THE X-RAY CLUSTER BARYON CRISIS

Nucleosynthesis in the standard hot big bang cosmology offers a successful account of the production of the light nuclides during the early evolution of the Universe. Consistency among the predicted and observed abundances of D, $^3$He, \hel4 and \li7 leads to restrictive lower and upper bounds to the present density of nucleons. In particular, the upper bound ensures that nucleons cannot account for more than a small fraction $(<0.06 h{^{-2} _{50}})$ of the mass in a critical density (Einstein-de Sitter) Universe. In contrast, x-ray observations of rich clusters of galaxies suggest strongly that baryons (in hot gas) contribute a significant fraction of the total cluster mass $(\ge 0.2 h{^{-3/2} _{50}})$. If, indeed, clusters do provide a ``fair" sample of the mass in the Universe, this ``crisis" forces us to consider other ways of mitigating it, including the politically incorrect possibility that $\Omega < 1$. The options, including magnetic or turbulent pressure, clumping, and non-zero space curvature and/or cosmological constant, are discussed.