The Cosmic Web Forms

13.8 billion years of gravity turning near-uniformity into filaments, knots, and voids

z ≈ 49 — almost smooth

Matter is spread nearly evenly; the only structure is a whisper of density ripples about one part in a thousand, inherited from the universe's first instants.

Collapse begins

Gravity amplifies the whisper. Slightly denser regions pull material from slightly emptier ones; voids open and drain, and matter streams onto sheets, then into filaments.

z = 0 — the web

Filaments channel matter into the knots where they cross — the sites of galaxy clusters. Nearly half of all matter now lives in the filaments; the voids hold almost none.

Structure From Within

No blueprint, no boundary, no outside architect: the web assembles itself as each region follows the gradients its neighbors create. The thermodynamic record of that assembly is measurable — entropy production per unit mass climbs steadily from void to sheet to filament to knot.

Particle-mesh N-body simulation: 2.1 million particles, 128³ mesh, 100 Mpc/h box, ΛCDM (Ωm = 0.31, σ8 = 0.81), Zel'dovich initial conditions at z = 49. Large-scale power growth verified against the linear growth factor D(a). An illustration of standard gravitational structure formation — evocative for the book's argument, but not evidence for it: the cosmic web cannot be run without gravity's rules, so it illustrates self-organization rather than testing any claim about it.

Grounding: The void → sheet → filament → knot entropy-production gradient is measured in IllustrisTNG (KC#CWEB-ENTROPY-ROBUST: monotonic in Ṡ/M across TNG50-4 and TNG100-3, all redshifts tested; filaments hold ~48% of baryon mass, matching Ch. 14's "forty to fifty percent"). Simulation code: research/experiments/cosmic_web_pm_animation.py; growth validation: analyze_cosmic_web_pm_validation.py. Interpretive ceiling per KC#CWEB-ABLATION-BOUNDARY: illustrative, not evidential.