Abstract
The origin of non-baryonic cold dark matter (DM) and baryon asymmetry in the Universe are among the most long-standing puzzles in cosmology as well as in particle physics today. In particular, the question why the energy density of baryons fib and that of dark matter ODM are within an order of magnitude from each other remains a mystery. A standard candidate for the non-baryonic cold dark matter is a weakly interacting massive particle (WIMP). Perhaps the most popular WIMP is the lightest supersymmetric particle (LSP) which remains stable owing to the conservation of R parity, in particular the lightest neutralino in the Minimal Supersymmetric Standard Model (MSSM) . We often assume that the neutralino freezes out of thermal equilibrium in the early Universe, however this economical framework does not explain why fib and C I D M are so close to each other. There are other well motivated candidates, gravitino,' axion which may solve the strong CP problem in QCD by Peccei-Quinn mechanism,2 and its fermionic supersymmetric partner ax in^.^^^
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