A HYDRODYNAMIC APPROACH TO COSMOLOGY - THE PRIMEVAL BARYON ISOCURVATURE MODEL

The primeval baryon isocurvature (PBI) model for the origin of cosmolo gical structure is explored with the aid of detailed numerical simulat ions. 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 ran ge OMEGA0 = 0.1-0.2. The amplitude of the primeval entropy fluctuation s is normalized to the COBE measurements, and the spectrum of the entr opy fluctuations is taken to be a power law with index in the range m = -0.5 to 0.0. We use H-0 = 80 km s-1 Mpc-1. Shortly after decoupling in this model, a large fraction of the mass must condense to a dark co llisionless component such as low-mass stars or massive black holes. W e follow the two-component (gas+collisionless) mixture with a hydro+pa rticle-mesh (PM) code in a (64 h-1 Mpc)3 box containing 128(3) = 10(6. 3) cells and particles, and a full nonequilibrium treatment of radiati on, ionization, heating, and cooling. Additional large PM simulations are made in a box with size 400 h-1 Mpc containing 250(3) = 10(7.2) pa rticles. The PBI model has more primeval density fluctuation power at both long and short wavelengths (and less at inter-mediate wavelengths ) than the standard cold dark matter (CDM) model, and it has a peak at approximately 600 h-2 Mpc, the Jeans mass prior to decoupling. The po wer spectrum, when convolved with the temperature history of the gas, allows gravitational growth of structure at three characteristic epoch s: at 1200 > z > 250 the collapse of mass concentrations at DELTAM alm ost-equal-to 10(5)-10(8) M. produces the assumed dark collisionless ba ryonic component; at 40 > z > 6 galaxy spheroids form at masses in the range DELTAM almost-equal-to 10(10.5)-10(12) M.; and at z < 1 large-s cale structures form at AM greater than or similar to 10(12.5) M.. For our parameters, 10%-15% of the residual baryons collapse to form gala xies at 10 greater-than-or-equal-to z greater-than-or-equal-to 5, givi ng an acceptable mean luminosity density for M(stellar)/L(B) = 3-4 (so lar units), and an acceptable galaxy mass function for M(tot)/L(B) alm ost-equal-to 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 ric h clusters also are acceptable, with modest amounts (10%) of merging c ontinuing at low redshift (z < 0.3). Principal differences between thi s model and the standard OMEGA = 1 cold dark matter model are (1) the small-scale relative velocity field is much lower (300-350 km s-1 vers us 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 coh erence length of the peculiar velocity field is considerably greater ( bulk flow in 100 h-1 Mpc sphere of 500 km s-1 versus approximately 200 km s-1 in CDM); (3) early ionization makes it much more likely that t he thermal cosmic background radiation has been scattered well after d ecoupling, considerably reducing cosmic background radiation fluctuati ons on scales smaller than about 2-degrees; and (4) galaxies and clust ers of galaxies are assembled at much higher redshifts. The central pr oblem for the PBI model is the dynamical evidence from large-scale pec uliar motions that the density parameter may be close to unity. If thi s is so, the PBI model is uninteresting. In all other cases we can see observational advantages for PBI over CDM and other OMEGA = 1 models.