote>M 87</quote> to find the Virgo Cluster, and on
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<quote>NGC 4884</quote> to find the Coma Cluster. You may have to
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zoom in to see the galaxies. Note that the Virgo Cluster appears to
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be much larger on the sky. In reality, Coma is the larger cluster;
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it only appears smaller because it is further away.
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</para>
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</tip>
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</sect1>
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