Dark matter. There is an overwelming evidence that most of the matter in the universe is not made of ordinary atoms. None of the particles discovered so far can be the dark matter. Hence, there is a discovery to be made. Theorists have tried different ways to tackle this problem and to study the possible candidates, as well as ways to detect them. A theoretically appealing model, supersymmetry, brings about a number of new particles. Some of these new particles, for example, neutralinos and SUSY Q-balls can be the dark matter.

Of course a number of other possibilities remain. The dark matter may consist of axions, which are very light bosons associated with the breaking of the Peccei-Quinn symmetry. Superheavy relic particles can also be dark matter; their decays can produce cosmic rays of ultrahigh energies. Another candidate is a singlet neutrino with mass in the keV range. If such a particle exists, it could be emitted asymmetrically from a cooling neutron star in the event of the supernova explosion, and the recoil momentum impacted on the neutron star would explain the observed pulsar velocities. The list of possible candidates is long, and there are many possibilities. However we may be at the brink of a big discovery that will unveil the most commont form of matter in the universe.

Ultra-high-energy cosmic rays and neutrinos. The origin of ultrahigh-energy cosmic rays presents a tanalizing puzzle. Experiments have observed cosmic ray particles, each of which carries as much energy as a bullet shot from a rifle. The problem is twofold: it is difficult to accelerate particles to such enormous energies, and it is also hard to understand how these particles could reach us from large distances, despite their interactions with the cosmic microwave background radiation. Theorists have explored various possible explanations, some of which point to new phyisics. Theorists in our group work closely with the experimentalists; in particular, Graciela Gelmini and Alex Kusenko are members of the Pierre Auger collaboration.

Baryogenesis. Although every particle has its antiparticle, there is no antimatter in the observed universe. The process in which the matter-antimatter asymmetry was produced in the early universe, called baryogenesis, remains a mystery. A number of solutions have been proposed. One possibility, the so called Affleck-Dine baryogenesis, could have taken place in the early universe if supersymmetry is right and if cosmological inflation took place. This process could produce both ordinary matter and dark matter.