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Tuesday, April 5, 2011

Dark matter

Over the past few years there has been much talk about dark matter in the popular scientific press.  A major reason for having to  hypothesize that such a material exists is the fact that galaxies rotate faster than they should based on the estimated  amount of visible matter in them.  The heavier a galaxy the faster it rotates.  The total weight of visible matter in a galaxy is derived by interpreting  the amount of matter that must exist in stars in order to produce the amount of observed electro-magnetic radiation (light and wave lengths both longer and shorter).  From what I can gather from the popular literature, the scientists are not talking about objects such as planets or dust clouds which are made out of ordinary matter or even of black holes which are made of highly compressed conventional matter,  all of  which are dark.  They are talking about  some totally different type of material which has weight such, possibly,  as the elusive WHIMPS.

Right at the outset, I have to admit that I am a Newtonian physicist.  I taught senior physics in high school and that is the limit of my knowledge of the subject.  Quantum mechanics, relativity, string theory and such topics are total mysteries to me.  Therefore, take what you read, below, with  a pinch or two of salt. 

What triggered off this blog was reading an article in New Scientist (reference coming as soon as I can find it again) which reported that by looking at the infra red spectrum  from other galaxies, they discovered that there are a very large number of red dwarf stars that had not being taken into account in previous estimates of the weight of said galaxies.  The thought struck me: how much less dark matter is it now necessary to hypothesize, to explain the rotational speed of these galaxies because of this finding.  Then I got to thinking about some other possible sources of conventional dark matter (rather than the hypothesized exotic kind)

How about cosmic rays.  Has anyone taken them into account.  Cosmic rays are not rays any more than alpha rays are rays*.  Cosmic rays are nuclei of atoms, some of them at least as far up the periodic table as iron, traveling through space at relativistic speeds.  The flux of these particles out here on the fringes of our galaxy, the Milky Way, is not large but consider that our nearest star is 4.3 light years away.  Lets say that cosmic rays from that distance heading our way will reach us in 5 years (they are relativistic meaning that they are traveling at a considerable portion of the speed of light).  That is a considerable weight of matter.  Add to that, that in the center of the milky way, the flux is undoubtedly much larger and that all of space between the stars have these nuclei whizzing around and it adds up to quite a bit of weight.  It might even be that since they are relativistic, they are significantly 'heavier' than a similar nuclei at rest (Einstein relativity) and hence have a greater gravitational effect than the same number of nuclei at rest.

*Alpha rays are helium nuclei emitted during the decay of certain radioactive isotopes.  They typically are ejected at about 5% of the speed of light.  About a tenth of cosmic rays are helium nucleii (alpha particles) Others are much heavier.  Some cosmic rays are traveling much faster than  alpha 'rays' derived from radioactive sources.

How about light itself.  It has been shown that as a ray of light passes by heavy objects**  it bends.  This is why lensing, which is used to study distant stars and galaxies, works.  If light bends as it comes near an object, it has weight.  Again all the light which is traveling within our galaxy adds weight to it.  How many tons per second does even our modest sized sun put out.  How much matter is in the form of electromagnetic radiation at any given time in our galaxy. If a ton of matter is turned into electromagnetic radiation, is it's Newtonian weight a ton (or more or less).

**One of Einsteins predictions, proven during an eclipse of the sunI have a problem with this also.  Could it not be that this bending is simply light being bent as it passes through the outer atmosphere (solar/stellar wind) similar to the phenomenon that produces our red sun rises and sun sets.

Then there are black holes.  Apparently it is thought that a very large black hole exists at the center of each galaxy.  Black holes by definition do not give out light.  The only way we can infer the weight of black holes is by the speed of rotation of stars around the hole.    The heavier the black hole, the faster the rotation.  It is also hypothesized that there are black holes within our galaxy (not in the center).  These are also undetectable and give weight to the galaxy but no light*.

*New Scientist 7 May, 2011, p18

Then we have some more prosaic considerations.  How about simple shading.  If we observe a distant galaxy, many stars are behind other stars.  The stars behind are shaded by nearer stars so we have to make some sort of estimate of how much of the light output of the galaxy is not visible to us.  I wonder what assumptions are made to  estimate this factor.

Of course there are all the conventional planets, free planets (not rotating around a sun), asteroids, dust, dead suns and so forth.  Are we basing the estimates of the total aggregate weight of these on the concentration in our local part of the Milky Way galaxy.  If we can't see them because they only show up by reflected light, it is hard to estimate how much there is.


At the bottom of the whole controversy is the fact that we can only factor in what we have already found and we are finding new things about our cosmos all the time (like the aformentioned red dwarfs).  I wonder if, as we find more and more, as yet undiscovered sources of conventional mass, the need for exotic dark matter to explain the rotational speed of galaxies will decrease or even disappear.

Remember Ocams Razor.

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