The Dark Matter

 The Dark Matter

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Two people have been credited for determining the need for dark matter.  The first was astronomer Jan Oort, who back in 1932 measured the perpendicular motions of nearby stars relative to the disk of our galaxy.  By studying the gravitational influence of the disk on these stars he was able to calculate the mass of the disk.  The value he determined was twice as much as that inferred by the visible stars and nebulae in that region.  A year later in 1933 astronomer Fritz Zwicky estimated the mass of a group of galaxies by measuring their brightness.  When Zwicky used a different method of calculating the mass of the cluster he found it to be 400 times the value from directly inferring the mass from the brightness of the cluster.  In both cases Oort and Zxwicky came to the conclusion that the visible galaxies only accounted for 10% of the mass needed to keep them gravitationally in the cluster.    

            Based on 70 years of accumulated observations of the motions of galaxies and the expansion of the universe, most astronomers believe that as much as 90% of the stuff constituting the universe may be objects or particles that cannot be seen.  In other words, most of the universe’s matter does not radiate – it provides no glow that we can detect in the electromagnetic spectrum.        

            Now in the 21st century, Zwicky and Oort’s theories have been further enhanced.  We have already observed in clusters of galaxies that the motion of galaxies within a cluster suggests that they are bound by a total gravitational force due to about 5-10 times as much matter as can be accounted for from luminous matter in said galaxies.  But it is also theorized that within an individual galaxy, you can measure the rate of rotation of the stars about the galactic center of rotation.  The resultant “rotation curve” is simply related to the distribution of matter in the galaxy.  The outer stars in the galaxies seem to rotate too fast for the amount of matter that we see in the galaxy.  Again, we need about 5 times more matter than we can see via electromagnetic radiation.  These results can be explained by assuming that there is a “dark matter halo” surrounding every galaxy.

            Exactly what dark matter is really composed of is purely theoretical at this time.  There are two main groups of plausible explanations.  The first group is made up of Massive Compact Halo Objects (MACHOs).  Machos are non-luminous objects, meaning they do no reflect light, that make up halos around galaxies.  Astronomers believe that Machos are made up primarily of red dwarf stars and black holes.  Red dwarf stars are an accumulation of hydrogen which has collected under it’s own gravity.  Due to the relatively low mass of red dwarfs, they do not have enough gravity to ignite when they form.  Therefore they are non-luminous, but they do have a large enough mass which can account from gravitational effects.  Black Holes on the other hand have an over abundance of matter.  Due to it’s own enormous gravity all that matter collapses into a relatively small area.  Since the black hole is so dense, nothing that comes close to it can escape it’s immense gravitational field, including light.  For this reason, Black Holes are truly black.  Together in a large halo, Black Holes and Red dwarfs theoretically would comprise enough matter to account for the observed motion of galaxies and the luminous objects within them.

            The second theory group for dark matter are called Weakly Interactive Massive Particles (WIMPs).  Particle physicists theorize that WIMPs are tiny non-baryonic particles that usually only interact with baryonic particles gravitationally.  Therefore they would pass through normal matter.  The problem with detecting WIMPs is that they rarely interact with normal matter other than gravitationally.  In order for WIMPs to be a plausible candidate for the explanation of dark matter there must be millions of WIMPs passing through normal matter every few seconds.  MACHO’s are sought after by astronomers, and WIMPs by physicists.  But most astronomers would agree that MACHOs alone do not make up enough accountable mass. 

            From NASA’s Hubble telescope, astronomers have been able to estimate that red dwarfs comprise of only 6% of the mass making up galactic halo matter.  Another way of detecting dark matter halo objects is by observing a phenomenon called Gravitational Lensing.  It occurs when a red dwarf or black hole passes between a star or galaxy and an observer on Earth.  The MACHO focuses the light rays due to the MACHOs high gravitational field causing the light source to appear brighter to the observer.  This effect was actually proven by Einstein in 1919. 

            Whatever dark matter turns out to be, we know for certain that the universe contains large amounts of it.  For every gram of glowing material we can detect, there may be tens of grams of dark matter out there.


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