Cosmic Walls: Surface Density, Face-On Columns and Collapse Timescales of the Sheet Component

Walls — the sheet-like panels of the cosmic web — hold a substantial share of matter in the lowest densities at which structure is still organised, yet they are the least directly measured component. This paper computes the basic budget of a stated wall: mean gas overdensity 5.0, thickness 3 Mpc. Its baryon surface density is 0.0932 solar masses per square parsec, a face-on sightline crosses a hydrogen column of 8.85e+18 per square centimetre and gains 3.32 pc per cubic centimetre of dispersion, and the sheet's free-fall time at its mean total density is 18.2 billion years against a Hubble time of 14.5 billion years — walls, at this density, are still assembling rather than collapsed. A deliberately crude two-phase estimate of the wall mass share, given a volume share of 0.16, returns 0.488, well above classification-based shares near a quarter, and the paper identifies why the crude estimate overstates: it prices the remaining volume at the cosmic mean density although most of that volume is underdense void.