Image via NASA

According to our model of cosmology, dark matter is thought to make up 26.8% of the total energy density  of the observable universe.  This is in stark contrast to regular matter, which makes up a mere 4.9% of the universe. Thus, in order to truly understand the cosmos, we need to define and understand the existence of dark matter—this fact is inescapable.

The ambiguous name "dark matter," reflects our inability to  figure out what this matter is. We can't see it, so we can't identify it; however, we do have a few ideas about what this form of matter could be made of.  One of these hypothetical particles is called axions.  Sometimes called sterile neutrinos, they were first proposed in 1977.  These particles are described as having no spin and no charge; in addition, they are described as interacting noticeably with the strong and weak force and having meager mass (considerably less than an electron).

The Search for Axions:

Science is all about testable predictions, and a lot of our insights come from failures of the past. One such "failure" is what ultimately allows us to text axions. This related to a 2004  experiment involving S/N/S Josephson junction.  This setup involves a loop of two superconductors separated by a thin metal junction that are connected to a sensor.  Electrical signals encounter no resistance on the superconducting material, until they briefly travel through a thin metal that is connected to the sensor. However, this experiment had "problems" with it, in the form of  inexplicable background noise.

Theoretical physicist Christian Beck, from Queen Mary University of London, has made a startling prediction: Dark matter, in the form of axions, can be detected in these small bench-style detectors (as oppose to the massively expensive underground or space-based experiments) by passing through these S/N/S Josephson junction setups.  According his research, these axions may condense together like Bose-Einstein Condensates (think of a whole bunch of axions stuck together with inherently similar properties). This can leave a detectable signal in the metal junctions of these tabletop experiments.

And back in 2004, the team working on a particular experiment like this may have unknowingly already found signals of this BEC of dark matter in the form of axions, detected as part of this anomalous background noise.  Beck calculated that, if this were true, axions would have to have mass of four-billionths of an electron.  He has published his details in Physics Review Journals.  Ultimately, future experiments, which are similar to the S/N/S Josephson junction setup but more elaborate and shielded from other forms of background noise, will be able to discern whether or not this prediction is true.

There are 3 major vexing problems that plague the standard model (in addition to the cosmological problem of dark matter).  They are the cosmological constant prediction, the naturalness problem (which you can read about here) and the strong CP problem (the latter of which is what we are interested in here).

If axions exist, they will have a corresponding axion field, which may help explain the strong CP problem (the sole reason why they were hypothesized in the first place).  This topic is a bit too technical to explain briefly, but it involves the enormous discrepancy between the theoretical prediction of the electric dipole moment of a neutron and its experimentally observed results.  In short, this is in regards to the relative stability of the neutron, and it is said in physics jargon that it doesn't violate CP (charge conjugation and parity).  Which is in stark contrast to other hadrons subjected to the strong force (like mesons) that are ephemeral and decay right away, which do violate charge conjugation and parity.

A 3D map depicting where dark matter resides in our universe. Image credit: ESA/Planck

Source via axiv physics site here


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