1996/04/30 by Joel R. Primack, Primack, Joel R.
Computer Science · Physics and Astronomy · #Computational Physics and Python Applications #Cosmology and Gravitation Theories #astro-ph #hep-ph
paper · pdf · doi:10.48550/arxiv.astro-ph/9604184
31 pages. To appear in International School of Physics ``Enrico Fermi'', Course CXXXII: Dark Matter in the Universe, Varenna 1995, eds. S. Bonometto, J.R. Primack, A. Provenzale
arxiv created 1996/05/01 · arxiv updated 2009/11/30
The cosmological parameters that I discuss are the Hubble parameter H0 ≡ 100 h km s-1 Mpc-1, the age of the universe t0, the average density Ω0, and the cosmological constant Λ. To focus the discussion, I concentrate on the the value of Ω0 in currently popular models in which most of the dark matter is cold, especially Cold + Hot Dark Matter (CHDM) and flat (Ω0 + ΩΛ=1) low-Ω CDM with a Cosmological Constant (ΛCDM). The evidence would favor small Ω0 ≈ 0.3 if (1) the Hubble parameter actually has the high value h ≈ 0.75 favored by many observers, and t0 ≥ 13 Gy; or (2) the baryonic/total mass ratio in clusters of galaxies is actually ∼ 15%, about 3 times larger than expected for standard BBN in an Ω=1 universe, Ωb ≈ 0.0125 h-2, despite the recent measurement by Tytler of D/H=2.4× 10-5 in two high-redshift Lyman limit systems, implying Ωb≈ 0.024 h-2. The evidence would favor Ω=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- Ω 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-Ω0 ΛCDM model. Statistics on gravitational lensing of quasars provide an upper limit on Λ, and the preliminary results on the deceleration parameter q0=Ω0/2-ΩΛ on very large scales from high-redshift Type Ia supernovae suggest that Ω0 ∼ 1 and ΩΛ is small.