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Status of Cosmological Parameters: $Omega_0approx 0.3$ vs. $Omega=1$   Date: 
1 May 1996  Abstract:  The cosmological parameters that I discuss are the Hubble parameter $H_0 equiv 100 h$ km s$^{1}$ Mpc$^{1}$, the age of the universe $t_0$, the average density $Omega_0$, and the cosmological constant $Lambda$. To focus the discussion, I concentrate on the the value of $Omega_0$ in currently popular models in which most of the dark matter is cold, especially Cold + Hot Dark Matter (CHDM) and flat ($Omega_0 + Omega_Lambda=1$) low$Omega$ CDM with a Cosmological Constant ($Lambda$CDM). The evidence would favor small $Omega_0 approx 0.3$ if (1) the Hubble parameter actually has the high value $h approx 0.75$ favored by many observers, and $t_0 geq 13$ Gy; or (2) the baryonic/total mass ratio in clusters of galaxies is actually $sim 15$\%, about 3 times larger than expected for standard BBN in an $Omega=1$ universe, $Omega_b approx 0.0125 h^{2}$, despite the recent measurement by Tytler of $D/H=2.4 imes 10^{5}$ in two highredshift Lyman limit systems, implying $Omega_bapprox 0.024 h^{2}$. The evidence would favor $Omega=1$ if (1) the POTENT analysis of galaxy peculiar velocity data is right, in particular regarding outflows from voids or the inability to obtain the presentepoch non Gaussian density distribution from Gaussian initial fluctuations in a low $Omega$ universe; or (2) the preliminary LSND report indicating neutrino mass $gsim 2.4$ eV is right, since that would be too much hot dark matter to allow significant structure formation in a low$Omega_0$ $Lambda$CDM model. Statistics on gravitational lensing of quasars provide an upper limit on $Lambda$, and the preliminary results on the deceleration parameter $q_0=Omega_0/2Omega_Lambda$ on very large scales from highredshift Type Ia supernovae suggest that $Omega_0 sim 1$ and $Omega_Lambda$ is small.  Source:  arXiv, astroph/9604184  Services:  Forum  Review  PDF  Favorites 


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