A Hydrodynamic Approach to Cosmology: The Primeval Baryon Isocurvature Model

The primeval baryon isocurvature (PBI) model for the origin of cosmological structure is explored with the aid of detailed numerical simulations. In this model we assume there is no exotic dark matter and that we live in an open universe with baryonic content initially in the range Ω = 0. 1-0. 2. The amplitude of the primeval entropy fluctuations is normalized to the COBE measurements, and the spectrum of the entropy fluctuations is taken to be a power law with index in the range m = -0. 5 to 0. 0. We use H0 = 80 km s-1 Mpc-1. Shortly after decoupling in this model, a large fraction of the mass must condense to a dark collisionless component such as low-mass stars or massive black holes. We follow the two-component (gas + collisionless) mixture with a hydro + particle-mesh (PM) code in a (64 h -1 Mpc) ^3 box containing 1283 = 106. 3 cells and particles, and a full nonequilibrium treatment of radiation, ionization, heating, and cooling. Additional large PM simulations are made in a box with size 400 h-1 Mpc containing 2503 = 107. 2 particles. The PBI model has more primeval density fluctuation power at both long and short wavelengths (and less at intermediate wavelengths) than the standard cold dark matter (CDM) model, and it has a peak at ∼ 600 h-2 Mpc, the Jeans mass prior to decoupling. The power spectrum, when convolved with the temperature history of the gas, allows gravitational growth of structure at three characteristic epochs: at 1200 > z > 250 the collapse of mass concentrations at ΔM 105-108 Msun produces the assumed dark collisionless baryonic component; at 40 > z > 6 galaxy spheroids form at masses in the range ΔM 1010. 5-1012 Msun and at z 12. 5 Msun. For our parameters, 10%-15% of the residual baryons collapse to form galaxies at 10 ≥ z ≥ 5, giving an acceptable mean luminosity density for Mstellar/LB = 3-4 (solar units), and an acceptable galaxy mass function for Mtot/LB ≈ 200. The position correlation function of galaxies in the model is acceptable with a modest bias of galaxy candidates over dark matter (b = 1. 1). The abundance of and correlation function among rich clusters also are acceptable, with modest amounts (10%) of merging continuing at low redshift (z Principal differences between this model and the standard Ω = 1 cold dark matter model are (1) the small- scale relative velocity field is much lower (300-350 km s-1 versus 800-1000 km s-1 at 5 h-1 Mpc), reflecting a systematically smaller value of the true dynamical density parameter; (2) the large-scale coherence length of the peculiar velocity field is considerably greater (bulk flow in 100 h-1 Mpc sphere of 500 km s-1 versus ∼200 km s-1 in CDM) ; (3) early ionization makes it much more likely that the thermal cosmic background radiation has been scattered well after decoupling, considerably reducing cosmic background radiation fluctuations on scales smaller than about 2° and (4) galaxies and clusters of galaxies are assembled at much higher redshifts. The central problem for the PBI model is the dynamical evidence from large-scale peculiar motions that the density parameter may be close to unity. If this is so, the PBI model is uninteresting. In all other cases we can see observational advantages for PBI over CDM and other Ω = 1 models.