Thursday, December 24, 2009

Dark Matter

This spat of commentary on dark matter and the rumors of possible detection provide us with a useful staring point to discuss the subject.


I personally have always felt disquiet over the hypothesis itself.  It is and was a deus ex machina lowered unto the empirical data to explain away a failing cosmological theory.  The universe is behaving as if there is more mass as per our physica, ergo there is more mass, ergo let us call it dark matter. 


Now nothing has really changed since.  We still have not detected dark matter convincingly although a generation of physicists has marched past.  This means that the first cohort of skeptics have left the scene to be replaced with a fresh cohort who learned already in high school about the existence of dark matter.  Two more generations and a vast castle of theory supported by legions of physicists will be elevated to truth.


I apologize for the rough handling, but I am trying to reintroduce the concept of critical skepticism before we tackle this subject.


Of course I am dealing from a different perspective and understand that observed dimensionality is distorted in the direction suggested by the problem that gave rise to dark matter in the first place.  Thus I am very interested in any indication that it can be properly discovered and identified since I suspect that it may not even exist except in the imaginations of physicists.


It is easy to feel smug when you think that a theory is bunkum and wait twenty years to be proven wrong.


Anyway, I plan to chew on this bone for a while to see if it goes anywhere.



December 23 2009

Dark Matter Particles and Dark Galaxy


If they are confirmed by further observations that will begin next year, they would rank as one of the most important recent advances in physics and understanding of the cosmos. 

The particles showed as two tiny pulses of heat deposited over the course of two years in chunks of germanium and silicon that had been cooled to a temperature near absolute zero. 

The detectors are place half a mile down to avoid them being effected by background radiation. 

But the scientists still said there was more than a 20 percent chance that the pulses were caused by fluctuations in the background radioactivity of their cavern, so the results were tantalizing, but not definitive.

In this new data set we indeed see two events with characteristics consistent with those expected from WIMPs. However, there is also a chance that both events could be due to background particles. Scientists have a strict set of criteria for determining whether a new discovery has been made. The ratio of signal to background events must be large enough that there is no reasonable doubt. Typically there must be fewer than one chance in a thousand of the signal being due to background. In this case, a signal of about five events would have met those criteria. We estimate that there is about a one in four chance to have seen two backgrounds events, so we can make no claim to have discovered WIMPs. Instead we say that the rate of WIMP interactions with nuclei must be less than a particular value that depends on the mass of the WIMP. The numerical values obtained for these interaction rates from this data set are more stringent than those obtained from previous data for most WIMP masses predicted by theories. Such upper limits are still quite valuable in eliminating a number of theories that might explain dark matter.

What comes next? While the same set of detectors could be operated at Soudan for many more years to see if more WIMP events appear, this would not take advantage of new detector developments and would try the patience of even the most stalwart experimenters (not to mention theorists). A better way to increase our sensitivity to WIMPs is to increase the number (or mass) of detectors that might see them, while still maintaining our ability to keep backgrounds under control. This is precisely what CDMS experimenters (and many other collaborations worldwide) are now in the process of doing. By summer of 2010, we hope to have about three times more germanium nuclei sitting near absolute zero at Soudan, patiently waiting for WIMPs to come along and provide the perfect billiard ball shots that will offer compelling evidence for the direct detection of dark matter in the laboratory.

Soudan Underground Lab

Dark Galaxy Evidence: New evidence has been discovered by an international team led by astronomers from the National Science Foundation’s Arecibo Observatory and from Cardiff University in the United Kingdom that VIRGOHI 21, a mysterious cloud of hydrogen in the Virgo Cluster 50 million light-years from the Earth, is a Dark Galaxy, emitting no starlight. Their results not only indicate the presence of a dark galaxy but also explain the long-standing mystery of its strangely stretched neighbour. Skeptics of the dark-matter interpretation argue that VIRGOHI21 is simply a tidal tail of the nearby galaxy NGC 4254

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