A Supersymmetric Universe?

What is the universe made of? This age-old question is still the subject of heated discussion in particle-physics circles. A large body of astrophysical observations now clearly points to a beginning for our universe about 14 billion years ago in a cataclysmic outpouring of elementary particles. There is, in fact, no evidence that any of the particles of matter with which we are now familiar existed before this great event. Most of the (apparently) stable particles that we now see around us -- photons, electrons, quarks, and several species of neutrinos making up all of the visible universe -- were created at this time. In addition to these, it is now known that several additional species of quarks and electron-like "leptons," as well as other particles ("gauge bosons") that serve as the force carriers of physics, existed in equilibrium for the first fraction of a microsecond after the beginning. These particles, which then decayed away, are briefly produced again in high-energy experiments.

However, it is now becoming clear from a variety of astrophysical observations and from hints emerging from accelerator experiments that all of these particles and all of the matter visible to telescopes operating from radio to gamma-ray frequencies constitute no more than 5 percent of the actual matter in the universe with less than 1 percent residing in stars and planets . The great bulk of the matter of the universe, now known as "dark matter," seems to interact only gravitationally or through the weak force with the particles of which we and our planet are made. Another component known as "dark energy" is also far more important than the "normal" matter that makes up our planet and our bodies. This dark energy is due, at least in part, to virtual particles that are constantly bubbling up from the vacuum and then evaporating in obedience to the uncertainty principle. It accounts for about 70 percent of the universe.

The leading candidate theory for dark matter and dark energy, now being actively pursued at UA and at many other universities around the world, is known as supersymmetry (susy). This is an idea of great aesthetic beauty, although it is supported at present by only a few suggestive but indirect experiments. It suggests that each of the known particles of nature is related to an as-yet-undiscovered "partner" particle. The lightest of these supersymmetric partners is expected to be stable and to account for most of the dark matter. If supersymmetry were exact the susy particles would have the same mass as their "normal matter" partners. In this case, apart from possible effects from higher dimensions, the dark energy would vanish.

The observation of a positive dark energy in the universe suggests that there might be a supersymmetric universe, nearby in parameter space, with vanishing vacuum energy. Since physical systems tend to go to the state of minimum energy, it is likely that our universe will ultimately undergo a phase transition to such a supersymmetric universe. The properties of this susy universe have been the subject of a recent investigation. A prominent question remaining to be resolved is whether life could exist in such a background. It is also possible that a supersymmetric bubble universe could exist at present in some distant part of our universe. The theory allows that such a bubble could be stable against collapse and could be confined to a region of higher density than the surrounding vacuum.