Sunday, July 20, 2014

What Could Cold Dark Matter Be and Future US Searches

A fun thing to read about in physics is cold dark matter (CDM).  From our current understanding of the big bang creation of the universe, known as the lambda-CDM model. cold dark matter is what about 25% of our universe is made of.  But what is cold dark matter?  What is it made of?  From what is known at the present time it is not any known particle at all.  It is not part of the standard model of particle physics.  It must be part of physics beyond the standard model.  That is why there have been many dark matter searches in the past.  All of them unsuccessful in detecting CDM.  In fact the US recently announced the three projects that it will fund in our future attempts to detect cold dark matter.  A good article describing these is here 
 I've blogged about CDM in the past here, and here,
CDM's presence is only known from the gravitational attraction that it has to other matter.  It doesn't interact with the known particles and forces other than gravity.  All attempts to detect it in the lab have failed.  A few days ago I posted on the following question "Could dark matter particles that don't couple to quarks or leptons have been produced?".  Here's the statement of my question and one of the answers I got.
"With what we know about physics, is it possible that when the universe 'began', around when quarks and leptons were produced, another particle, which doesn't couple to either quarks, leptons or photons was also produced ? The only other way that we can observe its existence is via the effects of its gravitational field. In others words, some ''dark-matter-particle'' that doesn't interact with known forms of matter, except through gravity?"
Answer: Yes, there have been suggestions that such particles exist, and an example is the sterile neutrino.
But your question is a little more involved than you might think at first sight. For example if the sterile neutrino only interacts through gravity what interaction caused it to be created in the first place? There is nothing in the Standard Model that could create such particles. However we expect that the Standard Model is a low energy approximation and as we work backwards in time towards the Big Bang and the energies get higher we'll need a grand unified theory like SO(10) and ultimately a quantum theory of gravity (which may or may not be String Theory). These contain interactions that can create particles like sterile neutrinos. However this remains a speculative area of Physics and at the moment we can't say definitely whether such particles exist or if they exist how they were created.
So yes, such a particle the sterile neutrino might exist.  The problem is then:if this particle only interacts by gravity how are we going to measure it?  How are we every going to determine if this particle actually exists other than by its influence on other particles through gravity?  A very difficult experiment to carry out, if ever it can be carried out.  This is sort of like Freeman Dyson's question about detecting an individual graviton. Posted online in an IAS newsletter here, scroll down in the newsletter to Dyson's article (there are other interesting subjects in the newsletter).  Articles about  Dyson's question are here, an arxiv paper abstract here

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