Monday, April 8, 2013

Dark Matter - after 80 years, a mystery endures

Dark matter is one of the enduring mysteries of the universe.  The first signals were observed by Fritz Zwicky back in the 1930's.  Zwicky and others observed that there was not enough visible mass in galaxies to provide the gravitational attraction required to keep them bound as they are.  By 'visible mass,' he meant the mass that could be seen i.e. stars.   He also determined that most of the'missing mass' was more broadly distributed than the visible mass.    Initially, this was thought to be likely due to dust clouds or other non-luminous mass.  However, over the past 80 years, scenarios where the hidden mass consists of 'normal' matter - protons, neutrons and electrons - have been largely ruled out. Among other things, if dark matter were 'normal,' it would have altered the density of heavy (heavier than hydrogen) nuclei produced in the early universe.   We have also observed many different signatures of dark matter, at very different distance scales - individual galaxies, galaxy clusters, and signs from the big bang.

For example, the recent findings from the Planck satellite (right) shows that dark matter is needed to explain the observed fluctuations in the cosmic microwave background radiation.  Further, this dark matter must be 'cold,' that is moving at much less than the speed of light.  In other words, it is relatively heavy; the most popular theories predict masses from 10 to 1,000 times the mass of a proton.   So, by process of elimination, most scientists believe that dark matter must be made of some form of as-yet-unseen particles.  A modification of the rules of gravitation cannot be ruled out, but we have not yet found a satisfactory modification.

There are many efforts to look for dark matter, using diverse techniques.  High-energy physicists are working hard, looking for signs that these particles have been produced in proton-proton collisions at the LHC.

Others physicists are looking for the signature of dark matter interacting in a laboratory detector - "direct detection."  In essence, a dark matter particle will bounce off a target nucleus, causing it to recoil.  The details of the recoil energy spectrum depend on the type of dark matter particle and its mass.  Because the cross-sections are tiny and because very little energy is transferred in these interactions, this is best done using large detectors cooled to cryogenic temperatures, and operated in deep underground laboratories, to avoid the background from cosmic rays. Many different types of dark matter detectors are being pursued, with rather different experimental strategies, involving different types of target material, and different strategies to observe the recoil energy.

Finally, many astrophysical experiments are searching for new signs of dark matter in the cosmos.  These searches rely on the idea that dark matter is likely to be its own antiparticle, so two dark matter particles can collide and annihilate, producing a shower of normal-matter particles, which can then be detected using conventional detectors.    The idea that dark matter is its own antiparticle may seem very strange, but it happens naturally in many theories, such as supersymmetry.   These searches look for dark mater annihilation anywhere that dark matter is expected to cluster.  In other words, anywhere that gravity is strong.  Locally, IceCube has searched for dark matter annihilation in the center of the Sun; a similar search from the center of the Earth is coming soon.   IceCube has also looked for signs of dark matter annihilation in the center and halo of our galaxy, and, coming soon, from nearby spheroidal dwarf galaxies.  The latter are of interest because they are believed to have a very high ratio of dark matter to normal matter.  Of course, for galactic searches, many other particles can be studied.   The Fermi observatory (right) has looked for photons coming from the galactic center and galactic halo.  Last year, there were some reports of photons with an energy of 130 GeV coming from the galaxy, but further instrumental studies are required to know if this is real.

The AMS experiment on  the space station (right)  has recently published a report of an excess of positrons (compared with theoretical expectations), with energies up to 350 GeV.  This excess is of particular interest because the positron fraction appears to increase with energy, while theory predicts that it should decrease.  Of course, the same behavior could be because there is a relatively local source of cosmic-ray positrons.

This is a lot of different approaches to dark matter.   One may wonder if there is a coherent strategy here.  The answer is 'sort-of.'   In the absence of a single clear idea what dark matter is, no single approach is known to work.  So, we are pursuing a large number of different approaches, based on what different scientists (and funding agencies) find attractive.  For the current scope of experiments, the multiple approaches are technically and financially feasible.  If none of the current efforts bear fruit, a larger experiment may be needed; this will require a clear strategy choice.  However, to inject a note of caution, different models of dark matter predict widely varying behaviors (interaction and annihilation cross-sections, etc.), and not all of these models lead to experimentally observable consequences, with current or planned future detectors. 

In short, dark matter is an enduring mystery.  After 80 years of effort in diverse areas, we know a great deal about its affect on the universe.  However, we still don't have any direct evidence for its existence, and we really don't know what it is.

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