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author | toma <toma@283d02a7-25f6-0310-bc7c-ecb5cbfe19da> | 2009-11-25 17:56:58 +0000 |
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committer | toma <toma@283d02a7-25f6-0310-bc7c-ecb5cbfe19da> | 2009-11-25 17:56:58 +0000 |
commit | ce599e4f9f94b4eb00c1b5edb85bce5431ab3df2 (patch) | |
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Copy the KDE 3.5 branch to branches/trinity for new KDE 3.5 features.
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diff --git a/doc/kstars/darkmatter.docbook b/doc/kstars/darkmatter.docbook new file mode 100644 index 00000000..7f094770 --- /dev/null +++ b/doc/kstars/darkmatter.docbook @@ -0,0 +1,119 @@ +<sect1 id="ai-darkmatter"> + +<sect1info> +<author> +<firstname>Jasem</firstname> +<surname>Mutlaq</surname> +<affiliation><address> +</address></affiliation> +</author> +</sect1info> + +<title>Dark Matter</title> +<indexterm><primary>Dark Matter</primary> +</indexterm> + +<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: <citation>It's a fairly embarrassing situation to admit that we +can't find 90 percent of the universe</citation>. </para> + +<para> The term given this <quote>missing mass</quote> 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 +<quote>Matter</quote>, because we can see the effects of its +gravitational influence. However, the matter emits no detectable +electromagnetic radiation at all, hence it is <quote>Dark</quote>. +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 <quote>missing +mass</quote> 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 cannot 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 <acronym>WIMP</acronym>s (Weakly Interacting +Massive Particles), <acronym>MACHO</acronym>s (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 action="simul">&Ctrl;<keycap>F</keycap></keycombo>) to +center on <quote>M 87</quote> to find the Virgo Cluster, and on +<quote>NGC 4884</quote> 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> |