A Lower Bound on the Cosmic Baryon Density

作者：Weinberg, D H, Miralda-Escudé, J, Hernquist, L E, Katz, N · 发表于：CERN Document Server (European Organization for Nuclear Research) · 年份：1997 · 被引用次数：147 · 研究领域：Galaxies: Formation, Evolution, Phenomena、Astrophysics and Star Formation Studies、Cosmology and Gravitation Theories

We derive analytic lower bounds on the cosmic baryon density by requiring that the high-redshift IGM contain enough neutral hydrogen to produce the observed \\lya absorption in quasar spectra. The key theoretical assumption is that absorbing structures are no more extended in redshift space than in real space. This assumption might not hold if \\lya clouds are highly overdense and thermally broadened, but it is likely to hold in the gravitational instability picture for the \\lya forest suggested by cosmological simulations, independently of the details of the cosmological model. The other ingredients in these bounds are an estimate of the UV background from quasars, a temperature $T~10^4K$ for the "warm" photoionized IGM that produces most of the the mean \\lya flux decrement $\\Dbar$ or, for a more restrictive bound, the distribution function $P(\\tau)$ of \\lya optical depths. With plausible parameter values, the mean decrement bound implies a baryon density parameter 0.005/h^2$, but the required clustering of the IGM is then incompatible with other properties of quasar spectra. A recent observational determination of $P(\\tau)$ implies $Ømb \\ga 0.0125/h^2$ even for a conservative estimate of the UV background, and $Ømb \\ga 0.018/h^2$ for a more reasonable estimate. These bounds are consistent with recent *low* estimates of the primordial deuterium-to-hydrogen ratio $\\dtoh$, which imply $Ømb \\approx 0.025/h^2$, but the $P(\\tau)$ bound can only be reconciled with high ...