The Low-Redshift Baryon Census by Direct Addition: Component Shares, Their Sum, and the Residual

The baryon census is usually presented as a list of components measured by different communities with different instruments. This paper treats the list as an arithmetic object: it adopts published shares of the cosmic baryon density for six components — stars 0.07, cold interstellar gas 0.017, circumgalactic gas 0.05, hot intracluster gas 0.04, the photoionised intergalactic phase 0.28, and the warm-hot phase 0.43 — and adds them. The accounted fraction is 0.887 with a quadrature uncertainty of 0.12 from the stated component errors, leaving a residual of 0.113 of all baryons, which differs from zero by 0.94 standard deviations under those errors: the census, at face value, is consistent with closure and also consistent with a tenth of the baryons still unlocated. In density terms the warm-hot component carries Omega = 0.0212 and the residual Omega = 0.005571. A cubic volume 100 Mpc on a side contains 6.22e+15 solar masses of baryons in total and 2.67e+15 in the warm-hot phase alone.