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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 high-redshift 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 present-epoch 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 $q_0=Omega_0/2-Omega_Lambda$ on very large scales from high-redshift Type Ia supernovae suggest that $Omega_0 sim 1$ and $Omega_Lambda$ is small. | | Source: | arXiv, astro-ph/9604184 | | Services: | Forum | Review | PDF | Favorites | |
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