Values for the physical baryon density shown in the figure are dominated by determinations based on two separate methods: 1) CMB determinations, in which the derived baryon density depends primarily on the amplitudes of the first and second peaks of the temperature power spectrum (e.g., Page et al. 2003), and 2) primordial deuterium abundance (D/H) measurements coupled with Big Bang Nucleosynthesis (BBN) theory.
We quote four separate determinations of baryon density which use D/H+BBN to analyze independent observations of Lyα clouds in QSO or damped Lyα systems (Pettini & Bowen 2001, Kirkman et al. 2003, Pettini & Cooke 2012, Cooke et al. 2016). Error estimates depend both on observational and modeling uncertainties. Kirkman et al. (2003) note that some early determinations may have underestimated the errors; see also e.g. Dvorkin et al. (2016) for further discussion. With continued improvements in data quality and reduction of observational systematics, Cooke et al. (2016) raise the near-term prospect of D/H+BBN determinations of Ωbh2 with uncertainties rivaling those derived using CMB data. However, both Cooke et al. (2016) and Dvorkin et al. (2016) note the increasing importance of accurate nuclear cross section data in the BBN computations. We illustrate this in the figure by quoting two estimates for Ωbh2 from Cooke et al. (2016), which are derived from the same D/H data but use two different estimates of a key nuclear cross section in the BBN computation.
We also include a determination using "present day" measurements. Steigman, Zeller & Zentner: (2002) employed SNIa data and the assumption of ΛCDM to obtain Ωm, and then computed the baryon density by combining Ωm with an estimate of the baryon fraction fb derived from X-ray observations of clusters of galaxies (e.g. Ωb = fb Ωm).
There is general good agreement between the recent CMB and D/H+BBN determinations shown in the figure. As noted above, reduction of baryon density uncertainties determined via D/H+BBN will require an increased accuracy in modeling inputs.
Most of the visible mass in the universe is composed of baryons (protons and neutrons). Values for the physical baryon density in the universe have generally been determined using either observations of the CMB or deuterium abundance in distant gas clouds with near-primordial composition. Big Bang Nucleosynthesis (BBN) models the production of light elements such as deuterium in the early universe, and given an observed deuterium abundance (D/H), BBN can predict baryon content. We include four such determinations in the plot; these serve as an important independent check on baryonic densities derived from ΛCDM model fits to CMB power spectra. Recent determinations of baryonic content based on CMB and deuterium abundance analyses are in reasonable agreement: see text for discussion of the 2016 D/H+BBN values. Only about 4-5% of the present-day universe is comprised of baryons. The gray vertical line, representing the weighted average of WMAP and Planck data points, is positioned at 100Ωbh2 = 2.239.
Contributed by the NASA / LAMBDA Archive Team.