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Alexander Mitov

On the problem of Dark Matter

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Perhaps the most talked about problem in theoretical physics these days is the so called Dark Matter problem. Put simply, Dark Matter is an unknown that parameterizes our inability to describe the observed Universe (at Galactic scales and up). The most compelling evidence for the existence of Dark Matter is the inability of the Newtonian gravity to describe the dynamics of the rotation of Galaxies. The term Dark Matter naturally derives from this context since, assuming that the Newtonian mechanics describes the dynamics, the rotation only depends on the distribution of the gravitational masses in the Galaxies. At present the astronomers believe that we are able to determine fairly well the amount of matter in the Galaxies. Therefore, to reconcile the discrepancy between experiment and theory, we need to 'add' more gravitational matter. And this additional matter has to be 'dark' in the sense that we do not detect it even in our most advanced telescopes.

The interpretation of Dark Matter as some sort of non-standard matter is the most popular nowadays. Such interpretation is appealing from the following point of view: all theoretical models of physics beyond the Standard Model predict new particles and forces which, for a variety of reasons, are presently undetectable. Some of these 'new' particles may even be stable or long lived and could constitute the Dark Matter.

While the above interpretation might well be correct, it is not perfect. I give brief account below. But first let me mention that matter interpretation of Dark Matter is not the only possible solution; the other option is to have a modification to the Newtonian (and Einstein to that effect) dynamics of gravity. While at first this appears as a rather radical approach, it turns out it is both viable and in some sense more natural. I'll return to that below.

So, here are the problems I see with the matter/particle interpretation of Dark Matter:

  1. The Standard Model of elementary particles is otherwise perfect. Various observations of the Universe lead to the rather firm conclusion that at present Dark Matter and Dark Energy account for around 95% of the gravitational mass in the Universe. In other words the matter as we know it accounts for only 5% of whatever's in the Universe! While this stunning fact is an exciting motivation for research, it also has a dark side: it suggests that our current fundamental models fail miserably in describing the real world! At the same time all test aiming at testing these same fundamental lows directly are in perfect agreement with the Standard Model! Let me stress that we are talking about several generations of collider experiments and in certain cases the precision achieved by experiment and theory, and therefore their agreement, is astounding. It is the success of precision physics (see my Blog article) that makes the Standard Model more successful that we ever anticipated.
  2. Supersymmetry and Grand Unification.

(to be continued)

Last Updated ( Wednesday, 24 November 2010 00:20 )